Mixing
Why are two stages used for an instrumentation amplifier?
Clash Royale CLAN TAG#URR8PPP
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty margin-bottom:0;
up vote
10
down vote
favorite
When we have a two-stage instrumentation amplifier, such as the following.
Why do we need the first stage of the two Operational Amplifiers? Couldn't we just input V1 and V2 into the differential amplifier?
amplifier instrumentation-amplifier diff-amp
asked Sep 9 at 20:17
Bee
1148
 |Â
show 1 more comment
up vote
10
down vote
favorite
When we have a two-stage instrumentation amplifier, such as the following.
Why do we need the first stage of the two Operational Amplifiers? Couldn't we just input V1 and V2 into the differential amplifier?
amplifier instrumentation-amplifier diff-amp
asked Sep 9 at 20:17
Bee
1148
2
Look at the input impedances
– PlasmaHH
Sep 9 at 20:19
Infinite for the first stage, and R2 for second stage, right?
– Bee
Sep 9 at 20:20
Input resistance is one point which is sometimes important but increasing CMRR electronics.stackexchange.com/questions/343096/… is another key benefit
– carloc
Sep 9 at 20:34
No, different input impedences for the two inputs of stage 2
– Scott Seidman
Sep 9 at 20:42
If you have any imbalance in capacitance on Vin+ versus on Vin-, the common-mode rejection is in peril.
– analogsystemsrf
2 days ago
 |Â
show 1 more comment
up vote
10
down vote
favorite
up vote
10
down vote
favorite
When we have a two-stage instrumentation amplifier, such as the following.
Why do we need the first stage of the two Operational Amplifiers? Couldn't we just input V1 and V2 into the differential amplifier?
amplifier instrumentation-amplifier diff-amp
asked Sep 9 at 20:17
Bee
1148
When we have a two-stage instrumentation amplifier, such as the following.
Why do we need the first stage of the two Operational Amplifiers? Couldn't we just input V1 and V2 into the differential amplifier?
amplifier instrumentation-amplifier diff-amp
amplifier instrumentation-amplifier diff-amp
asked Sep 9 at 20:17
Bee
1148
asked Sep 9 at 20:17
Bee
1148
asked Sep 9 at 20:17
Bee
1148
asked Sep 9 at 20:17
Bee
1148
asked Sep 9 at 20:17
Bee
1148
1148
2
Look at the input impedances
– PlasmaHH
Sep 9 at 20:19
Infinite for the first stage, and R2 for second stage, right?
– Bee
Sep 9 at 20:20
Input resistance is one point which is sometimes important but increasing CMRR electronics.stackexchange.com/questions/343096/… is another key benefit
– carloc
Sep 9 at 20:34
No, different input impedences for the two inputs of stage 2
– Scott Seidman
Sep 9 at 20:42
If you have any imbalance in capacitance on Vin+ versus on Vin-, the common-mode rejection is in peril.
– analogsystemsrf
2 days ago
 |Â
show 1 more comment
2
Look at the input impedances
– PlasmaHH
Sep 9 at 20:19
Infinite for the first stage, and R2 for second stage, right?
– Bee
Sep 9 at 20:20
Input resistance is one point which is sometimes important but increasing CMRR electronics.stackexchange.com/questions/343096/… is another key benefit
– carloc
Sep 9 at 20:34
No, different input impedences for the two inputs of stage 2
– Scott Seidman
Sep 9 at 20:42
If you have any imbalance in capacitance on Vin+ versus on Vin-, the common-mode rejection is in peril.
– analogsystemsrf
2 days ago
2
2
Look at the input impedances
– PlasmaHH
Sep 9 at 20:19
Look at the input impedances
– PlasmaHH
Sep 9 at 20:19
Infinite for the first stage, and R2 for second stage, right?
– Bee
Sep 9 at 20:20
Infinite for the first stage, and R2 for second stage, right?
– Bee
Sep 9 at 20:20
Input resistance is one point which is sometimes important but increasing CMRR electronics.stackexchange.com/questions/343096/… is another key benefit
– carloc
Sep 9 at 20:34
Input resistance is one point which is sometimes important but increasing CMRR electronics.stackexchange.com/questions/343096/… is another key benefit
– carloc
Sep 9 at 20:34
No, different input impedences for the two inputs of stage 2
– Scott Seidman
Sep 9 at 20:42
No, different input impedences for the two inputs of stage 2
– Scott Seidman
Sep 9 at 20:42
If you have any imbalance in capacitance on Vin+ versus on Vin-, the common-mode rejection is in peril.
