Frequency Peak Detecting Probe

This is a handy companion for a digital voltmeter. Its allowed me to do a lot of things I used to use my oscilloscope for, and in addition it measures voltages to much greater precision.Using an LM324 quad op amp, this peak detector provides 2% measurement accuracy on square waves from 5 Hz to 20 kHz. Taking the circuit from left-to-right, the first op amp (lower-left corner, inputs pins 8 and 9) is a power supply splitter. It splits the 9 volt battery to make plus and minus 4.5 volts, providing a virtual ground at its output. At the upper-left a voltage divider allows the circuit to operate with up to a 15 volt full-scale input.
Frequency Peak Detecting Probe - schematic

The 10k resistor in series with the switch following the voltage divider limits current in case the input voltage exceeds the power supply current. The two diodes after the 10k resistor limit the input voltage at the input (pin 12) of the op amp. This op amp is a unity gain buffer and serves no immediate use, though it could be connected as a gain stage to increase the peak detector's sensitivity, at the cost of bandwidth. The next op amp (pins 1,2, and 3) drives two diode peak detectors. The dpdt switch selects which of the two peak detectors is buffered by the last op amp, thus closing the loop around the output voltage for either the positive peak or the negative peak. (Note that in the schematic there appear to be two SPDT switches, but they are really ganged as DPDT.) The other section of the dpdt switch connects a diode from the output of the op amp to the input to prevent the op amp from going open loop -doing so would cause an accuracy problem at high frequencies because the op amp takes some time to come out of saturation and close the loop. The tricky part of the circuit is the two-stage peak detector. So...what's special about this peak detector? This peak detector's quick response time while maintaining a low droop rate, which is necessary to maintain accuracy at low frequencies is owed to the presence of the dual time constant peak detector. The 100 uf capacitors and 100 K resistor provide a 500 ms time...

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