Description:

Because the Grounded-Gate Buffer is connected between the DAC output and the passive I/V resistor,
normally the I/V resistor is connected between the DAC output and GND,
you must get rid off the approx. 4V DC on each I/V resistor.
Therefore we must get a new reference GND for each channel.

The circuit is connected to the ref.+5V at the output of the buffer and GND,
with a decoupling cap for each channel.
By trimming each voltage so it exactly matches DC voltage on the I/V resistor,
the DC voltage between I/V resistor and reference becomes zero.
In other words, the DC component is removed without using a coupling cap.