– analogsystemsrf
2 days ago
If you have any imbalance in capacitance on Vin+ versus on Vin-, the common-mode rejection is in peril.
– analogsystemsrf
2 days ago
 |Â
show 1 more comment
5 Answers
5
active
oldest
votes
up vote
14
down vote
One of the biggest benefits of the 3 op amp INA is the equal and high input impedance. The op amp's non-inverting pins' input impedance can be up in the $TOmega$ range. I'll leave it as an exercise for you, but if you look at the difference amplifier circuit, the input impedance of the negative input varies with the positive input.
answered Sep 9 at 20:21
Matt Young
12.1k42459
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
6
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
add a comment |Â
up vote
12
down vote
The 3 op-amp design has three main advantages over a single op-amp differential amplifier.
- The input impedance is much higher, since the inputs drive directly into an op-amp input rather than into a resistive divider.
- The gain can be set by changing a single resistor, so the critical parts can be easily integrated on to one chip (maximizing symmetry) with a single external resistor for setting the gain.
- In high gain configurations the common mode rejection is much better because the gain of the first stage effectively multiplies the common mode rejection of the second stage.
Note that in general it is better to use a specific instrumentation amplifier chip than to try to build it yourself out of separate parts. Having everything on one chip improves symmetry and hence common mode rejection.
answered 2 days ago
Peter Green
10.7k11837
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
add a comment |Â
up vote
3
down vote
In addition to input impedance concerns, gain in two stages offers better frequency response.
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
1
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
add a comment |Â
up vote
1
down vote
There is a single stage, high input impedance, differential amplifier.
simulate this circuit – Schematic created using CircuitLab
Without Rg, this has a gain of (f+1). Rg can be used to increase the gain.
However it has performance compromises over the 3 amplifier version.
a) It has less open loop gain, so very high gains are not as stable
b) The two signal paths have different phase shift, so common mode rejection only works to low frequencies. You can improve this a bit with a strategically placed capacitor though
c) At low gains, common mode range is limited by the power supply headroom
d) It's more complicated to draw correctly than the 3 amplifier version. It's only recently that I've created a mnemonic to get the resistors in the right places from memory.
But, if you have only 2 amplifiers left, then it does work. It does allow you control of the gain with a single variable resistor, just like the 3 amplifier version, but unfortunately, just the 3 amp version, this resistor is floating.
answered 2 days ago
Neil_UK
69.3k272152
add a comment |Â
up vote
-2
down vote
The above answers are reliable, but i want to add something. Consider the differential amplifier: 
When a person wants to vary the amplifier gain (for instance to exploit the maximum resolution of ADC) the 2 resistors with value K⋅R must be adjusted perfectly synchronously, via electromechanical adjustable resistors so an slight runout or wearing of these resistors results in imbalance between the value of this two resistors then it results in not to be neglected common mode factor. Usually the common mode in the previous stage (Wheatstone bridge for example) is way bigger than differential mode then it results in false measurements.
edited 2 days ago
Elliot Alderson
2,5801310
answered Sep 9 at 20:58
Sam Farjamirad
16919
5
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
2
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
 |Â
show 5 more comments
5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
14
down vote
One of the biggest benefits of the 3 op amp INA is the equal and high input impedance. The op amp's non-inverting pins' input impedance can be up in the $TOmega$ range. I'll leave it as an exercise for you, but if you look at the difference amplifier circuit, the input impedance of the negative input varies with the positive input.
answered Sep 9 at 20:21
Matt Young
12.1k42459
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
6
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
add a comment |Â
up vote
14
down vote
One of the biggest benefits of the 3 op amp INA is the equal and high input impedance. The op amp's non-inverting pins' input impedance can be up in the $TOmega$ range. I'll leave it as an exercise for you, but if you look at the difference amplifier circuit, the input impedance of the negative input varies with the positive input.
answered Sep 9 at 20:21
Matt Young
12.1k42459
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
6
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
add a comment |Â
up vote
14
down vote
up vote
14
down vote
One of the biggest benefits of the 3 op amp INA is the equal and high input impedance. The op amp's non-inverting pins' input impedance can be up in the $TOmega$ range. I'll leave it as an exercise for you, but if you look at the difference amplifier circuit, the input impedance of the negative input varies with the positive input.
answered Sep 9 at 20:21
Matt Young
12.1k42459
One of the biggest benefits of the 3 op amp INA is the equal and high input impedance. The op amp's non-inverting pins' input impedance can be up in the $TOmega$ range. I'll leave it as an exercise for you, but if you look at the difference amplifier circuit, the input impedance of the negative input varies with the positive input.
answered Sep 9 at 20:21
Matt Young
12.1k42459
edited Sep 9 at 20:32
answered Sep 9 at 20:21
Matt Young
12.1k42459
answered Sep 9 at 20:21
Matt Young
12.1k42459
answered Sep 9 at 20:21
Matt Young
12.1k42459
12.1k42459
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
6
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
add a comment |Â
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
6
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
Why do we want a high input impedance?
