Hi
regarding my last little project for my friend (Short wave AM test transmitter), I am trying to complete a test PCB for the DDS. However I have become quite confused about how to design the output circuitry properly, or at least in an appropriate manner.
So here's the datasheet of AD9834, so you don't need to look it up:
AD9834 Datasheet Click HereI have tried looking at the EVAL board for it to get an idea, however that was of no help, as they just terminate the DAC outputs with a 200ohm resistors and send it down directly to a 50ohm SMA cable. Oh well.
These are the important parameters from the AD9834 datasheet:
Output compliance voltage: Maximum VDD-1.5V
(haven't they meant VDDA instead, see page 17?)Maximum DAC output current: 4mA.
So where's the confusion coming from? I see many options how to implement the output circuitry. My requirement is to optimize for maximum signal level (obviously) and end up with a 50ohm output. Also to use a de-symetrization transformer to obtain the most power and best performance.
To maximize output power, the DAC has to be terminated with larger impedance, than 50 ohm. (DAC outputs are high impedance current sources). This probably makes sense.
Datasheet suggests to terminate with 200 ohm, which gives the maximum voltage (still within the compliance range) at the minimum supply voltage of 2.3V. (I don't care about minimum supply voltage, I will always supply more than 3V. )
So the options now I have are these:
a) Selecting the load impedance for the DAC. Loading 2x200ohm yields 400ohm differentially, resulting in a weird turn ratio down to 50ohm. Selecting too high impedance makes for a large turn ratio in the transformer, which probably isn't any good either.
b) How to wire up the transformer. There are two possibilities: First one to terminate both DAC outputs directly to gnd with resistors. Second, use a center tapped transformer, with the CT connected to GND and with a single loading resistor on the secondary. Not sure which one is better and/or why.
c) Where to lower the impedance to 50ohm? I can either use the transformer itself for the impedance match (then the filter will be at 50ohm), or make the filter asymmetrically terminated, 200ohm input and 50 ohm output. Or, use another trickery at the output of the filter (another transformer or matching circuit, for example implement a signal buffer with appropriate Zin).
d) Filter itself. Probably 7th order is "good enough", however shall one prefer a standard Butterworth, or try implementing an Elliptic one? (I think Elliptic may be better option, when pushing the output frequency compared to the MCLK). However implementing Elliptic filter at low impedance seems problematic (too sensitive for component values).
I have also tried surfing the web, what people use, but confusion was just getting bigger, because most circuits I have found were obviously flawed, like for example this one, where there are numerous things just wrong.
http://kitsandparts.com/minikits/DDS_1.gif (7mA DAC current, 100 MHz clock, pins 5-6 shorted - wtf? some overclocking hackery?, etc)
What would you suggest as the preffered options and why?
I would like to try an elliptic filter (therefore I'd need higher design impedance of the filter like 200ohm) and then a buffer would be needed to isolate the load and buffer the output to 50ohm.
Thanks for suggestions, I think other may find this topic useful, as it could be applied to other DDS chips too.