Using ADPD1080 with a photodiode with a 180kΩ shunt resistance

Hi VictorCov, Thank you for your inquiry.  Mentioning Neal.Kurfiss for support. 

There are two issues that can come up when reading out input devices with shunt resistances in the kilo-ohm range or lower.

issue 1:  the 180k shunt will create a DC current if the PD is held to a reverse bias.  This DC current could saturate the read-outcircuitry.

issue 2: a current pulse can be attenuated by the resistor dividor created by the PD shunt and the input impedance of the analog front end.

For issue #1 the ADPD1080 has a zero-bias mode between the Anode and the cathode.  This can be set up in registers 0x3C or alternatively in register 0x54.  While the MAX series AFEs will generally hold a reverse bias of ~400mV many of these AFEs hae a large amount of DC current cancelation.  The  MAX86181 hs 200uA of DC current cancelation.  As a result it is possible to use these AFEs to read out this PD even though they cannot put zero volts.

For issue #2 No AFE has zero input impendance, and often this is not a parameter that is well characterized because most silicon photododes have a shunt resistance that is measured in 100s of mega-ohms so this impedance is not relavant to the use case.  ADPD1080 has DC input impedance values near 1k ohms so while the attenuation of the received pulse is not zero it is relatively low, and photo-current is generally observable, however there is some part to part variability in this input impedance which could affect a system that needs to set up a calibrated optical response from part to part.  In the ADPD4100 the input impedance was reduced into the 100s of ohms and made to be more consistant from part to part.  This part also allows to have a zero PD bias.   The MAX series AFEs do not have an input impedance that is specified so as of right now I am not sure how much such a pulse would be attenuated by these AFEs.  An carefully constructed Op-Amp design will allow you to get the most precision with repsect to low imput impednance.  

The ADPD1080/ADPD10801 and the ADPD4100 /ADPD4101 have been used to receive Germanium and other 3-5 PDs that have much much lower shunt resistance than that of Si.  The ADPD4100 family would allow for greater perciesion than the ADPD1080 family but but systems will work so long as the default reverse biase setting on the PD is changed to a zero volt bias.