I have gotten request for optical VOA test equipment that want to control current Iout from Few uA to mA depend on different VOA because Voltage output of VOA module is tolerance range huge.
VOA(U1) generates a Voltage output to ROSA, and then ROSA generate a fix Iout.
When Iout current range is drift, customer can adjust Vin voltage to calibration Iout and Vout in order to let whole optical module stable and good performance.
P.S: Vin is from a precision DAC(16bit, Vout:0~5V)
I have reference CN-0179 and CN-0099 for current sourcing monitor and control.
a. The purpose of this circuit is to have accuracy Iout by control of Vin.
b. Vin is generating by DAC (Vout: 0~5V, 16bit).
c. U4 (AD8276) and U6 (AD8603) construct a loop control based on CN-0099 application note.
d. Iout is current flow cross ROSA device. Iout will be controlled by U1. U1 is control by Vout. (Changing Vout will change Iout equivalently);
e. As Iout will be very small (range from 1uA to 1mA). I have not much idea how to resistors value R6/R7/R11/R12??
I have draw a brief schematic as attached, and I need your suggestions.