I need help with the instrumentation amplifier

[antonydublin]

Hello everyone. Sorry for my bad English.
I need some help with the instrumentation amplifier.
Look at the analog part of my scheme below.
I'm using 3-OpAmp scheme on MCP602 operational amplifiers (U1:A, U1:B, U2:A), filtered reference (U2:B), active filter
(U3:A), single supply on ADP3303 - 3.3V, 1% resistors. But independently from value of the potentiometer RV1 gaine
is always near "2". I have tried different reference voltage and other amplifiers. The same result on the breadboard and
board. What is wrong?

P.S. I understand that the discrete design for the instrumentation amplifier are not greet in our days,
and it is better to use something like AD620 or AD8220 , but this scheme is cheaper in terms of repair.
View attachment MCP_InAmp.pdf

 

[LvW]

Sorry, the attachement cannot be opened.

 

[antonydublin]

I'm sorry. I upload new one.

 

[erikl]

Sorry, the attachement cannot be opened.

@LvW: It's a simple PDF. Here's the corresponding PNG image:

 

[FvM]

What is wrong?

Apparently your real circuit isn't corresponding to the shown schematic. Or the input voltage isn't within the common mode range of MCP602.

 

[LvW]

@LvW: It's a simple PDF. Here's the corresponding PNG image:

Thank you erik. However, the original link from antonydublin was not pdf. Now, he has changed the format.

 

[antonydublin]

Thanks, FvM.
I'm already made a PCB (only with smd 1206 components), it checks with scheme, so my circuit is correct. There is also a digital part, that works fine. Also grounding is correct. But anyway, I do not expect that amplifier will not work. After all I use the prototyping board with MCP602 in DIP - the same result. MCP6002 - the same. About common mode range. To test circuits I used different sources from 1mV to 3V (high/low impedance), always higher then ground.

 

[LvW]

Antonydublin, what is the output dc voltage of U2-B (reference supply) in terms of Vdd?

 

[antonydublin]

Output voltage of U2:B is 1.65V. The output of U2:A in this case 1.65 (when inputs of U1:A/B are connected together).
When signal occurs on inputs of U1:A/B the output changes U2:A like 1.65+"2"*(V1-V2). This means that the differential part work correct, it seams that input amplifiers U1:A/B are not working, and gives only input buffering, something like that.
Also I try to set R5 to ground. Output of U2:A close to ground - 1.5 mV when inputs of U1:A/B the same potential.
Today I'll try to use LM358 instead of MCP6XX.

 

[FvM]

Sounds like a wiring error (open circuit R25/R26/RV1) or shorted R1/R2. MCP602, although somewhat noisy, should be basically fine. Common mode range is very similar to LM35, means no input rail-to-rail, input range about 0 to 2V with 3.3V supply.

 

[antonydublin]

I'm looking at breadboard right now. Wirings correct. I change resistor values. Nothing.
Some mysticism.

 

[LvW]

Output voltage of U2:B is 1.65V. The output of U2:A in this case 1.65 (when inputs of U1:A/B are connected together).
When signal occurs on inputs of U1:A/B the output changes U2:A like 1.65+"2"*(V1-V2). This means that the differential part work correct, it seams that input amplifiers U1:A/B are not working, and gives only input buffering, something like that.
Also I try to set R5 to ground. Output of U2:A close to ground - 1.5 mV when inputs of U1:A/B the same potential.
Today I'll try to use LM358 instead of MCP6XX.

Antony, are you aware that in this case the dc output (operational point) of U3:A (filter stage) is 1.65*2=3.3 volts ?

 

[antonydublin]

I've test it without filter stage.
In fact, filter gain in circuit equal to 1 (instead putting R12 to the ground I left it sealed off, R11 = 0r).
Thanks for your replies. I made some replacement today: instead of U1:A/B, U2:A I use AD623.
It works fine.