1
\$\begingroup\$

I just finished building and testing a board using an LTC2440 24Bit analog to digital converter.

It functions, but there is a noise problem that I don't understand. The signal I am trying to digitize is very clean (for what I need.) When I connect it to the LTC2440, however, I get a very large amount of noise. It seems to come from the LTC2440 itself, although the noise on the input is larger than the noise on the 5V supply to the LTC2440.

What I am trying to measure is a slowly changing DC from the MAX2015. I need the DC voltage accurate to 0.00001 or better. The voltage represents the level of 13GHz RF received by a satellite dish LNB. Right now, the circuit far outperforms the original solution I had that used a SatFinder to provide the level reading voltage.

I expect to have to improve the circuit and layout in order to get the performance that the LTC2440 is capable of. This question is NOT about that, though. Here I am only trying to find the cause of the enormous peaks that you can see below in the picture labeled "Noise on LTC2440 Input."

Information:

Schematic: Schematic

Noise on LTC2440 Input (pin 5): Noise

Signal (Output from U5 MCP6231 pin 6, when disconnected from LTC2440 pin 5): Signal This picture doesn't do the real signal justice. The camera has a problem capturing the trace. If it is bright enough to be sharp, the camera captures the glare around it. If the trace is dim enough that the camera doesn't capture the glare, then it also doesn't capture the trace. In reality, the trace is a razor thin line with small, slow, excursions. If you look closely, there is a thin, almost white line down the middle of the fuzz. That's the signal. The rest fuzzy from the glare. Even so, that is a small fraction of 1mVolt as compared to the approx. 10mV peaks in the noise picture above.

The MCP6231 was added during testing because I found that the MAX2015 puts out a lot more noise than I expected. The simple RC filter clears that up, but doesn't make the glitches go away.

5Volt to LTC2440: enter image description here

3.3Volt to MCP6231: enter image description here

\$\endgroup\$
  • \$\begingroup\$ Are your scope traces labelled correctly? It looks like the first two (U4 pin 5 and U5 pin 6) are on the same net, but are very different. If that's actually the case, then the parasitics on that connection are playing a big part and it would help to see a photo. \$\endgroup\$ – Jack B Jul 28 '16 at 21:31
  • \$\begingroup\$ "This picture doesn't do the real signal justice. The camera has a problem capturing the trace. If it is bright enough to be sharp, the camera captures the glare around it." - Be aware that the camera might be showing noise that is actually there but your eyes cannot see (similar to how some people complain digital scopes are noisier because the same noise isn't really visible on an analog scope). It looks like a decent signal to me either way, but still ;) \$\endgroup\$ – marcelm Jul 28 '16 at 21:35
  • \$\begingroup\$ @JackB: U5 was added as I was experimenting, and wasn't in the original schematic. It is a bit hard to read in the picture, but U5 pin 4 only crosses the conenction from U5 pin 2 to U5 pin 6. \$\endgroup\$ – JRE Jul 28 '16 at 21:36
  • \$\begingroup\$ @marcelm: I'm sure. The camera has a problem with the brightness, and it seems to capture multiple scans as well. The signal doesn't vary a lot, but is does vary enough that each scan is different. When they are all overlapped, and the camera does its fuzzy bit, the true, sharp lines disappear. \$\endgroup\$ – JRE Jul 28 '16 at 21:38
  • 1
    \$\begingroup\$ @TomBrendlinger: The frequency of the peaks on the input is constant, and is at the sampling rate I'm using on the LTC2440. I've posted an answer myself, since a lot of things became clearer to me as I gathered info to post this question. I'm pretty sure I know what is wrong, and how to fix it. In short: pay more attention to the datasheet. \$\endgroup\$ – JRE Jul 28 '16 at 21:48
3
\$\begingroup\$

The main problem is that the voltage reference (LT1019) is not suited for that ADC. The ADC requires substantial decoupling on the REF inputs because it is an unbuffered input and will glitch merrily at whatever sampling frequency you are running at. However, the LT1019 likes to run without output decouplers. It's a very old device and the data sheet is poor and just does not tell you how good the LT1019 is at dealing with high frequency switching noise caused by the DAC. More modern devices show a graph of output impedance versus frequency to give you better facts to work with.

This is also true of the inputs but at least you are using a buffer amp (MCP6231). However, that buffer amp has a very poor equivalent input noise (50 nV per root(hertz)) and, now that I've checked, does not specify what output impedance it has (versus frequency) either. Poor choice of op-amp and poor voltage reference.

There may be other things wrong too but these stick out to me as bad design choices.

