AD9705 FS ADJ

FS ADJ is the same voltage as REFIO. So if you use internal reference (1.025V not 1.25V) then FS ADJ is also about 1.025V. If you use external reference, it can go up to 1.25V.

We have been unable to see this behavior on our chip.  We have tried three different chips. Two on our board, and one on a breakout. We are using the example circuit with the AD5620 shown in this circuit note. https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0156.pdf

Here is our circuit

We have been running the AD9705 in pin mode.

Ahh, I see. That is what you meant by adjusting voltage across Rset. Yes, it should work.

It looks like you are using internal reference. If you measure REFIO and FS ADJ, are they both around 1.025V? What is Vout of AD5620? It must be less than the voltage at FS ADJ, such that current flows out of FS ADJ for the DAC to work. What is Rset? Can you change Vout of AD5620 by SPI writes? (V(FS ADJ)-V(VOUT))/Rset must be within 31.25 μA and 156.25 μA .

Ahh, I see. That is what you meant by adjusting voltage across Rset. Yes, it should work.

It looks like you are using internal reference. If you measure REFIO and FS ADJ, are they both around 1.025V? What is Vout of AD5620? It must be less than the voltage at FS ADJ, such that current flows out of FS ADJ for the DAC to work. What is Rset? Can you change Vout of AD5620 by SPI writes? (V(FS ADJ)-V(VOUT))/Rset must be within 31.25 μA and 156.25 μA .

The REFIO is at 1.022V. We started with the vout of the AD5620 at zero or as close as it gets. It is at 16mv right now. We can change it over SPI. The FS ADJ just goes to whatever voltage we put out of AD5620. RSET is at 6.8k.

Is DAC_SLEEP/CSB at 0V?

Yes it is

Is Rset the resistor between AD5620 and AD9705? You said FS ADJ is same as VOUT at 16mV? Do you get anything from the DAC? If you set DAC_SLEEP/CSB to Vdd, does Vout change?

What value are the resistors around the transformer? what impedance is to the right of the transformer? What is OTCM set to?

RSET is between the two dacs. Yes, FS ADJ is the same. Nothing from the DAC. Vout does not change. The resistors around the transformer are DNI. Right of the transformer is a high Impedance buffer. OTCM is floating, nc.