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I have a board that connects two analog sensors to an ADC via several op-amps (all MCP6042). One circuit is behaving as expected but the second (virtually identical) circuit on the same board is showing severe oscillation on two op-amp outputs. I don't have much experience with op-amps so I'm not sure how to approach debugging it. Both circuits share the same power supply, ground plane, and ADC. I've already tested for connectivity/shorts, reflowed the joints, and cleaned the board. Any ideas what could be causing this behavior?

The yellow trace is the clean output from the first circuit, blue is the same for the second (problematic) circuit. Both circuits were probed at the output of the final unity-gain stage (U3.1:B, pin 7); probing the output of the previous stage (U3.1:A, pin 1) gives similar results.

Trace Measured values
Oscilloscope trace Measurements of oscilloscope trace

I'm seeing similar oscillations on the output of the second circuit's first stage (U2.1:A, pin 1) as well; these are mostly damped though by the RC network after it (R4.1, C2.1).

Here's the schematic of the problematic circuit. It's the reference design from the sensor's manufacturer. The only difference between the working and wonky circuits are the values of R2.1, R6.1, the position of the two Bias +/- jumpers, and the actual sensor used.

Schematic

All parts on the problematic circuit (shown) have the suffix .1 (e.g. R6.1 or U3.1:A); those on the working one have the suffix .0.

On the working circuit R2.0=2kΩ, R6.0=100kΩ, bias=-, U1.0=SPEC 110-109 (CO sensor).
On the wonky circuit R2.1=16.2kΩ, R6.1=499kΩ, bias=+, U1.1=SPEC 110-407 (O3 sensor).
The two circuits are otherwise the same.

Bias jumpers XY1.1 and XY2.1 are connected pin 1 to 2 on the working circuit and pin 3 to 2 on the troublesome one.

My understanding of the sensors' design is that when the gas is present a current flows out of the sensor's W1 and W2 terminals. Current flows into terminal C to replenish the supply of electrons and terminal R is used to monitor the charge level within the sensor (i.e. a feedback loop).

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  • \$\begingroup\$ Hm bad routing can also cause problems. youtube.com/watch?v=ySuUZEjARPY \$\endgroup\$ Commented Mar 22, 2021 at 21:55
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    \$\begingroup\$ @ChristianidisVasileios That video looks interesting; I'll take a look at it later. As far as my board goes it's 2 layers with the bottom being a solid ground plane (no tracks). When signals need to cross I'm using 0Ω links (mostly 1206). \$\endgroup\$ Commented Mar 22, 2021 at 22:02
  • \$\begingroup\$ You know what? I have never thought of using a 0R resistor to avoid using tracks on bottom. Thanks alot! haha \$\endgroup\$ Commented Mar 22, 2021 at 22:04
  • \$\begingroup\$ What is U1.1? Which circuit is first and which second? Kind a weird feedback C3.1 to the first stage of XYZ(??) circuit. \$\endgroup\$ Commented Mar 22, 2021 at 22:29
  • \$\begingroup\$ You've told us R2 and R6 are different, but not what the difference is... \$\endgroup\$ Commented Mar 22, 2021 at 22:40

2 Answers 2

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U2.1A has no efficient negative feedback path at high frequencies. This will often lead to oscillations. A usual remedy is to connect the output and inverting input with a capacitor as it is done for U3.1A. This will close the feedback path for high frequencies. Then, to decouple the outputs of U2.1A and U2.1B, there should be a resistor between the new capacitor and C3.1. For values, start with 0.1uF for the new cap and 1kOhm for the new resistor.

Also, depending on what you connect to V_O3, also U3.1B will oscillate. Namely if there is substantial capacitance on the output node, the amplifier could start oscillating. In this case a small series resistor usually solves this issue as it is done for U2.1B.

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Your circuit is not the same as I can find on the datasheets or application notes.

In particular, the phase shift in R4.1/C2.1 together with the phase shift in C3.1 and the sensor is almost as if it has been designed to be an oscillator.

Can you link to the recommended circuit?

Does it oscillate if the sensor is disconnected?

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  • \$\begingroup\$ I'll have to unsolder the sensor to see if it still oscillates. Here's a link to the reference design (in the SDK Gerbers Schematic & BOM folder). I copied it pretty much verbatim; the only parts omitted are those not directly related to the sensor (i.e. the EEPROM, voltage regulator, and temperature sensor plus the gain trimpot R7). \$\endgroup\$ Commented Mar 23, 2021 at 0:03
  • \$\begingroup\$ It (output of U2.1:A) does still oscillate with the sensor removed. I'm going to see if replacing the caps makes a difference. \$\endgroup\$ Commented Mar 23, 2021 at 0:41

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