For instrumentation amplifier LT6370A, input applying from 0-28VDC. When the input is 0V, generated output is 2.229VDC. As simulation results shows that the out linear.

Question

Hi Team,
Is this circuit being correct? Because we are facing the issue of amplifier out is 2.25V constant from input 0V to 4V. If the input is greater than the 4V to till 32V its output increasing linearly.
Here, Issue is that the input is lesser than 4V, the output is 2.25V constant. In the simulation out is linear and there is no fixed voltage.
Is there any IC common mode supply (greater than 2.4 Volts ) which interfering in this case?
As per the simulation amplifier output is linear. But in actual circuit, getting 2.25 fixed output if the input is less than 4V.

kristinehoffman · Accepted Answer

Hi CTS , 
 
 Could you specify your target Vdiff and Vcm? is 0-4V and 4-32V your target input signal per input? 
 For example, if your target Vdiff is 4V, you'll see in the diamond plot tool that its not operating within its limits. You will need to lower your target Vdiff and higher Vcm.

For 4-32V, definitely this is beyond the range of the instrumentation amplifier with a 0-15V Vs. 
 
 Please let me know your specific target input Vdiff and Vcm so we can confirm or suggest a configuration for LT6370. Or you can also use the diamond plot tool to verify if your instrumentation amplifier is operating within its limits. 
 Regards, 
 Kristine)

kristinehoffman · Answer

Based on your attached simulation, your input Vdiff is around 3V and input Vcm is around 7-8V. It is within the operating range of the LT6370, hence this configuration will work. 
 
 I'm not quite sure what you meant by VCM clamping. LT6370 does not have a separate Vout clamp. It uses the Vs rails to clamp/know the limits of the Vout signal. But for your current configuration, no it does not clamp the signal. 
 Regards, 
 Kristine