When using the AD9705 with the 1.25V internal reference what should be the voltage on the FS ADJ pin?
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 .
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?
Hmm, how is that possible? I assume the center tap of the transformer is grounded. Can you sent a enlarged picture of the area around the transformer, and point out which resistors are open and which are short? BTW, you need SOME termination for the DAC, at least the resistor across the coil with center tap.