DRV8811: Oscillation on Decay input Pin

Hi Michael,

Can you confirm the scope captures signal names are correct. In the title and scope capture, the signal oscillating is DECAY but in the question the signal oscillating is VREF. Which one is oscillating?

Assuming this is the DECAY input, is the DECAY signal crossing the AOUT1  or running parallel to AOUT1 for some distance? There could be coupling there from AOUT1 to DECAY. If this is the case, you could try separating the two traces as an experiment.

Hi Rick,

Thanks for the response.

Yes the two names are correct.  The DECAY signal is the main one with an issue, I misspoke above saying VREF.

The signal will look like this whether the motor is moving or static, so I'm not entirely sure if this is due to coupling.

The chip is on a PCBA, so it will be tough to separate the traces.

Any idea where this pure 75khz ring might be coming from? We are stepping the motors much slower (2000sps).

When I remove the 0.1uF cap on DECAY this is what the signals look like:

Thanks,

Mike

Hi Mike,

Thanks for the confirmation.

When the DRV8811 is enabled, the outputs are switching. This could lead to coupling on the DECAY pin if the layout is not ideal. It is still confusing where the 78kHz comes from.

The AD5308 states:

The output amplifier is capable of driving a load of 2 kΩ to GND or VDD, in parallel with 500 pF to GND or VDD. The source and sink capabilities of the output amplifier can be seen in the plot in Figure 14.

Is it possible the capacitor is overloading the AD5308 and causing this?

Can you cut the trace between the AD5308 plus capacitor and DECAY pin, and measure the voltage? You will need to supply the desired voltage on the DECAY pin for proper current regulation.

Hi Rick,

You found the root cause, the DAC had too much capacitance, changing it to 1000pF completely removed the oscillation.

I'd just like to confirm there should be no operational issues with the DRV8811 if the Decay pin only has a 500pF cap instead of the 0.1uF suggested in the datasheet.

Thanks,

Mike

Rick

So, you do not expect the input C load on the VREF pin to be > 500pF?   I ask this because Mike was showing me oscillation on the VREF net the other day.

Dave W

Hi Dave,

I don't think the VREF input pin capacitance will be >500pF. I expect it to be <20pF. 

Rick,

Hmm.. again I walked into the lab earlier this week and Mike showed me VREF was oscillating.  Oscillation was not on the X axis in this post but another axis which is the same design.  We all understand why DECAY can oscillate.  And I had seen the oscillation vanish when I remove the 0.1uF cap maybe a week or so ago.  But if  the input load on DRV8811 pin VREF is indeed  < 20pf - we do not have an explanation for why it oscillates as well. 

We have a DAC driving both (VREF/DECAY) analog inputs to the DRV8811 - both have shown oscillation. One input is terminated with a cap of 0.1uF the other input is not terminated.    

Dave

Hi Dave,

Can you provide a snippet of the schematic and layout from the DAC to the VREF pin for the VREF?

Are there any high current signals passing near the VREF trace?

What is the p2p oscillation and frequency?

Rick

What I remember is the VREF oscillation was very similar to the DECAY oscillation - when viewed on the scope.  About 1V pk-pk and in the ~ 75kHz range.

I do not control these items you requested.  But I will request them tomorrow (9/20).  It may take some days to review the layout. 

Thank you for the help. 

Dave 

Hi Dave,

We will await the layout.

If you have a zoomed in version of the second scope capture also, please provide it also. Noise on VREF is not an issue as long as VREF is relatively stable when the current is at the point to be regulated.

Rick

Mike was planning on taking a more detailed look at VREF soon

Dave

Vref looks stable, relative to Decay.

- Mike