Instrumentation Amplifier output

Thread Starter

Dritech

Joined Sep 21, 2011
869
Hi all,
I did a basic circuit on simula ion, where I used the AD621 and provided a common input.
Why is the output of the IA not measuring 0V? I also tried another IA and I got similar results.
Any help would he highly appreciated.
 

MikeML

Joined Oct 2, 2009
5,444
Try it with +- split supplies. The data sheet doesn' t say anything about its rail-to-rail operation with a single supply. In fact, the data sheet implies that split supplies are required.
 

Thread Starter

Dritech

Joined Sep 21, 2011
869
Thanks for the reply. I want to use the IA with a bridge. With reference to the schematic below, since all resistors are equal, shouldn't the output of the IA be 0V?
 

hp1729

Joined Nov 23, 2015
2,304
Thanks for the reply. I want to use the IA with a bridge. With reference to the schematic below, since all resistors are equal, shouldn't the output of the IA be 0V?
That is almost right out of the data sheet. Yes, it quotes only split supplies but an example in the data sheet shows single supply operation in a similar circuit. Have you tried modeling it with another (simpler) IC that says it is designed for single supply operation?
 

Alec_t

Joined Sep 17, 2013
11,419
:confused: I think there is something wrong with the simulator or with the AD621 model: the amp output at pin 6 is apparently -3.49V with a single-polarity supply !
 

Thread Starter

Dritech

Joined Sep 21, 2011
869
Try it with +- split supplies. The data sheet doesn' t say anything about its rail-to-rail operation with a single supply. In fact, the data sheet implies that split supplies are required.
When I connected pin 4 to -12V using the circuit shown in post #4, the meter measures -15V :confused:
 

JohnInTX

Joined Jun 26, 2012
4,114
One problem is that Vref pin 5 is strapped to what the amp considers the negative PS rail (Vs). The specs say this pin should be 1.6v above Vs. Try setting it to Vcc/2 for single supply operation.
 

Thread Starter

Dritech

Joined Sep 21, 2011
869
One problem is that Vref pin 5 is strapped to what the amp considers the negative PS rail (Vs). The specs say this pin should be 1.6v above Vs. Try setting it to Vcc/2 for single supply operation.
Thanks all for the replies. I did as JohnlnTX suggested (pin 5 set to 6V) and it worked.
What method should I use to make the output go to 0V when both inputs are equal?
I tried using a dual supply (12V and -12V) and setting the reference pin to ground, but it did not work.
 

hp1729

Joined Nov 23, 2015
2,304
:confused: I think there is something wrong with the simulator or with the AD621 model: the amp output at pin 6 is apparently -3.49V with a single-polarity supply !
Okay, assuming the simulator is wrong ... what do you get if you actually breadboard the design?
 

hp1729

Joined Nov 23, 2015
2,304
One problem is that Vref pin 5 is strapped to what the amp considers the negative PS rail (Vs). The specs say this pin should be 1.6v above Vs. Try setting it to Vcc/2 for single supply operation.
re: returning Ref to V/2 ... Super! That agrees with the example in the data sheet.
 

hp1729

Joined Nov 23, 2015
2,304
I've no idea. I've never had/used an AD621.
Very pricey, about $10 a piece. Bipolar input. Well less than 1 MHz frequency range. I would need a good reason to justify it. Low noise, low CMRR, precise gain?
Is it really needed in this design?
 
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