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Build A Digital Audio Millivoltmeter

Versatile unit indicates signal levels in mV, dBv & dbm

Want to measure small signals at audio frequencies? Here's a low-cost digital audio millivoltmeter which will allow you to measure audio signals from below 5Hz to above 100kHz. As well as indicating the level in both millivolts and dBV, it also shows the corresponding dBm level into 600 ohms.

by Jim Rowe


• Main Features: a low-cost audio millivoltmeter based on a logarithmic amplifier/detector coupled to a digital metering circuit using a programmed PIC microcontroller and an LCD readout.

• Input Impedance: 100kΩ (balanced input can be changed to 600Ω)

• Measuring Frequency Range: from below 5Hz to above 100kHz

• Maximum Input Signal Level: 1.4V RMS (+3.0dBV, +5.2dBm/600Ω)

• Minimum Input Signal Level: 160μV RMS (-76dBV, -73.8dBm/600Ω)

• Measurement Linearity: approximately ±0.3dB

• Measurement Accuracy: approximately ±3%

• Power requirements: 12-15V DC at <200mA with backlit LCD

THIS NEW AUDIO millivoltmeter design is an adaptation of the RF Level & Power Meter described in the October 2008 issue of SILICON CHIP. Like that design, it makes use of a logarithmic amplifier/detector IC (an AD8307) to provide a very sensitive detector. This has a DC output which is closely proportional to the logarithm of the audio input voltage.

Click for larger image
Fig.1: block diagram of the Digital Audio Millivoltmeter. The audio signal is first fed to an impedance transformer stage (IC1) and then to a log amplifier/detector via a resistive attenuator. Its output is then fed to three different DC amplifiers which in turn feed a digital voltmeter stage based on PIC microcontroller IC4 and an LCD module.

We have combined one of these Analog Devices AD8307 chips with an instrumentation amplifier to provide it with a high input impedance and also added an “intelligent” metering circuit based on a PIC microcontroller. In operation, the PIC processes the detector’s logarithmic DC output voltage to indicate signal level and the equivalent dBV and dBm levels.

The PIC micro uses some fairly fancy maths routines to work out the signal level, which is then displayed on a standard 2-line LCD display. All the circuitry is on a single PC board and fits in a compact diecast aluminium case. The whole set-up works from an external 12V battery or plugpack, drawing less than 200mA (most of which is drawn by the backlighting in the LCD module).

How it works

The block diagram of Fig.1 shows how the new meter works. At far left are the two input sockets, one for a balanced input and the other for an unbalanced input. Switch S3 allows one of these inputs to be selected, with the desired input fed to an impedance transformer stage. This uses an AD623 instrumentation amplifier (IC1) to provide a relatively high input impedance of 100kΩ and operates with a gain of two (+6dB).

The output of the impedance transformer stage is then fed to the AD8307 log amplifier/detector (IC2) via a 10:1 resistive attenuator. This attenuator is formed by the 5kΩ resistors in series with each input and the AD8307’s own input resistance of 1100Ω.

The output of the log amp/detector is essentially a DC voltage, with a value closely proportional to the logarithm of the AC input voltage. In fact, the slope of the detector’s output is very close to 25mV per decibel rise or fall in the input. By adjusting the log detector’s load resistance via trimpot VR1, we can set the slope to 20mV/dB (for calibration).

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