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AD9959 - no AC output, improper termination?

Question asked by sofiam on Aug 27, 2015
Latest reply on Aug 28, 2015 by KennyG

Hello,

 

I am setting up an AD9959, and I have a basic question about how to properly extract the signal out of a DAC output.

 

My understanding is that the DAC outputs are current sources. Hence, if I want my signal to be a voltage, I need to place a resistor between the center pin and the ground of the DAC output.

 

At the same time, the manual advises to terminate the DAC outputs to AVDD at 1.8V. Of this I understand that I should connect a resistor (~50 - 400 Ohms, to keep the output current with the range set in the specs) between the center pin of the DAC and the center pin of the AVDD.

 

Following this logic, I have a 100 Ohm resistor (R1) between AVDD and the DAC, and a 100 Ohm resistor (R2) between ground and the DAC. Then I connect the ends of R2 to my oscilloscope, with a 50Ohm terminator. But I only see a DC voltage that matches more or less the voltage I would expect from this sort of voltage divider I have between 1.8V and ground.

 

What is wrong with this picture?

 

I should mention that I have an external clock reference, and that I do load the parameters on the device. I believe that there is communication between the AD and the software, since the status lights are blinking and when I turn on the multiplier the AD draws 10mA more current. A hand-wavy sign, but a repeatable one.

 

Thanks a lot!
Sofia

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