I have to design constant current source for Quantum Cascade Laser ,4-6V 100mA max, 10kHz sine modulation frequency.
Source will be modulated from FPGA and differenatial current DAC LTC1668. I choosed this DAC due to compromise between AC and DC accuracy.
To meet high precision in DC, high CMRR, low noise and low distortion sine requirements I have designed differential composite transimpedance current source based on howland current source topology.
I have performed simulation in LTSpice and with resistor load it works perfectly but with LED load i have strange DC current offset with I cannot cancell (U2 AD8421 ref voltage manipulation change nothing with LED load). Output impedance is too small ? But why? in differential configuration should be huge.
If anybody have idea what is happening or mayby know how to build better circuit that will meet my requirements i will be very appreciated.
Single ended version works with offset control. But I need higher CMMR so I need differential current source.