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.,
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"
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
Figure 22. Buffer with x10 gain.
NUTS & VOLTS
Circle #147 on the Reader Service Card.