OPA855: OPA855 as transimpedance amp

Giora Eytan

Part Number: OPA855

Hello,

I am trying to simulate the OPA855 as transimpedance amp - very similar to fig. 55 in the data sheet.

See attached.

When simulate DC I got at the output 3.12V instead of 3.5V - as the vref source connected to the positive input of the amp.

Any explanation? I am doing something wrong?

Giora

over 3 years ago

0 Michael Steffes over 3 years ago

Guru 48395 points

Morning Giora

At DC this will act like a unity gain follower, you are violating the input range limits so the output just sits at that limit instead of your input voltage,you might try ramping that DC input voltage and see where the output clips off.

0 Michael Steffes over 3 years ago in reply to Michael Steffes

Guru 48395 points

Oops Giora, this is simple, I forgot the OPA855 was bipolar input with about 10uA of input bias current, that explains your output shift,

We usually put a matching R on the V+ side for bipolar op amp Zt which will reduce the output error to Ios *Rf, you want to filter that R noise with a cap as shown here,

0 Giora Eytan over 3 years ago in reply to Michael Steffes

Expert 1235 points

Michael,

Thanks.

Did not saw it.

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OPA855: OPA855 as transimpedance amp