OPA2182: Design Question of Input Error

Hi Francis,

1) If available, can you please provide a schematic, including the photodiode bias connections, and available op-amp supplies 

2) Please clarify the photodiode minimum and maximum current range; and resolution required in the application.

3) Please provide the photodiode junction capacitance, and/or if available, the photodiode data sheet

4) Kindly let us know the required signal bandwidth requirements in the application

5) If possible, also let us know the resolution/noise and  accuracy required in the photodiode current measurement. 

On most Transimpedance applications, we recommend the use of linear CMOS input or JFET input amplifiers since these op-amp amplifiers provide the lowest input bias current. Depending on the minimum current range of the photodiode, and photodiode current measurement resolution, the input bias current, and input current noise can be in some cases a dominant source of error.

The OPA2182 is a high-precision zero-drift (chopper) amplifier.  However, the input bias current is not as low as a standard linear CMOS or JFET amplifiers. Chopper amplifiers have many benefits such as extremely low input offset voltage (Vos), ultra-low offset drift, and no flicker noise. However, these benefits are achieved through the use of an internal calibration circuit that uses MOSFET switches to commutate the inputs. The caveat, is that this calibration technique generates small current input transients within the amplifiers input bias current. These transients of current increase the average input bias current, and can cause errors on some high-gain photosensor/transimpedance applications when requiring the measurement of small photosensor currents.

For this reason, in the great majority of precision transimpedance applications, we recommend the use of precision linear (non-chopper) CMOS or JFET input amplifiers. These precision amplifiers can still offer relative low offset, low offset drift and low noise with very small input bias currents and low current noise. 

However, we can provide targeted op-amp suggestions after we learn about your supplies, and photodiode measurement requirements listed above.

Thank you and Best Regards,

Luis