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Instrumentation amplifier gain, twice than expected

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sudil99

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I am trying to test the attached ciruit. I am using INA128 which has the gain of 1 + 49.4k/Rg. When is use two 2.7 kohm resistors, I was expecting gain of 10 but I am getting 20. I tried with AD620, using different resistors too and in different frequencies (0 - 200 Hz). And, the output is amplified by 2 * theoretical gain.
What could be the reason?
analog.png
 

I am giving signal from function generator, at positive input terminal. The negative input terminal is grounded.
 
Why do you split RG in two resistors?
What about the midpoint?
Look into the data sheet: One resistor RG between both nodes (pins).
 

i used two resistors in series. since nothing is connected to the node, total resistance is 5.4 kohm
 

sudil99 said:
i used two resistors in series. since nothing is connected to the node, total resistance is 5.4 kohm
Click to expand...

There is a pin called Vref that should be at ground potential. Did you consider this?
 

yes. it is grounded.
it looks very weird to me, why this is happening
 

my problem is with actual circuit. simulation is fine.
 

I see two possible explanations:
- the part is not INA128
- the center of the two gain setting resistors is not unconnected

Third explanation (out of competition) is wrong measurement.
 

sudil99 said:
my problem is with actual circuit. simulation is fine.
Click to expand...
That's weird. David was getting double the expected output in simulations. Maybe you could email the manufacturer and ask for clarification / explanation?
 

sudil99 said:
I am giving signal from function generator, at positive input terminal. The negative input terminal is grounded.
Click to expand...

Try the opposite: Ground the pos. inpiut and apply the sgnal at the inverting terminal. Same result?
 

Maybe you could email the manufacturer and ask for clarification / explanation?
Click to expand...
If you were with the TI support stuff, how would answer the question?
- Refer to application circuits in datasheets
- Ask for the exact circuit and conditions the customer uses

In other word, asking the same questions that we didn't get yet answered exactly.
 

The way I see it is if the measurement doesn't agree with the datasheet, then either the measurement is wrong or the datasheet is wrong.

I've seen enough mistakes in datasheets not to trust them blindly. Hence I'd want to confirm if the data in the datasheet is correct.

OTOH, if you've used these chips and found them to work as advertised, then that's another story - it would confirm the datasheet's accuracy.
 

LvW said:
Try the opposite: Ground the pos. inpiut and apply the sgnal at the inverting terminal. Same result?
Click to expand...

May I predict something? I suppose (hope) that the measurement will be according to the data sheet. In this case, the resistor in the last stage is not grounded.
 

either the measurement is wrong or the datasheet is wrong
Click to expand...
Or the real circuit is no corresponding to intended/expected one. I can confirm that INA128 is generally behaving according to the datasheet.

I suppose (hope) that the measurement will be according to the data sheet. In this case, the resistor in the last stage is not grounded.
Click to expand...
With about factor 10 gain, the first stage output voltage is almost symmetrical, independent of input common mode to differential relation. Then output is about factor 1.5 high with floating REF pin, not factor 2.
 
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FvM said:
With about factor 10 gain, the first stage output voltage is almost symmetrical, independent of input common mode to differential relation. Then output is about factor 1.5 high with floating REF pin, not factor 2.
Click to expand...

Sorry, my error. My way of thinking was a bit to simple (hopefully, in this case only).
 

Sorry, my error. My way of thinking was a bit to simple (hopefully, in this case only).
Click to expand...
I think, it's pointing in right direction. Which external circuit fault could achieve the observed gain?
 

Maybe it's due to a misunderstanding.
e.g. Maybe he sets "amplitude = 100mV" on the signal generator, and assumes that gives 100mV peak to peak, but it's actually 200mV peak to peak. If the input is double what you expect, but you're only checking the output, you'd think the gain is double.
 
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