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Metrology, standardization and certification |Метрология, стандартизация и сертификация

Lesson 8

Read the text: Precision Audio Frequency Peak Detecting Probe.

This is a handy companion for a digital voltmeter. It’s 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.

Circuit description: 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. 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 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  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 constant peak detector. This 500 ms peak detector is coupled through a diode to the 22 uf capacitor pair. Normally, the 100 uf capcitor pair is kept charged at one diode drop above the signal's peak voltage, which is the same voltage as the 22 uf capacitors are chaged to. The time constant on the 22 uf capacitor is very long, primarily determined by the capacitors and the input leakage on the op amp. This long time constant is what makes the high accuracy possible at low frequencies. The voltage on the 100 uf capacitors droops beween pulses, but not until the frequency drops below 5 Hz does the ripple get to be large enough to "reach through" the series diode between the 100 uf pair and the 22 uf pair, and affect the voltage on the 22 uf pair.

1. Match the left part with the right:

2. Complete the sentences with the suggested words: plus, splitter, first, virtual

Taking the circuit from left-to-right, the _______ op amp (lower-left corner, inputs pins 8 and 9) is a power supply _______. It splits the 9 volt battery to make _______ and minus 4.5 volts, providing a _______ ground at its output.