I am trying to generate some line code using my fpga board and output the signal through DAC5682EVM. And the sequence sometimes contains a string of DC component.
Currently my EVM is configured to transformer output. I observed severe baseline wander where the DAC output for a string of DC value start to approaching the mean.
I assume that is because the transformer poor response for low frequency? I have been kindly been told that I should remove the transformer to get DC coupled DAC output. But I can't find any reference on how exactly what need to be done on the board. Could anyone suggest?
I would also appreciate if I could discuss the following with you guys
1. Would J3 still be working solely after removing the transformer?
2. What would the impedance matching issue be?
Thanks a lot!
I'm not surprised about the baseline wander since transformers cannot passing DC values. The transformer allows us to convert the differential output of the DAC into a single-ended output that can be used with most spectrum analyzers and oscilloscopes. So the question is, can you handle a differential output or do you need single-ended. Depending on this answer there are a couple of approaches we can take.
So first things first, what is it that you're driving? Does it have differential inputs or a single-ended input?
If it is differential, we simply need to remove the transformer and install some zero ohm jumpers and adjust the output termination resistors to keep the output common mode centered at 3.3 V. The termination resistors will depend on the load at the input to your next stage.
If you need a single-ended output, there are a few choices. You can use a single leg from the DAC, but at the expense of output power, distortion, and noise. This is not recommended, however the process would be similiar to the previous case. You would need to remove the transformer and install the zero ohm jumpers for ONE of the outputs. You would then have to set the resistor termination similar to the previous case. On the opposite leg, you would need to change the termination to match the other leg however also take into account the load seen by that leg, such that both legs are driving the same TOTAL impedance. The total impedance would be leg 1's resistor termination (at the DAC) in parallel with the load down stream at the next device. Again, care would need to be taken to keep the 3.3V common-mode.
The other choice is to use the differential output and drive an amplifier to convert to single-ended. This would be much preferred over the previous method. The DAC output would be setup the same as the differential case. You would then need an amplifier with a differential input and a single-ended output.
Thanks for your reply. It makes perfect sense.
I will need single ended output therefore will give a try on the differential amplifier approach. Does TI have any module? I am looking at 1GHz and Gain is not big problem. Currently I can't deal with chips unfortunately.
I'm not an expert on amps, but take a look at the THS4275 or the OPA836. Both EVMs have differential inputs. You'll likely need to modify the boards/components to get it setup correctly.
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