I need help with the instrumentation amplifier

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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.

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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.

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What is the part number for ADS1?

What voltage are you supplying the op-amps with?

What exactly are you trying to acheive with this design(other than a working circuit ;))

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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ADP1 - ADP3303-3.3, 3.3 V (I've already wrote it).
Jgmdesign, what do you mean "other than a working circuit"? What working circuit? I do not want to use
AD or INA inst. amplifiers, and I do no need high precision and CMR bigger then 20-30dB.

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You noted that your circuit does not work...

Quote:
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

This tells me that the circuit is not working.

What I am asking is what is the circuit's function. Are you looking to read a therrmocouple? A strain gauge? An accelerometer, etc.?

A little more information can go a long way

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Nicely drawn schematic. I think it would work great if you run all the amps from +-5V. Otherwise, you need to bias all the plus inputs at 2.5V.

Imagecraft compiler user

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Bob,
IIRC from the datasheet, these opamps cannot run on split supplies, besides the OP has mentioned that the PCB is already etched.

JIm

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Greetings antonydublin,

I can barely remember that amplifier topology back from college....

Anyway, after taking a quick look at it: that *should* work, if it is failing, I would first try this:

1. Build, maybe on your breadboard, just the instrumentation part, without *any* of the filters, just leaving: U1A, U1B, U2A, R1, R2, R3, R4, R6, R25, and RV1, and, off course, the power supply, and add a 10k resistor between U2A pin 1 and ground (to add some load).

2. Using your preferred method, feed 1V to J1 and 1.5V to J3 (do not reverse them, J1 goes to - and J3 goes to +).

3. Set RV1 to maximum value (30k).

Measure output voltage: you should get 1.5V. Now, while measuring, change RV1 value: voltage should go up (gain will go up).

Please, let me know how it goes (long time not fiddling with op-amps).

I hope this helps, sincerely,

Ildefonso Camargo

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jgmdesign wrote:
Bob,
IIRC from the datasheet, these opamps cannot run on split supplies, besides the OP has mentioned that the PCB is already etched.

JIm

And what is the difference between running the op-amp at say with V+=5V and V-=0V while biasing the +in halfway to (V+ - V-)/2=2.5V, and running the op-amp with V+=2.5V and V-=-2.5V supply while biasing the +in halfway to (V+ - V-)/2=0V?

None, it is a matter of reference.

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Soulhunter. I did as you told. The output of U2:A without load is 2.8V, and change on couple mV when RV1 varies from 0 to 30k. When U2:A loaded with 11k it gives 100mV and the same little changing with changes of RV1. When difference between U1:A and U1:B goes upper then 1,5V, differential amplifier U2:A gives 3.24V (rail-to-rail works :-)). IS this means that the differential part work correct and input amplifiers U1:A/B are not working, and gives only input buffering, something like that.

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You should scour the datasheet to determine the maximum input voltage for the amplifier input pins. Rail-to-rail output doesn't always mean rail-to-rail input. Often for single-supply amplifiers, the common-mode input range does include ground but may not go any where near the +ve rail. [This is a microchip part after all] :)

If the output buffers are reaching their input pin limits, this will cause you some problems.

--
Nigel

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From MCP601/1R/2/3/4:

Quote:
4.1.3 NORMAL OPERATION
The Common Mode Input Voltage Range (VCMR)
includes ground in single-supply systems (VSS), but
does not include VDD. This means that the amplifier
input behaves linearly as long as the Common Mode
Input Voltage (VCM) is kept within the specified VCMR
limits (VSS–0.3V to VDD–1.2V at +25°C).

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Quote:
And what is the difference between running the op-amp at say with V+=5V and V-=0V while biasing the +in halfway to (V+ - V-)/2=2.5V, and running the op-amp with V+=2.5V and V-=-2.5V supply while biasing the +in halfway to (V+ - V-)/2=0V?

None, it is a matter of reference.

You are correct. I was simply commenting on something Bob mentioned regarding the use of a split supply.

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Quote:
This means that the amplifier
input behaves linearly as long as the Common Mode
Input Voltage (VCM) is kept within the specified VCMR
limits (VSS–0.3V to VDD–1.2V at +25°C).

There you go ... for your 3.3V system the maximum input voltage you can apply to any input pin (and retain linear operation) is (3.3 - 1.2) = 2.1V.

On your prototype; apply the conditions where your error appears then check the voltage on each amplifier input pin including the output buffer. Does any rise above 2.1V ?

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antonydublin: take a look at this link

http://en.wikipedia.org/wiki/Ins...

It seems pretty much correct about that kind of amplifiers. The gain expression seems to be correct.

Also, this, certainly, is a differential amplifier, and thus: common mode voltage will be rejected (that's expected).

Ildefonso.

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antony,
A few questions:

A. What is the purpose of the "100K ressitor" at J2?
B. What is the purpose of tying R5 to U2B?
C. What are you using to measure hte voltages you are quoting in these posts? DVM or Scope?

As commented above, nicely drawn schematic.
But:
A. Annotate it with function names for each of the major circuit sections.
B. Don't use generic voltage labels (e.g. "VCC") if the circuit is designed to run on a fixed voltage.

This will help others understand your circuit more quickly and easily when you need their help.
Thanks

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The scheme above work proper.
I do not know what it was :(.
100k resistor gives me the voltage middle point for output. Measurements - Scope and DVM. In that way amplifier could operate with different sensors that have unipolar/bipolar signals around "VCC/2" to get maximum resolution for ADC with fixed reference (in case of resistive/magnetic bridge, etc).
Thanks to everyone.