I'm using a precision 16-bit DAC (AD5689) to create DC bias current source, as indicated in the following picture and I'm detecting a high level of low frequency noise.
I represented L1 as an inductor for simplicity, but it is a filter with an high output inductance. Vout is a signal DC (controlled by the DAC) + AC (from L1).
At the moment I removed the AC component so that I could analyze the noise. I connected Vout to a opamp chain with gain of ~19k compensating the DC offset. From the datasheet of the DAC, the integrated noise (0.1 10Hz) is 6uV. That would give an output noise of 0.114V at this gain (without any filtering). I simulated the circuit with the opamp chain and I get 76mV of output integrated noise (0.1Hz 100MHz).
Scoping the output from the opamp chain, I got the following picture: (2V/div 1s/div)
I managed to track down the noise to Vout, since I probed the signals along the opamp chain and they are coherent. Given that the problem presents at low frequencies, I tried increasing C1 and C2 and adding a capacitor to the DAC output (300uF), but the noise level remained similar. I also resoldered the DAC just in case.
Is there a way to reduce this noise?
Eddit: Shorting Vout to ground reduces the noise level as seen in the following picture. Unfortunately I used a probe to short Vout which may have introduced some noise.
Simulation of current and voltage requirements @ DAC , C1, C2
DAC: V(n006) I(v1)
C1: V(n001) I(C1)
C2: V(n002) I(C2)