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I am designing a circuit that should convert a unipolar differential audio signal using an ADC. The signal varies between 200mV - 28V peak to peak and I'm planning to use the TI PMC6020 ADC that accepts differential inputs up to 10V RMS.

Unfortunately, the circuit may accidentally be connected to a bipolar signal (-20V to 20V peak to peak at maximum) during installation (this condition could last for several minutes). The ADC only tolerates -0,3V negative input, so there should be some protection against continuous negative voltage.

My first idea was to use a Zener diode connected from the ADC input to ground, but that will clip the signal at -0,7V.

A normal diode in series will not work either, as the voltage drop will affect the reading of the signal when the amplitude is low.

Any suggestion how to clamp the negative voltage in this case? Are there any ideal diode circuits that can be used for this purpose?

I have no split rails available, only 3.3V.

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    \$\begingroup\$ If you can tolerate the idea, you could use a half-wave rectifier (schottky diode and cap.) The output will be automatically boosted (bootstrapped) by the cap after the first few charging cycles are over. This is just a simple idea to work from. It's not a solution. But it may give you some thoughts to consider further. \$\endgroup\$
    – jonk
    Feb 26, 2021 at 3:12

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You're so close:

just use a Schottky diode to gnd that's in reverse bias during normal operation, and in forward bias if the input voltage is negative. Depending on your diode, that would clip to -0.1 to -0.2 V, ca.

The only "normal" usage current would be the reverse leakage current, and that shouldn't be very high if you use an appropriately sized diode.

It is also worth noting that any diode in reverse bias is in fact a bit of a capacitor (that's used, in fact, in voltage-controlled RF filtering/attenuation circuitry), but the capacity here should be very small.

It might make sure to consider whether you can add a bit of series resistance to your audio inputs without reducing the quality, just to limit the current through that diode.

Are there any ideal diode circuits that can be used for this purpose?

Yes, but you'll in the end need a negative supply voltage to make these work. There's some op-amps that are "beyond rail-to-rail input", by using an internal charge pump to generate a small negative supply voltage. But it really doesn't sound like you'll need this – a diode should be fine.

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  • \$\begingroup\$ This will protect the ADC. But the signal generator may not be happy during negative swings. \$\endgroup\$
    – user57037
    Feb 26, 2021 at 1:20
  • \$\begingroup\$ Wouldn't the proposed series resistances take care of the outside potentially (pun alert) high diode shunt currents? \$\endgroup\$
    – mmmm
    Feb 26, 2021 at 2:00
  • \$\begingroup\$ Excellent! I will be able to put in some series resistance, so that should limit the current. \$\endgroup\$
    – JohnDonut
    Feb 26, 2021 at 11:47
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    \$\begingroup\$ Nah, I meant parallel but that may be wrong. Lets say that the Schottky has reverse breakdown of 50V and the Zener has reverse breakdown of 20V. If an AC signal of 30V peak comes in, wouldn't the Zener breakdown happen first? I agree that a Schottky to positive rail would be best, but the only rail I have is 3.3V and the input signal can be much higher. \$\endgroup\$
    – JohnDonut
    Feb 26, 2021 at 14:00
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    \$\begingroup\$ Ah, Ok, yeah, then that might be sensible, indeed. \$\endgroup\$
    – Marcus Müller
    Feb 26, 2021 at 14:26

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