0
\$\begingroup\$

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

\$\endgroup\$
  • \$\begingroup\$ Hi! What's wrong? I'm not quite sure I understand what you're asking. \$\endgroup\$ – Marcus Müller Oct 24 at 17:30
  • \$\begingroup\$ and: at which of the points in the schematic are you measuring? \$\endgroup\$ – Marcus Müller Oct 24 at 17:34
  • 2
    \$\begingroup\$ what does it look like once it's been through the necessary reconstruction filter? \$\endgroup\$ – Neil_UK Oct 24 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\$ – Mohammed Al-Abdullah Oct 24 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\$ – user1850479 Oct 24 at 18:11
3
\$\begingroup\$

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!

| improve this answer | |
\$\endgroup\$
  • \$\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\$ – Mohammed Al-Abdullah Oct 25 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\$ – Marcus Müller Oct 26 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\$ – Mohammed Al-Abdullah Oct 26 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\$ – Mohammed Al-Abdullah Oct 26 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\$ – Marcus Müller Oct 26 at 11:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.