Electronic – RC peak detector calculation

Several ways to measure peak-to-peak voltage with a MCU:

• undersampling: Directly sample with an ADC, even though you know you are getting less than once sample per cycle of the signal. This requires (a) the signal repeats over and over the same waveform, and any changes to that pattern occur relatively slowly, and (b) the actual sample-and-hold part of the ADC has a high enough bandwidth, even though you only get one sample once every few cycles of the signal, and (c) the exact sampling instant of the sample-and-hold doesn't always hit the same point on the repeating signal, but drifts along it, eventually hitting the highest peak and later the lowest valley of the signal. (Often this drift happens without you having to do anything; but to guarantee it will always work, you may need to lower your sampling rate with some signals). High-speed digital oscilloscopes often use this undersampling technique to inspect amazingly high frequency signals.

• use maximum peak detector and minimum valley detector to hold the maximum and minimum voltages on sampling capacitors long enough for you to sample them. Either use a resistor on each capacitor with a "long enough" RC time constant that the change in voltage from the actual maximum to your ADC sample is insignificant, or use a transistor on each capacitor to reset the voltage immediately after you sample it. If you can estimate the diode voltage drop in software, the simplest hardware is a diode-capacitor-resistor peak detector and a diode-capacitor-resistor valley detector ... but using an op-amp precision peak detector rather than a single diode may be simpler than trying to estimate the diode voltage drop.

• use a DC blocking capacitor, followed by a diode to ground (to clamp the minimum voltage to 0) and a diode to a peak detect sampling capacitor. The voltage on that capacitor gives you the peak-to-peak voltage directly. Somehow estimate and compensate for the diode voltage drops in software, or else use op-amps to compensate for them in hardware, similar to the peak detector.

• use a frequency mixer (analog multiplier) to shift high-frequency signals down to an easier-to-handle frequency well inside your ADC's capability. (Like undersampling, this assumes the signal repeats over and over the same waveform, and any changes to that pattern occur relatively slowly).

I'm not sure why you want a precision rectifier. I assume you really meant "precision peak detector" (which is admittedly almost the same circuit).

A RC filter with a long RC time constant will give you the average voltage of the signal. As pingswept says, it's probably best to use a fixed cutoff frequency (for the peak detect, valley detect, and the average detect) to handle the high frequencies with analog circuits, and deal with low frequencies in software.

As long as the MCU is sampling the highest peak voltage and lowest valley voltage, consider also reporting the midway voltage halfway between the two -- this is not always the same as the true average voltage of the signal.

So let me see if I understand this. You give it a peak of 1.0v and it detects it correctly. Then you back it off to 0.5v and it still reports a 1.0v peak. What's the problem? 0.5v is not the peak if the voltage also goes up to 1.0v.

I suspect that what you want is to detect a peak "over some amount of time", and not a peak "over an infinite amount of time".

If I am following your schematics correctly, you need some way to discharge C1 so that it can detect the next pulse. There are several ways to do that.

One way is to have a transistor/MOSFET/relay actually short C1 to GND. Then have some circuit that will periodically trip that.

Another way is to simply put a resistor in parallel with C1. This resistor would cause the peak value to decay over time. Change the value of the resistor to change how fast it decays. If the resistor is connected to GND then the decay will be a somewhat inverse-square curve. Use a constant current source to make it more linear.