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Design >> Analog Design >> Instrumentation Amplifier problem
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Message started by neuro11 on Jul 5^th, 2013, 12:23am

Title: Instrumentation Amplifier problem
Post by neuro11 on Jul 5^th, 2013, 12:23am

Hi,
I am using INA 129 as an Instrumentation amplifier. Can someone tell me the exact purpose of Pin 5 (Ref). I have shorted this with output. Is that right to do.....
Also i am recieving a significant difference in the results if i connect the input to non-inverting terminal(the other is grounded) than connecting it to inverting terminal. Am i not supposed to reciev same output ??

Title: Re: Instrumentation Amplifier problem
Post by aaron_do on Jul 5^th, 2013, 1:32am

Hi,

pin 5 should not be shorted to the output. It is a reference pin (AC ground).

since you are referencing your input to ground, then can I assume you are using both positive and negative supplies? If so, pin 5 should be tied to ground. If you are using only a single supply (+ve), then I'm guessing you want to DC shift in+, in- and pin5 by VCC/2.

cheers,
Aaron

Title: Re: Instrumentation Amplifier problem
Post by neuro11 on Jul 5^th, 2013, 2:59am

Thanks a lot for the reply.
I am using dual supply and has connected pin5 to circuit ground. But can you shed some light on the reasons i am getting different results from 2 inputs.
I have checked with function generator and its working fine with both input giving same output. Its only when i connect it to a sensor. Do you think adding a small resistor to the input would compensate the problem???

Title: Re: Instrumentation Amplifier problem
Post by aaron_do on Jul 5^th, 2013, 6:32pm

Hi,

Apart from one input being inverting and the other being non-inverting (i'm guessing that's not it), I'm not really sure without looking at how everything is connected up.

How big is the difference. I guess you can make sure that the supply rails are really + and - opposites. Also you could probe vout when both inputs are grounded to make sure it is also ground. Also how small is your input? There is some DC offset between the inputs but according to the datasheet its very small...

Aaron

Title: Re: Instrumentation Amplifier problem
Post by neuro11 on Jul 8^th, 2013, 1:50am

aaron_do wrote on Jul 5^th, 2013, 6:32pm:

hi....thanks again for your reply...i have done some check accordingly
ground potdential (both input grounded) : 2~4 mv
the difference ranges from 100 to 200 mv
thats a lot because sensor input is normally very small. the unity gain output is around 29-30 mv.