Ultra high precision current controlled current source question


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 will be very appreciated.

Single ended version works with offset control. But I need higher CMMR so I need differential current source.

[edited by: KoRba88 at 8:28 PM (GMT 0) on 1 Jul 2019]
  • I didn't simulate the circuit but I think taking the laser forward voltage as feedback to regulate the current is not good idea.

    Forward voltage changes a lot between devices and is hugely affected by temperature.

    I would suggest you to introduce a resistance and measure the current using that.

    Furthermore your circuit is putting two current generators in series, this approach simply doesen't work unless they are absolutely identical, and you cannot guaranty that at 100%.