Matching DAC output impedance

[Ardni]

Hi all,
I have a question regarding matching the impedance of a digital to analog converter to the entry stage of an RF board. The DAC we are using is the DAC5687 from TI. (**broken link removed**)

The output of the DAC is differential and in our board we have the track on the PCB set for an impedance of 50 ohms (22 for each differential pin). We measure a signal with 0.6V pk-pk on the output (0.3 on each differential pin) giving a current of 6mA when measured across a 50ohm load (please correct me if this in incorrect). Now the problem is that initially the RF board is expecting a signal with a limit of +/- 10mA. The problem is that their board is set with an impedance of 25 ohms in total. (12 ohms in each differntial pin). My question is, how does the output current of the DAC change when connected to this load?
If the tracks on the DAC board were also changed by the same amount i.e. to give an impedance also of 25 ohms then I assume the current would double to 12mA. Please confrim if my thinking is correct here.
But what happens when the impedance on the DAC board is 50 ohms in total and on the RF board its 25 ohms in total. How would this affect the output current?

Many thanks for any advice.

 

[crutschow]

That DAC has a current output (not a voltage output) which can be set from 2mA to 20mA full scale. It is independent of the load resistance until the output voltage equals the supply voltage. So going from 25 ohms to 50 ohms on the output load will change the voltage accordingly, but not the output current. For example, if you had the output set for 10mA, then the full-scale output voltage would go from 0.25V to 0.5V.

 

[Ardni]

Hi again,
Thanks for the reply. So from what I understand, the DAC will always try to supply the amount of current configured regardless of the impedance. Please correct me if I am wrong on this.

I have attached an image of the circuit diagram we have. The poblem I have understanding is that we measure about 6mA on the output even though the DAC is configured for 20mA. My question is, how do the various pull up resistors that can be seen in the attached diagram affect the current entering the following stage.

Many thanks for any help.