– Bee
Sep 9 at 20:29
6
6
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
@Bee so that we can measure from sources that have a high output impedance
– BeB00
Sep 9 at 20:33
add a comment |Â
up vote
12
down vote
The 3 op-amp design has three main advantages over a single op-amp differential amplifier.
- The input impedance is much higher, since the inputs drive directly into an op-amp input rather than into a resistive divider.
- The gain can be set by changing a single resistor, so the critical parts can be easily integrated on to one chip (maximizing symmetry) with a single external resistor for setting the gain.
- In high gain configurations the common mode rejection is much better because the gain of the first stage effectively multiplies the common mode rejection of the second stage.
Note that in general it is better to use a specific instrumentation amplifier chip than to try to build it yourself out of separate parts. Having everything on one chip improves symmetry and hence common mode rejection.
answered 2 days ago
Peter Green
10.7k11837
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
add a comment |Â
up vote
12
down vote
The 3 op-amp design has three main advantages over a single op-amp differential amplifier.
- The input impedance is much higher, since the inputs drive directly into an op-amp input rather than into a resistive divider.
- The gain can be set by changing a single resistor, so the critical parts can be easily integrated on to one chip (maximizing symmetry) with a single external resistor for setting the gain.
- In high gain configurations the common mode rejection is much better because the gain of the first stage effectively multiplies the common mode rejection of the second stage.
Note that in general it is better to use a specific instrumentation amplifier chip than to try to build it yourself out of separate parts. Having everything on one chip improves symmetry and hence common mode rejection.
answered 2 days ago
Peter Green
10.7k11837
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
add a comment |Â
up vote
12
down vote
up vote
12
down vote
The 3 op-amp design has three main advantages over a single op-amp differential amplifier.
- The input impedance is much higher, since the inputs drive directly into an op-amp input rather than into a resistive divider.
- The gain can be set by changing a single resistor, so the critical parts can be easily integrated on to one chip (maximizing symmetry) with a single external resistor for setting the gain.
- In high gain configurations the common mode rejection is much better because the gain of the first stage effectively multiplies the common mode rejection of the second stage.
Note that in general it is better to use a specific instrumentation amplifier chip than to try to build it yourself out of separate parts. Having everything on one chip improves symmetry and hence common mode rejection.
answered 2 days ago
Peter Green
10.7k11837
The 3 op-amp design has three main advantages over a single op-amp differential amplifier.
- The input impedance is much higher, since the inputs drive directly into an op-amp input rather than into a resistive divider.
- The gain can be set by changing a single resistor, so the critical parts can be easily integrated on to one chip (maximizing symmetry) with a single external resistor for setting the gain.
- In high gain configurations the common mode rejection is much better because the gain of the first stage effectively multiplies the common mode rejection of the second stage.
Note that in general it is better to use a specific instrumentation amplifier chip than to try to build it yourself out of separate parts. Having everything on one chip improves symmetry and hence common mode rejection.
answered 2 days ago
Peter Green
10.7k11837
answered 2 days ago
Peter Green
10.7k11837
answered 2 days ago
Peter Green
10.7k11837
answered 2 days ago
Peter Green
10.7k11837
10.7k11837
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
add a comment |Â
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
Excellent point about using a single IC; even using 1% discrete resistors lowers the theoretical CMRR to no more than 34dB
– Peter Smith
2 days ago
add a comment |Â
up vote
3
down vote
In addition to input impedance concerns, gain in two stages offers better frequency response.
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
1
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
add a comment |Â
up vote
3
down vote
In addition to input impedance concerns, gain in two stages offers better frequency response.
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
1
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
add a comment |Â
up vote
3
down vote
up vote
3
down vote
In addition to input impedance concerns, gain in two stages offers better frequency response.
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
In addition to input impedance concerns, gain in two stages offers better frequency response.
