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I have a sensor that can output 0 to 12 VDC. However, the ADC I am using can only convert 0..3.3 V. I am trying to limit the sensor output to 3.3 V to protect the ADC from damage.

The sensor output range of interest is 0..0.1 V and it is important to minimize noise there as much as possible. I have seen circuits with diodes used to limit the voltage but I believe they add significant noise.

Would a voltage follower op-amp circuit using a 3.3 V positive supply be able to limit the voltage, while adding less noise than the diode voltage clamps?

I don't have a negative voltage regulator so I am unable to provide a -3.3 V negative supply but do have a -12 V supply. Will using 3.3 V and -12 V supplies for the op-amp affect its output in any way I need to be concerned about?

enter image description here

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  • \$\begingroup\$ What is the signal frequency/bandwidth and what resolution do you need? You can express the required noise level over the bandwidth in µV for example, or ADC LSBs, etc. \$\endgroup\$
    – bobflux
    Jan 19, 2022 at 10:26
  • \$\begingroup\$ Signal frequency would be 500Hz. I am using a 16-bit ADC and averaging the data that I get in order to increase the resolution/accuracy as much as possible. \$\endgroup\$
    – Sank
    Jan 19, 2022 at 10:30
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    \$\begingroup\$ What do you mean with the area of interest? is that the interesting portion of the analog sensor signal? What kind of sensor are you measuring? Also is it alright to just cut off this 12V analog signal? That voltage follower opamp will need input protection just as much as your ADC \$\endgroup\$
    – Thijs
    Jan 19, 2022 at 10:33
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    \$\begingroup\$ I have limited availability of op-amps due to the country that I live in. Due to the current situation I am not sure when I can order better op-amps. \$\endgroup\$
    – Sank
    Jan 19, 2022 at 10:59
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    \$\begingroup\$ @TonyM thank you. I had not noticed this. \$\endgroup\$
    – Sank
    Jan 19, 2022 at 11:03

2 Answers 2

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enter image description here

I used this schematic with good results.

It was an instrumentation opamp powered from +/-15V, because that was required due to the input voltage common mode. So its output would range from about -13V to +13V. The ADC had to be protected from that. You can use any other opamp, of course.

R1 limits current. If frequency is low, you can use a higher value. Then D1/D2 are the usual diode clamps. Schottky diodes are preferable. The 3V zener diode is kept in on-state by R2, which keeps D1 off when it doesn't need to be on. This keeps capacitance low. I used a zener because the 3V3 supply was not able to sink the maximum current that could flow through R1.

You could also just use a zener, which works as a diode in one direction and limits negative voltage to about -0.7V, and works as a zener in the other direction to limit voltage. But zeners have quite large capacitance, which can distort your signal.

Note this circuit will not protect the micro if its 3V3 supply is missing. In this case it will limit input voltage to 3V3, but that's not low enough if the micro is unpowered. So the 3V3 supply should be derived from the 12V supply yo make sure they come up at the same time.

Another solution is to use a diode clamp to the power supply, but then you have to make sure it can sink the current, because the 3V3 LDO only works one way. A solution can be to use a TL431 wired as shunt regulator, set to 3.4V, and acting as overvoltage protection.

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A passive diode clamp will introduce far less noise than a voltage follower, as it won't be conducting in your 0-100 mV area of interest.

Providing a -12 V rail to your opamp means that's it's possible for it to produce a -12 V output which might also embarrass your ADC, perhaps during power up or power off when rails are rising and falling with unfortunate timings.

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