AD5791 with AD797 as Vref buffer oscillation

I have a DAC rack where I have a quad LTC6655 voltage reference with an inverter as my +-10V reference. The four LTC's are to minimize the noise on the reference over the square root addition trick (4*2.5V=10V, but the noise is sqrt(4)=2 times the single noise).

Anyhow, I have set this up and it looks ok so far. I have designed a tripple DAC PCB which is basically just the typical operating circuit from the datasheet page 26, fig. 53. Here's a screenshot of my circuit and the corresponding layout:

   I followed the design rules as closely as possible. Every IC is decoupled with both 4.7µ and 0.1µ and the board itself with 220µ and inductors in series. The RC lowpass directly after the VREF+ input is just as a backup solution and so far not in use.

The output of the DAC gets fed directly into the lower AD797, the top one is for Vref+. Both VrefPF and VrefPS get to the AD5791 in the middle layer (ground planes in the top and bottom layer).

The oscillation definitely appears after the AD797, as this screenshot shows:

Channel 4 is the VrefIN and Ch2&3 are the outputs of the AD797. There is a 8Mhz oscillation on my otherwise quite nice signal. The peakpeak voltage is almost 800mV which is above anything reasonable.

I tried following the AD797 datasheet page 13 fig. 38 and added a 100 Ohm resistor and a 22pF cap in series with the inverting input, but nothing changed. I also tried loading the output with a 1K resistor to ground and it also changed nothing.

Since I've now tried everything I'm asking you people for help and am looking forward to your ideas about the source of my problem.

Best,

P

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  • P,

      There is a reason why we suggest the AD8676 and AD8675 on the AD5791 data sheet.

    If you have not done RF/microwave pcb's, the AD797 is extremely difficult to tame.

    You need a clean four layer board with all metalization removed below the inputs and the

    smallest bypass cap right next to the part.  I can't tell from you post if this is correct.

    Switch to the AD8597, easier to use, or if you do a board spin, switch to the AD8676/AD8675

    combo or use AD8675's everywhere.

    Harry

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