# DC coupling - How can I reduce low frequency noise?

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.

Eddit2:

Simulation of current and voltage requirements @ DAC , C1, C2

DAC: V(n006) I(v1)

C1: V(n001) I(C1)

C2: V(n002) I(C2)

• First, short VOUT to GND (disconnecting it from U1) and look at the noise of the opamp chain alone. If that's fine, then .... one observation : U1 In+ is derived from 2 supplies, of unknown noise, with no decoupling...
– user16324
Oct 1, 2020 at 16:22
• Is the opamp U1 capable of driving that much capacitance even with 3Ω in series? Oct 1, 2020 at 16:22
• @BrianDrummond I eddited the post to include the the output of the opamp chain with Vout connected to ground
– user264107
Oct 1, 2020 at 17:23
• Looking at the datasheet I'd up it to at least 50Ω. Also, you may want to look at driving capacitive loads app notes. They usually put the feedback resistor on the other side of Rs. Oct 1, 2020 at 18:36
• With 6 uV noise from the DAC, * 1.5 (U1 gain) * 19000 (opamp chain) how did you calculate 114mV? Did you omit U1 gain?Did you try decoupling U1 In+? How is the DAC voltage reference generated? (You could use the amp chain to probe these points)
– user16324
Oct 1, 2020 at 18:37

As said in the comments and edit: Shorting $$\V_{out}\$$ to ground reduces noise.

Choose at least 50 Ω. Also in the application notes: feedback resistor on the other side of $$\R_s\$$:
Last, decoupling/bypassing $$\U_1\$$ In+ with a a 680uF capacitor did improve.