Hi Shawn,

The voltage references and/or the positive/negative bias voltages may allow you to create a positive or negative offset current, that can be used to substract to your PMT signal.  However, not sure if this will provide the logarithmic output you expect.  Since you require a logarithmic output as a function of the ~100pA  to 100μA PMT negative input current, you will likely need to implement a current inverter.  The current inverter circuits I am familiar with, involve op-amps.

The LOG114 datasheet has a couple of suggestions for current inverters on Figure 7 involving one op-amp and two BJTs, and Figure 8 involving a dual amplifier.

Another example of a precision current inverter using a dual amplifier is below, the first stage is a transimpedance amplifier, followed by a Howland Current source stage:

Is this circuit for a unipolar +5V supply or bipolar ±5V supplies? 

What is the stray capacitance at the input current inverter circuit / output of PMT sensor?

We do offer a couple of evaluation boards that may allow you to wire or setup op-amps circuits on different configurations, that could make the setup easier.  For example:

DIYAMP-EVM — Universal do-it-yourself (DIY) amplifier circuit evaluation module

Kindly let me know your supply requirements, and I can also look into suggesting a detailed LOG114 circuit accordingly.

Thank you and Regards,

Luis 

thanks Luis, Currently we only supply +5V, but we certainly could supply + and - supplies if needed.  Ideally a modification to the LOG114 a current invertor EVM if that might exist.

HI Shawn,

Connecting the LOG114 in the circuit below with bipolar ±5V provides a 0-4V output for a 100pA to 100uA input.

VBIAS is set to 1.5V using U2 (OPA365) on the LOG114EVM/

IREF is set to 1uA using the RREF=1MΩ.  OP AMP A4 is set as a difference amplifier on a gain of 1.778V as shown below.

The LOG114EVM will require some re-wiring as this is not the standard default configuration on the evaluation board.

For the current inversion circuit, the below is one possibility using the OPA192 supporting bipolar ±5V supplies.  I have not found a standard evaluation board that provides this circuit, but I believe it is possible a couple of the DIY modules.  If I find an existing eval board that easily allows to implement this circuit, I will update.

Thank you and Regards,

Luis

DIYAMP-EVM — Universal do-it-yourself (DIY) amplifier circuit evaluation module