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I want to use the MCP1501 precision voltage reference in an ADC circuit and have the issue that the reference produces a very noisy output. Much worse than an average LDO.

I have a VPP of 200mV when using the reference circuit shown in Figure 5-1 on page 19 of the Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/20005474C.pdf

How can this happen? Does the output have to be connected to a load?

Example Circuit

Here is the measurement on the scope: enter image description here

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  • \$\begingroup\$ I was going to suggest you try the schematic on page 21 (Figure 5-4) but then I saw that it has a 1 Ohm resistor from output to ground. I somehow don't think that's a really good idea. Figure 5-3 is more reasonable looking on the output side. Seems like you need some kind of load on the output. \$\endgroup\$ – JRE Sep 29 '16 at 14:27
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    \$\begingroup\$ Note that their noise is specified between 10 Hz and 10 kHz. You're currently measuring up to 500 MHz. \$\endgroup\$ – pipe Sep 29 '16 at 14:32
  • \$\begingroup\$ @JRE I think there's a misspelling in Fig. 5-4: The 1R resistor should be 1k, clearly. Anyways, circuit in Fig. 5-4 is ideal for OP's application. \$\endgroup\$ – Rohat Kılıç Sep 29 '16 at 14:39
  • \$\begingroup\$ @RohatKılıç: There's also a 50 Ohm in series with the 2.2µF, so I think I'd just rather skip that diagram than try to guess what they really meant. \$\endgroup\$ – JRE Sep 29 '16 at 14:44
  • \$\begingroup\$ I'd like to see a picture of your measurement setup and the model number of your scope. My Agilent DSOX 4054 produces easily noise figures in the same order of magnitude if I connect something wrong on similar settings. \$\endgroup\$ – Arsenal Sep 29 '16 at 14:54
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1st ensure you are not seeing capture measurement error across your decoupling cap near IC.

Probe ground and check you get a noise free flat line.

If not, arrange for 2 pins or short wires <1cm on Vref and gnd to be probed with tip and ground wire removed and use barrel ground to gnd pin and pin for signal to get the cleanest signal.

enter image description here

Convenient method below. enter image description here

Better method for SMD below.

enter image description here

You can buy these spring attachments for convenient accurate low noise measurements on 10:1 probes. ( or makeshift from pen spring ;)

Then ensure BW of scope is limited to range of spectrum expected. e.g. 10MHz here.

To locate EMI use the probe as a loop antenna then with gnd clip shorted to pin and wave over affected inductor current loop on SMPS to see what is being inductively coupled to high impedance tracks.

THen adding series choke or ferrite bead can raise track impedance >1MHz thus improve ZL/ZC LPF effects. Ensure Caps are low ESR.

Lastly ensure output cap value min/max and ESR min/max is added as recommended in datasheet.

ref: datasheet... Noise should be < 30nV per square root Bandwidth of signal captured or 6uV pp max with no load.

C load must be << 0.1uF for good phase margin if RL<1K.

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  • \$\begingroup\$ I think this is a good idea to check the ground if it's noisy. \$\endgroup\$ – zdun8 Sep 29 '16 at 14:44
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    \$\begingroup\$ The ground has the same noise present. So I guess I can assume I have a measurement error somewhere or that the cause is at least not the reference IC itself. \$\endgroup\$ – PTS Sep 29 '16 at 14:59
  • \$\begingroup\$ Ah a nice short answer +1 \$\endgroup\$ – Andy aka Sep 29 '16 at 15:03
  • \$\begingroup\$ They don't teach "Test Engineering" in university. But I have used this probe method for 40yrs. Last photo is how to get datasheet waveforms other improvements are possible. However your layout can see this if it has an equally large loop. \$\endgroup\$ – Sunnyskyguy EE75 Sep 29 '16 at 15:12
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The basic circuit configuration does not follow the datasheet section on the output capacitor (page 20):

The output capacitor from OUT to GND acts as a low-pass noise filter for the references and should not be omitted. The maximum capacitive load is 300 pF, however, larger capacitors may be implemented if a resistor is used in series with a larger load capacitor. Figure 5-1 illustrates a 1 kΩ resistor in series with a 2.2 µF capacitor.

(Emphasis mine)

They forgot to draw the mentioned capacitor in their schematic. Have you put an output capacitor there or not?

The other schematics in their datasheet have the mentioned 2.2µF capacitor in series.

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  • \$\begingroup\$ I did try both, just the 1k resistor and the 1k resistor in series with a 2.2uF cap. The output doesn't change at all, even if I remove both from the circuit. \$\endgroup\$ – PTS Sep 29 '16 at 14:44
  • \$\begingroup\$ 2,2uF seriously degrades phase margin for RL<=1k and may cause instability noise. But most likely Measurement error. Improve probe methods.....and read datasheet! \$\endgroup\$ – Sunnyskyguy EE75 Sep 29 '16 at 14:46

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