AD8675 Op-amps not following input at low voltages.

Hello Saif,

if I recognize it correctly in your schematic, you use the AD8675 with single supply (V+ = 12V, V- = 0V). Then your circuit cannot work: the AD8675 has a rail to rail output, but it doesn't have a rail to rail input. If you check the parameter "input voltage range" in the datasheet you'll see, that correct operation is only guaranteed, if the input voltage is ~2V above V-, just like you see it in your measurements.

Saif67 said:

Requesting advice on how to make the output linear across the entire range.

Either you have to use a different opamp,ensure that not only the output voltage range but also the input voltage range extends to the negative supply.

Or if you want to stay with the AD8675, you have to provide a negative supply voltage (must be below -2V).

best regards

Achim

Hello Achim,

Thank you so much for your prompt reply. I think since my board is otherwise ready and tested, my best course of action should be to swap out the Op amps.

I would appreciate it you can suggest an Opamp that would be a drop in replacement for the AD8675.

Thanks again and Regards,
Saif.

Hello Saif,

since I do not know, what other requirements you have (e.g. regarding accuracy, noise, bandwidth, ..), it might be better if you choose a suitable chip yourself. The parametric search gives you a complete overview on all available opamps:

https://www.analog.com/en/parametricsearch/11070#/sort=5055,asc

Set the filter for rail to rail property e.g. as shown in the picture:

All chips, which show either "Both" or "In to V-, Output" in the column should be good with respect to the voltage range. (The AD8675 would only show "Output" and will not appear in the list due to the filter settings.)

You may set Vs span max to a starting value of 12 for your 12V supply. And set the package according to the package you used with the AD8675. That should give you a good starting list where you may set the other filters for accuracy, noise, ... according to your requirements. 

best regards

Achim

Thanks a lot Achim. 

Hello Saif67 ,

I agree with Achim that a rail to rail input is recommended in your setup. I checked the closest amplifier to AD8675, and I found the ADA4510-2 which has a rail to rail input and rail to rail output. 

For more information, you can check ADA4510-2 Datasheet.

Thanks,

Mae

Hello,

It appears that the above question has been answered already so I'm closing this thread. In case there are additional questions, feel free to reply to this thread. Thank you!

Best regards,
Paul