Nuts & Volts - February 2004

Figure 19. Basic multi-range AC volt/millivolt meter circuit.

Figure 20. A useful AC volt/millivolt meter circuit variation.

mV FSD sensitivity (Rx = 470 Ohms); when set to give 10 mV sensitivity (Rx = 47 Ohms), the input impedance varies from 90 K at 15 kHz to 56 K at 150 kHz.

Figure 18 shows a simple x10 pre-

Figure 21. Unity-gain input buffer.

amplifier that can be used to boost the above circuit's FSD sensitivity to 1 mV; this circuit has an input impedance of 45 K and has a good wideband response. Note, when building highly sensitive AC millivoltmeters, great care must be taken to keep all connecting leads short, to prevent unwanted RF pickup.

A wide-range AC volt/millivolt meter can be made by feeding the input signals to a sensitive AC meter via suitable attenuator circuitry. To avoid excessive attenuator complexity, the technique of Figure 19 is often adopted; the input is fed to a high-impedance unity-gain buffer,

either directly (on "mV" ranges) or via a compensated 60 dB attenuator (on V ranges), and the buffer's output is fed to a basic 1 mV FSD meter via a simple low-impedance attenuator, which in this example has 1- 3-10, etc., ranging.

Note, when using this circuit that its input-to-unity-gain-buffer-output frequency response is virtually flat over the (typical) frequency range 20 Hz-150 kHz when used on the "mV" ranges, and that the primary attenuator's

15 pF trimmer must — when initially setting up the circuit — be adjusted on test to obtain the same frequency response on the basic "V" range.

Figure 20 shows a useful variation of the above technique. In this case, the input buffer also serves as a x10 amplifier, and the secondary attenuator's output is fed to a meter with 10 mV FSD sensitivity, the net effect being that a maximum overall sensitivity of 1 mV is obtained with a minimum of complexity.

Figures 21 and 22 show input buffers suitable for use with the above types of multi-range circuits. The Figure 21 design is that of a unity-gain buffer; it gives an input impedance of about 4.0 M. The Figure 22 buffer gives a x10 voltage gain (set by the R1/Rx ratio) and has an input impedance of 1.0 M. NV