Hi,

I was looking at the AD9910 evaluation board schematic and was wondering how they got the values for their resistors that tie both of the DAC outputs to ground. My circuit is exactly the same except that my sma connectors are rated at 50 ohms (not sure about the ones on the eval sch). I think we chose the 49.9 ohm terminating resistors at the DAC output just because the evaluation board schematic had them. My load impedance is thus the two smas in parallel which is 25 ohms and according to the app note AN912, my terminating resistors should be 12.5 ohms each. Is this right or am I missing something here?

Thanks,

Caleb

I am fairly certain the SMAs on the evaluation board are 50 ohm rated.

From an impedance matching perspective the DAC load resistors on the evaluation board should be 25 ohms. However, we have found that using a larger load increases the signal level and improves overall spurious performance, which generally is more desirable than satisfying the impedance matching requirement.

Just to muddy the waters a little more, the transformer on the evaluation board is a 75 ohm device. But even with a 50 ohm filter design and 50 ohm (instead of 25 ohm) DAC load resistors, there does not seem to be a negative impact on the overall performance.

The impedance rating is the "characteristic impedance", which is different than a load impedance. The impedance matching exercise minimizes reflections and ensures maximum power transfer from source to load. As such, the SMA impedance rating is really not an issue if the SMA is not terminated. The fact that your design has two parallel SMAs should not be much of an issue unless both are simultaneously terminated into 50 ohm loads, in which case you will have a total load of 25 ohms. As you point out, if you want to ensure impedance matching, then you will need 12.5 ohm DAC load resistors. Be aware, however, that using 12.5 ohm DAC load resistors will result in less output signal level.