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
edited Sep 9 at 21:37
Michel Keijzers
4,67152149
4,67152149
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
answered Sep 9 at 20:43
Scott Seidman
21.6k43180
21.6k43180
1
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
add a comment |Â
1
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
1
1
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
While this is true in principle. most instrumentation amplifiers have all the gain in the first stage with the second stage having unity gain.
– Peter Green
yesterday
add a comment |Â
up vote
1
down vote
There is a single stage, high input impedance, differential amplifier.
simulate this circuit – Schematic created using CircuitLab
Without Rg, this has a gain of (f+1). Rg can be used to increase the gain.
However it has performance compromises over the 3 amplifier version.
a) It has less open loop gain, so very high gains are not as stable
b) The two signal paths have different phase shift, so common mode rejection only works to low frequencies. You can improve this a bit with a strategically placed capacitor though
c) At low gains, common mode range is limited by the power supply headroom
d) It's more complicated to draw correctly than the 3 amplifier version. It's only recently that I've created a mnemonic to get the resistors in the right places from memory.
But, if you have only 2 amplifiers left, then it does work. It does allow you control of the gain with a single variable resistor, just like the 3 amplifier version, but unfortunately, just the 3 amp version, this resistor is floating.
answered 2 days ago
Neil_UK
69.3k272152
add a comment |Â
up vote
1
down vote
There is a single stage, high input impedance, differential amplifier.
simulate this circuit – Schematic created using CircuitLab
Without Rg, this has a gain of (f+1). Rg can be used to increase the gain.
However it has performance compromises over the 3 amplifier version.
a) It has less open loop gain, so very high gains are not as stable
b) The two signal paths have different phase shift, so common mode rejection only works to low frequencies. You can improve this a bit with a strategically placed capacitor though
c) At low gains, common mode range is limited by the power supply headroom
d) It's more complicated to draw correctly than the 3 amplifier version. It's only recently that I've created a mnemonic to get the resistors in the right places from memory.
But, if you have only 2 amplifiers left, then it does work. It does allow you control of the gain with a single variable resistor, just like the 3 amplifier version, but unfortunately, just the 3 amp version, this resistor is floating.
answered 2 days ago
Neil_UK
69.3k272152
add a comment |Â
up vote
1
down vote
up vote
1
down vote
There is a single stage, high input impedance, differential amplifier.
simulate this circuit – Schematic created using CircuitLab
Without Rg, this has a gain of (f+1). Rg can be used to increase the gain.
However it has performance compromises over the 3 amplifier version.
a) It has less open loop gain, so very high gains are not as stable
b) The two signal paths have different phase shift, so common mode rejection only works to low frequencies. You can improve this a bit with a strategically placed capacitor though
c) At low gains, common mode range is limited by the power supply headroom
d) It's more complicated to draw correctly than the 3 amplifier version. It's only recently that I've created a mnemonic to get the resistors in the right places from memory.
But, if you have only 2 amplifiers left, then it does work. It does allow you control of the gain with a single variable resistor, just like the 3 amplifier version, but unfortunately, just the 3 amp version, this resistor is floating.
answered 2 days ago
Neil_UK
69.3k272152
There is a single stage, high input impedance, differential amplifier.
simulate this circuit – Schematic created using CircuitLab
Without Rg, this has a gain of (f+1). Rg can be used to increase the gain.
However it has performance compromises over the 3 amplifier version.
a) It has less open loop gain, so very high gains are not as stable
b) The two signal paths have different phase shift, so common mode rejection only works to low frequencies. You can improve this a bit with a strategically placed capacitor though
c) At low gains, common mode range is limited by the power supply headroom
d) It's more complicated to draw correctly than the 3 amplifier version. It's only recently that I've created a mnemonic to get the resistors in the right places from memory.
But, if you have only 2 amplifiers left, then it does work. It does allow you control of the gain with a single variable resistor, just like the 3 amplifier version, but unfortunately, just the 3 amp version, this resistor is floating.
answered 2 days ago
Neil_UK
69.3k272152
answered 2 days ago
Neil_UK
69.3k272152
answered 2 days ago
Neil_UK
69.3k272152
answered 2 days ago
Neil_UK
69.3k272152
69.3k272152
add a comment |Â
add a comment |Â
up vote
-2
down vote
The above answers are reliable, but i want to add something. Consider the differential amplifier:
When a person wants to vary the amplifier gain (for instance to exploit the maximum resolution of ADC) the 2 resistors with value K⋅R must be adjusted perfectly synchronously, via electromechanical adjustable resistors so an slight runout or wearing of these resistors results in imbalance between the value of this two resistors then it results in not to be neglected common mode factor. Usually the common mode in the previous stage (Wheatstone bridge for example) is way bigger than differential mode then it results in false measurements.
