Designing a Wideband Analog Voltage and Current Meter

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Nicholas Presnell
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It may be surprising to offer a purely analog meter when digital meters are now so widely used. However, it isn’t well-known that digital meters have analog circuits preceding the analog-to-digital converter (ADC). Wideband analog voltmeters, with frequencies from below 20 Hz up to 200 kHz, have been widely used in the past and are still very useful.

Key Specifications for a Wideband Analog Meter

With modern operational amplifiers (op amps), the design of our analog meter can be greatly simplified. This design provides a flat bandwidth of 20 Hz to 1 MHz (-1 dB at 10 Hz) using reasonably-priced components.

It is not difficult to add the capability of measuring currents in circuits in which the insertion of a resistanceof 0.1 Ω at the grounded end does not reduce the current significantly. If the meter is battery-operated or of safety Class 2 construction, the grounded connection does not have to be an actual ground.

The six voltage ranges are 1, 10, 100 mV, and 1, 10 ,100 V full scale, while the four current ranges are 10, 100 mA, and 1, 10 A full scale. The design incorporates a wide-band peak detector, whose sensitivity can be switched to read the peak voltage or the root mean square (RMS) voltage of a sine-wave signal.

There is a provision to insert external filters to provide special frequency responses. This feature also allows the meter to be used as two separate amplifiers with switched variable gain.

The instrument will run from two 9 V batteries or a 9-0-9 V mains power unit. The current drain is below 25 mA for each battery (without any light-emitting diode (LED) indicators), so long life can be predicted.