DAC Output Noise Coupling

[linuswck]

Description

When I connect the DAC Output 0 to an oscilloscope and I always see a high frequency sin wave being coupled to the DAC output SMA port. This high frequency sin wave exists even when the fast servo is powered up without the SD Card installed (so nothing is initialized). I am not sure if a board issue or a measurement problem.

Connection Topology

Fast Servo DAC Output Port 0 -> SMA - to - SMA Cable -> SMA - to - BNC Adapter -> Oscilloscope BNC Port

Other Info

Fast servo is installed inside a crate.
I tried powering it via PoE and power jack. The output noise makes no differences.
I have tested GND continuity between fast servo's and the SMA cable's, adapter's GND.
Oscilloscope Input Bandwidth is set to Full. This sin wave is still visible even after setting bandwidth to 20MHz.

[mkiepiela]

Oscillations in the outputs come from an unstable amplifier in the DAC AFE.

This is caused by the parasitic inductance of the feedback capacitor and PCB tracks, as well as the parasitic capacitance in the summing node (amplifier inputs). This creates a filter pole and leads to excitation of the circuit.

When the feedback capacitors are removed, the oscillations disappear.
When they are mounted, the oscillation frequency is about 770-790 MHz (depends on the channel).

Experimentally, I added inductance to the feedback capacitors (soldering them on pieces of wire), expecting this to increase the problem (lowering the oscillation frequency).

The oscillation frequency decreased from 790 MHz to 600 MHz (green channel in the figures).

I'm going to test it with low ESL RF capacitors and update this topic.

Recommendations:

• use of RF capacitors with low ESL, preferably in smaller cases
• improved PCB layout (shorter and wider tracks and GND cutouts under the capacitor pads)

I am not closing this topic until the recommendations are implemented in the new revision.