OPA197: When using OPA197 as Transimpedance Amplifier, what is the max value for Rg?

Hi,

There isn't really a maximum value for Rg, except for whatever you define in your application. Since Rg sets the gain, you will want to make sure that your maximum signal level is still within the output limitations of the op amp. From a frequency response perspective, Rg creates a zero in the ac response due to interactions with input impedance, and the larger Rg is, the earlier that zero occurs. This means you have to throw away more bandwidth to compensate for the zero. So the maximum value for Rg is also a function of how much bandwidth you need.

If you need high accuracy keep in mind that the input bias current of the op amp will also be gained up by Rg, and if Rg becomes very large you may start to see noticeable offsets even with low bias current.

Something you may want to consider in your design is applying a small bias to the non-inverting pin so that the op amp does not saturate when there is no current flowing through the diode. The offset should be kept small though because it also biases the diode, and too much bias will degrade the linearity of the photodiode. For reference, here is a TI Design that walks through the design process: 

http://www.ti.com/lit/ug/tidu535/tidu535.pdf

Regards,

Zak Kaye

Precision Amplifiers Applications