1. I don't understand the issue. The IDAC_Write(1) should produce 0.125 uA in 32 uA full scale mode.
2. PSoC current source is a current mirror, which needs some headroom to operate; it can't go closer than ~ 0.7V to a corresponding rail. For example, IDAC source can't go above 4.3-4.4V in source mode (5V VDDA operation).
I loaded an IDAC8 on 47k resistor and see 6.1mV, which gives 0.13 uA current flow.
Thanks for the response. I too tried with the same current 0.125 uA through a variety of resistors and got similar results.
I try to take this current (125 nA) as the source and build a current divider and generate currents from 0.1 nA to 1 nA. Then drop them through TIA to check its correctness.