Need suggestions on attaching a ADC/DAC converter chip to an FPGA

[Shawan]

Hello, everyone

I'm currently trying to attach a rather fast ADC/DAC-converter chip to an FPGA to receive and transmit RF in the future, but getting the converter to run and attaching a signal generator and an oscilloscope for testing is my main goal right now.

I'm coming from the digital world. I did all kinds of digital circuits and used an AD converter for easy tasks with a microcontroller, but when it comes to analog high speed signals which are differential and sensitive to several factors like impedance and so on, I have basically no idea what I'm doing.

The chip I want to use for this project is the AD9862 was purchased from Kynix Semiconductor. It's rather old but they are not very expensive, easy to solder, and they have been used by Ettus Research in several models of their USRPs which I'm using as a reference platform. If you have a suggestion for a better chip, please tell me!

Now the main thing I'm concerned with is the whole analog domain. The AD9862 has 2 differential inputs which can be optionally buffered (which is what I should do, right?) and the datasheet says the input buffer has a constant impedance of 200 Ohm. Now what I want to do is, just take those two AD-channels out to an unbalanced SMA connector with 50 Ohm impedance to connect a signal generator or a radio front end later. So, I need a Balun for that.

Ettus did that too. They have several daughterboards which you can attach to the baseboard to have different frontends connected to the AD-/DA-Converter. Now, if I look at the BasicRX Daughterboard which does exactly what I want, I see that they are using a Balun called ADT1-1WT. When I look that one up, the datasheet tells me, that it has an impedance of 75 Ohm. Isn't that totally wrong? I thought I need a 50 Ohm unbalanced to 200 Ohm balanced transformer.

Also the input is terminated with a 50 Ohm resistor and the output which goes, directly without any more components except a connector, to the AD (VINP_A/VINN_A and is series terminated (right? Or is that a low pass filter with the 10pF capacitor? I read on a mailing list somewhere that the values for a low pass filter are wrong in this schematic BTW) with 50 Ohm. That doesn't match at all with the 200 Ohm input impedance of the AD input. It would be awesome if somebody could explain that to me! For me all of the values are totally off.

Also, what about the traces on a PCB? They also need to have the right impedance to prevent reflections and standing waves. So I need to match them, I guess? So the output of the balun should be differential traces with a differential impedance of 200 Ohm going to the AD input and on the other side of the balun, I need a 50 Ohm trace going to the SMA connector?

These are all things you only seem to learn at university if you take electrical engineering as a major and I took computer science and this is all just an amateurish hobby for me, so I'm kind of lost right now

Any suggestions will be grateful!

[jpeyron]

Hi @Shawan,

First and foremost, this question is beyond the forums intent as well as my experience and knowledge. Hopefully one of our more experience forum members will be able to help you with your question. Also, I have forwarded your question to our senior design, layout and software engineers. This is not a guarantee that they will have time to respond to your question. 

cheers,

Jon

[Notarobot]

Hi @Shawan,

You are asking question which are important from RF design standpoint. You need to find books about RF design, specifically, front end. You also need to use PCB design tools allowing for control of trace impedance, for example, Altium DXP.

Also you will need to have pleanty of spare time just to get RF design right. My question is do you really want to reinvent the wheel since there are so many well designed software define radio hardware on the market at prices a small fraction of Ettus. Hardware became a commodity, software rules.

This is just my opinion, good luck!

 

[zygot]

Shawan,

I do have the electrical engineering and hardware background that you mention but am struggling to figure out how to respond to your question. My short answer is that Digilent makes two boards where it's conceivable to add a high speed ADC or DAC using the FMC connector. None of their other boards have IO connectors suitable for such endeavours. Digilent did a nice job with the FMC connectors on these boards. Now, selecting the appropriate ADC mezzanine board is not as straight-forward as you might want to imagine. ADC vendors offer evaluation boards for their converter products but often, as is , they are mostly good for evaluating their converters. For ADCs there's a whole level of complexity driving the inputs properly for a particular converter architecture and user application apart from the strictly digital interface.

You should realize that what you want to do is fairly complicated,