\$\endgroup\$
  • \$\begingroup\$ Hi. Thanks. I didn't realize the LT1019 was that problematic. However poor the datasheet may be, however, it does mention how to add bypassing to the output. I didn't go that route because I didn't realize how necessary it was. This is a learning experience for me, and I don't mind changing the circuit and having a new board made to incorporate what I've learned. \$\endgroup\$ – JRE Jul 28 '16 at 22:04
  • \$\begingroup\$ Yes, the MCP6231 is pretty bad for that task. It was already mounted on a small board with its own regulator, so when I discovered that I needed a filter, I put it in place. The MCP6231 doesn't seem to do much about the spikes, though. Which is really not surprising, as it is for low power, low frequency use. I knew that when I put it where it is, but I didn't have anything better at hand. Next revision will include the buffer as shown in the LTC2440 datasheet. \$\endgroup\$ – JRE Jul 28 '16 at 22:07
  • \$\begingroup\$ Would you tell me where in the LTC2440 datasheet that is says the inputs are unbuffered? I didn't see anything like that, just that it likes low impedance sources. \$\endgroup\$ – JRE Jul 28 '16 at 22:09
  • \$\begingroup\$ @JRE try figure 16 in the DS. It's a picture and not words hence it doesn't actually "say". \$\endgroup\$ – Andy aka Jul 29 '16 at 0:03
  • \$\begingroup\$ OK. Thanks. Figure 16 is in the lead up to the information on buffering that I need to implement. \$\endgroup\$ – JRE Jul 29 '16 at 21:52
2
\$\begingroup\$

I see two things. One is noise where there should not be, and two is an absence of bypass/decoupling capacitors at your LT1019 voltage regulator, and pins 3 and 4 of your ADC ref inputs, and minimal filtering at your op-amp.

Your op-amp only has minimal filtering with a 100nF capacitor. I would add a 4.7uF across its power pins. Add a 100nF and 4.7uF capacitors to pins 3 and 4 of your ADC. Also your main power feed could use a 470uF capacitor at pin 2 of the voltage regulator.

Make sure the grounds leads from the RF receiver to the op-amp and ADC are as short as possible. Get the power feeds clean first, then check for signal clarity again.

\$\endgroup\$
  • \$\begingroup\$ Sorry, the op-amp actually has 100nF close by it and a 4.7µF on the regulator powering it. The MCP-6231 is actually mounted on a second board with its own regulator that I had laying around from other experiments. The schematic has been modified to show the additional op-amp and filter. \$\endgroup\$ – JRE Jul 28 '16 at 21:44
  • \$\begingroup\$ I'll add more bypassing on the reference voltage on the next go around. I can add a larger caps to the main power input. I'll see what I have here. That ought to get rid of the garbage on the 5V lines. \$\endgroup\$ – JRE Jul 28 '16 at 21:46
  • \$\begingroup\$ @JRE. That's good to know. But I am by nature one who goes heavy on the bypass/decoupling capacitors, even as far as using 10 ohm resistors on op-amp power feeds for isolation from the main rails. Maybe too extreme for you, but when I build a board I am paranoid about noise on the power feeds. \$\endgroup\$ – Sparky256 Jul 28 '16 at 21:50
  • \$\begingroup\$ I deliberately didn't bypass the LT1019, since the datasheet says it may oscillate when driving capacitive loads in a certain range. There's also a recommended way to add a bypass to the output and prevent oscillation, which I'll build into the next board. \$\endgroup\$ – JRE Jul 28 '16 at 22:00
  • 1
    \$\begingroup\$ @JRE. Usually the 'bad' values that cause ringing are simply too small. A 100nF capacitor may cause ringing while a 10uF or higher may be very stable. Follow the spec sheets for this stuff. \$\endgroup\$ – Sparky256 Jul 28 '16 at 22:04
1
\$\begingroup\$

I think I know where the problem is coming from, and why it isn't affecting my measurements as badly as I would expect from the size of the peaks.

While putting things together to post this question, I had to dig out the LTC2440 datasheet again. I also had good look at the pictures of the scope traces while cropping them.

The peaks occur at the sampling rate of the LTC2440. That is 880Hz the way I am operating it.

Pages 23 and 24 of the LTC2440 datasheet, explain what happens when the LTC2440 samples, and it also explains the peaks. The LTC2440 appears to need a very low impedance source to sample from. The datasheet also explains how to drive the LTC2440 so as to keep the peaks from being a problem.

Basically, driving directly from a simple non-inverting buffer won't cut it.
You need a hefty capacitor on the input (1µF) and an op-amp configured to drive that capacitor. The capacitor supplies the current needed by the LTC2440 input, and gets recharged to the correct voltage by the op-amp between samples.

So, my low power MCP6231 isn't up to the job of driving the LTC2440.


The interesting thing for me is that this explains why I'm not getting completely crap results. I tested the circuit with a small program that just reads a bunch of values and calculates the peak-to peak AC. The peak-to-peak values were in the range of 1mVolt, which wouldn't be possible if it were simply noise - the spikes are 15mV high.

I think what is happening is that my little MCP6231 manages to pull things within 1mV of the real signal before the sampling takes place in the flat spot between peaks.

\$\endgroup\$

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.