Circuit for small level voltage detection (microvolts)
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I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.
The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layouts
Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.
low-power medical
asked 8 hours ago
DodZiDodZi
162
$endgroup$
add a comment |
$begingroup$
I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.
The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layouts
Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.
low-power medical
asked 8 hours ago
DodZiDodZi
162
$endgroup$
$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
8 hours ago
1
$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
8 hours ago
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The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
7 hours ago
$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
5 hours ago
$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
4 hours ago
add a comment |
$begingroup$
I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.
The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layouts
Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.
low-power medical
asked 8 hours ago
DodZiDodZi
162
$endgroup$
I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.
The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layouts
Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.
low-power medical
low-power medical
asked 8 hours ago
DodZiDodZi
162
asked 8 hours ago
DodZiDodZi
162
asked 8 hours ago
DodZiDodZi
162
asked 8 hours ago
DodZiDodZi
162
asked 8 hours ago
DodZiDodZi
162
162
$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
8 hours ago
1
$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
8 hours ago
$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
7 hours ago
$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
5 hours ago
$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
4 hours ago
add a comment |
$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
8 hours ago
1
$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
8 hours ago
$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
7 hours ago
$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
5 hours ago
$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
4 hours ago
$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
8 hours ago
$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
8 hours ago
1
1
$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
8 hours ago
$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
8 hours ago
$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
7 hours ago
$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
7 hours ago
$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
5 hours ago
$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
5 hours ago
$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
4 hours ago
$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
4 hours ago
add a comment |
3 Answers
3
active
oldest
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$begingroup$
The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.
If you need that much gain, you need to do it elsewhere.
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
$endgroup$
add a comment |
$begingroup$
The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
$endgroup$
add a comment |
$begingroup$
Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.
The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.
Reduce the gain in that stage to at least 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
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add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.
If you need that much gain, you need to do it elsewhere.
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
$endgroup$
add a comment |
$begingroup$
The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.
If you need that much gain, you need to do it elsewhere.
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
$endgroup$
add a comment |
$begingroup$
The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.
If you need that much gain, you need to do it elsewhere.
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
$endgroup$
The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.
If you need that much gain, you need to do it elsewhere.
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
answered 3 hours ago
Dave Tweed♦Dave Tweed
118k9145256
118k9145256
add a comment |
add a comment |
$begingroup$
The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
$endgroup$
add a comment |
$begingroup$
The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
$endgroup$
add a comment |
$begingroup$
The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
$endgroup$
The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
answered 1 hour ago
WhatRoughBeastWhatRoughBeast
49.3k22875
49.3k22875
add a comment |
add a comment |
$begingroup$
Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.
The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.
Reduce the gain in that stage to at least 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
$endgroup$
add a comment |
$begingroup$
Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.
The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.
Reduce the gain in that stage to at least 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
$endgroup$
add a comment |
$begingroup$
Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.
The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.
Reduce the gain in that stage to at least 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
$endgroup$
Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.
The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.
Reduce the gain in that stage to at least 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
answered 1 hour ago
Edgar BrownEdgar Brown
3,729425
3,729425
add a comment |
add a comment |
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$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
8 hours ago
1
$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
8 hours ago
$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
7 hours ago
$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
5 hours ago
$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
4 hours ago