How to drive INA321

[Dlibip6]

Hi,
I would like to make this INA works in the best range of output voltages for input differentail voltages from 40mV to 80mV.

According to datascheet I should have 100 V/V gain. It isn't true even with Vref = GND (I get G=65). I thought I have to put +3V on Vref to decrease 8V to 5V on output when Vin = 80mV.

The full circuit. I have 0.5 mA current source, which drives PT100. Currently doesn't work in simulation.

Thanks in advance for any help.

 

[rjenkinsgb]

A differential amplifier in effect transfers the input voltage (x gain) to between the output and reference terminals.

At the input you should have 50mV, with 0.5mA and 100 Ohms.

That should translate to the output being 5V above the ref terminal; if you want to reduce the offset so 40mV in is 0V out, then the ref terminal would need to be set to negative 4V.

 

[danadak]

Use AD tool utility.

Instrumentation Amplifier Diamond Plot Tool | Analog Devices

Generate Common-Mode Voltage vs. Output Voltage (aka Diamond) plots, using your design criteria. Select your preferred In-Amp from a list of devices that will meet your signal range requirements.

tools.analog.com

https://www.analog.com/cpsearch/CrossReferenceSearch.aspx Cross for INA321

You might contact them as tool does not have AD cross in it, they might be able to give
you an equivalent.

There is also this tool, may be of some utility.

https://www.analog.com/en/design-center/interactive-design-tools/adi-diffampcalc.html

Regards, Dana.

 

[Dlibip6]

Nice. I would leave this circuit with Ref set to ground because of difficulties with generating negative voltage.
Can I connect straight VF1 into MCP3422 without additional aliasing filters due to C1 on feedback?

 

[danadak]

If you are looking forward to having the INA321 set the goals for offset, drift,
CMRR and PSRR, then using a LM358 in the signal path will impact the
great specs the INA321 presents. As well as noise and drift and offsets.

Consider a better OpAmp than a LM358.


Regards, Dana.