edited 2 days ago
Elliot Alderson
2,5801310
answered Sep 9 at 20:58
Sam Farjamirad
16919
5
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
2
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
 |Â
show 5 more comments
up vote
-2
down vote
The above answers are reliable, but i want to add something. Consider the differential amplifier:
When a person wants to vary the amplifier gain (for instance to exploit the maximum resolution of ADC) the 2 resistors with value K⋅R must be adjusted perfectly synchronously, via electromechanical adjustable resistors so an slight runout or wearing of these resistors results in imbalance between the value of this two resistors then it results in not to be neglected common mode factor. Usually the common mode in the previous stage (Wheatstone bridge for example) is way bigger than differential mode then it results in false measurements.
edited 2 days ago
Elliot Alderson
2,5801310
answered Sep 9 at 20:58
Sam Farjamirad
16919
5
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
2
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
 |Â
show 5 more comments
up vote
-2
down vote
up vote
-2
down vote
The above answers are reliable, but i want to add something. Consider the differential amplifier:
When a person wants to vary the amplifier gain (for instance to exploit the maximum resolution of ADC) the 2 resistors with value K⋅R must be adjusted perfectly synchronously, via electromechanical adjustable resistors so an slight runout or wearing of these resistors results in imbalance between the value of this two resistors then it results in not to be neglected common mode factor. Usually the common mode in the previous stage (Wheatstone bridge for example) is way bigger than differential mode then it results in false measurements.
edited 2 days ago
Elliot Alderson
2,5801310
answered Sep 9 at 20:58
Sam Farjamirad
16919
The above answers are reliable, but i want to add something. Consider the differential amplifier:
When a person wants to vary the amplifier gain (for instance to exploit the maximum resolution of ADC) the 2 resistors with value K⋅R must be adjusted perfectly synchronously, via electromechanical adjustable resistors so an slight runout or wearing of these resistors results in imbalance between the value of this two resistors then it results in not to be neglected common mode factor. Usually the common mode in the previous stage (Wheatstone bridge for example) is way bigger than differential mode then it results in false measurements.
edited 2 days ago
Elliot Alderson
2,5801310
answered Sep 9 at 20:58
Sam Farjamirad
16919
edited 2 days ago
Elliot Alderson
2,5801310
edited 2 days ago
Elliot Alderson
2,5801310
edited 2 days ago
Elliot Alderson
2,5801310
2,5801310
answered Sep 9 at 20:58
Sam Farjamirad
16919
answered Sep 9 at 20:58
Sam Farjamirad
16919
answered Sep 9 at 20:58
Sam Farjamirad
16919
16919
5
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
2
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
 |Â
show 5 more comments
5
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
2
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
5
5
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
I wouldn't call this an instrumentation amplifier, I would call it a differential amplifier. You need to provide a citation for the image you included. Also, please don't assume that all of the people using this site are male.
– Elliot Alderson
Sep 9 at 21:30
2
2
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson maybe the implication is that women are too smart to use this circuit design anyway ;)
– alephzero
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
@ElliotAlderson Yes this is a differential amplifier, the OP asked what if we use only differential amplifier and i tried what happen when we use only differential amplifier. And by men i meant 'man' , so all the human races regardless of their gender. It is perfectly possible to make such a mistakes, that's why there is 'EDIT' button.
– Sam Farjamirad
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
If you know it is a differential amplifier, you shouldn't say "consider the instrumentation amplifier:". It's true that others can correct your answers, but in the meantime some new users may get more confused.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
You need to add a citation for the graphic in your answer so the original creator may receive proper credit.
– Elliot Alderson
2 days ago
 |Â
show 5 more comments
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f395195%2fwhy-are-two-stages-used-for-an-instrumentation-amplifier%23new-answer', 'question_page');
);
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
2
Look at the input impedances
– PlasmaHH
Sep 9 at 20:19
Infinite for the first stage, and R2 for second stage, right?
– Bee
Sep 9 at 20:20
Input resistance is one point which is sometimes important but increasing CMRR electronics.stackexchange.com/questions/343096/… is another key benefit
– carloc
Sep 9 at 20:34
No, different input impedences for the two inputs of stage 2
– Scott Seidman
Sep 9 at 20:42
If you have any imbalance in capacitance on Vin+ versus on Vin-, the common-mode rejection is in peril.
– analogsystemsrf
2 days ago