TINA/Spice/INA332: Differrent Output of INA332 insrtumentation amplifier

Akshay,

The issue with your INA332 circuit has to do with you floating the reference voltage, Vref.  The reference pin MUST be set to a desired voltage because the output voltage is referenced with respect to Vref - allowing Vref to float results in the output voltage saturating close to positive rail.  

Applying Vin_diff results in the output, Vout=Vref+(Gain*Vin_diff)  

Therefore, for Vin_diff=0 and Vref=1.5V, VM1=Vref=~1.5V – see Fig 1

For Vin_diff=-1mV, VM2=1.5V+160*1mV=1.66V – see Fig 2

For Vin_diff=-10mV, VM3=1.5V+160*10mV->>3.1V but it cannot get there because of 3V supply – Fig 3.  

Lowering Vref to 0V allows the output to operate in a linear region: VM4=0V+160*10mV=~1.6V – simulation shows VM4=1.64V (see Fig 4) due to 40mV output voltage error caused by INA332 input offset voltage.

INA332 edited.TSC

Thanks a lot!! That was very useful!

But, I have another question.

Now, I have drawn insider design of INA332, to verify results. But, there was different result.

Schematic is here:

Here, Vref = 1V is used as you have mentioned, output voltage is referenced with respect to Vref.

Rf=68.1k and R1=2.2k, Vdiff.= 10mV, then output should be ~2.6V. 

But, it is showing 1.94V.

How is this possible?

Detailed INA332.TSC

Akshay,

Due to non-linearity of INA332 internal nodes, for Vsupply of 3V, the allowable maximum output voltage is 1.9V - see below.  

You may download the Vcm vs Vout calculator under following link: www.ti.com/.../ina-cmv-calc

Thus in order to be able to properly measure Vout, you will need a minimum Vsupply of 3.7V - see below.

 You may simulate this condition but in your circuit you have to use ideal op amps instead of the ones you used in your schematic- see below:

Detailed INA332 edited.TSC

It is working with ideal op-amps.

So, do you want to say that in reality i.e. while actually working with INA332 as a actual instrumentation amplifier on PCB, it will give maximum 1.9V output for configuration we have used earlier?

Configuration: Vdiff.=10mV, Gain =160, supply= 3V.

If yes, then is it possible at any time that INA332 will give output greater than 1.9V with other circuitry remaining the same for above configuration?

Akshay,
Correct. For Vdiff=10mV, Gain =160, supply= 3V, the output voltage linear range is from 0.09V to 1.9V and the only way to increase Vout to 2.6V is to increase the power supply voltage to at least 3.7V - this is shown on the Vcm vs Vout graphs above.