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I'm using DAC902 @ clock rate of 40 MSPS. The DAC is generating a 1 MHz sine wave. when I measure the output of the DAC, I get the following noisy sine wave: enter image description here

enter image description here

enter image description here

I'm not sure what's the reason behind this. I did notice that in the datasheet, they didn't mention a clock rate of 40 MSPS, so I'm not sure if that's the reason behind the problem. To see the datasheet, you can visit the link: https://www.ti.com/lit/ds/symlink/dac902.pdf?ts=1603505920654&ref_url=https%253A%252F%252Fwww.google.com%252F

Below is a picture of the schematics:

enter image description here

enter image description here

enter image description here

Can anyone give me any suggestions?

Thanks

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  • \$\begingroup\$ Hi! What's wrong? I'm not quite sure I understand what you're asking. \$\endgroup\$ Commented Oct 24, 2020 at 17:30
  • \$\begingroup\$ and: at which of the points in the schematic are you measuring? \$\endgroup\$ Commented Oct 24, 2020 at 17:34
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    \$\begingroup\$ what does it look like once it's been through the necessary reconstruction filter? \$\endgroup\$ Commented Oct 24, 2020 at 17:37
  • \$\begingroup\$ I'm measuring pin 2 of the LM7171 op-amp. There is a lot of noise that is being carried and amplified by the op-amps. I added a better picture to help illustrate my point. I tried to add a 27pf capacitor in parallel with the 1.2k feedback resistor of the first lm7171 op-amp and that actually worked to clean the noise. In any event, I'm not sure, but I don't think the output of the DAC should be that noisy. \$\endgroup\$ Commented Oct 24, 2020 at 18:04
  • \$\begingroup\$ Looks like you have some high frequency noise modulated on your output. An FFT view might give you a clue where it's coming from. \$\endgroup\$ Commented Oct 24, 2020 at 18:11

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Just eyeballing the ripples on your sine: that is 40 ripples per period of your sine - that's the sampling rate.

You forgot to add a proper reconstruction filter, as Neil already pointed out in the comments. You'll want to fresh up your discrete signal theory on why you need that!

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  • \$\begingroup\$ Thanks for the suggestion. My problem is I followed the datasheet blindly. I increased the value of the capacitor C52 and that solved my problem. \$\endgroup\$ Commented Oct 25, 2020 at 22:15
  • \$\begingroup\$ You had to increase that significantly, then, and probably are badly loading the DAC's output, leading to a bad linearity! Your signal will look very bad for sines of other frequencies! Bad idea. But: your large C52 also constitutes low-pass filtering (just not the right kind). You following the datasheet was right: The signal at that point simply isn't meant to already be reconstruction-filtered, so it's expected that it doesn't look "clean". Usually, that filtering does happen – later on \$\endgroup\$ Commented Oct 26, 2020 at 9:04
  • \$\begingroup\$ You are right again. I had to increase that cap value up to 1nf to make a noticeable difference. I knew that C52 is not a low pass filter, but the datasheet said that and that didn't make sense to me; why are they saying it's a low pass filter. In any event, the datasheet now makes more sense. They said to add this capacitor if the stepped output of the DAC is causing the op-amp to be slew-rate limited. I take that to mean that they know that the signal will look noisy, and the only reason to smooth out the signal a little is if it's causing your op-amp to be slew-rate limited. \$\endgroup\$ Commented Oct 26, 2020 at 11:17
  • \$\begingroup\$ I have never worked with a DAC before, so that's why I didn't know what the output will look like. I thought that the output would be smooth without the need for any filtering. I'm not sure it's a good idea to keep that noise in the signal for the circuit I'm working on. I might try to add a feedback capacitor to the LM7171 capacitor, and it will filter that noise. I tried that before, and it worked great. \$\endgroup\$ Commented Oct 26, 2020 at 11:24
  • \$\begingroup\$ @MohammedAl-Abdullah C52 together with the source impedance of the DAC is a low-pass filter, of course, but not the kind you want. When working with DACs, you really want to read up on DAC theory, and what a reconstruction filter is (I hope you've already done that, now that we've mentioned it four times). It's not hard to understand! \$\endgroup\$ Commented Oct 26, 2020 at 11:59

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