Electrical indicating and test instruments

 6.1.1 Voltage-to-time conversion digital voltmeter

This is the simplest form of DVM and is a ramp type of instrument. When an unknown voltage signal is applied to the input terminals of the instrument, a negative-slope ramp waveform is generated internally and compared with the input signal. When the two are equal, a pulse is generated that opens a gate, and at a later point in time a second pulse closes the gate when the negative ramp voltage reaches zero. The length of time between the gate opening and closing is monitored by an electronic counter, which produces a digital display according to the level of the input voltage signal. Its main drawbacks are non-linearities in the shape of the ramp waveform used and lack of noise rejection, and these problems lead to a typical inaccuracy of ±0.05%. It is relatively cheap, however.

6.1.2 Potentiometric digital voltmeter

This uses a servo principle, in which the error between the unknown input voltage level and a reference voltage is applied to a servo-driven potentiometer that adjusts the reference voltage until it balances the unknown voltage. The output reading is produced by a mechanical drum-type digital display driven by the potentiometer. This is also a relatively cheap form of DVM that gives excellent performance for its price.

6.1.3 Dual-slope integration digital voltmeter

This is another relatively simple form of DVM that has better noise-rejection capa[1]bilities than many other types and gives correspondingly better measurement accuracy (inaccuracy as low as ±0.005%). Unfortunately, it is quite expensive. The unknown voltage is applied to an integrator for a fixed time T₁, following which a reference voltage of opposite sign is applied to the integrator, which discharges down to a zero output in an interval T₂ measured by a counter. The output–time relationship for the integrator is shown in Figure 6.1, from which the unknown voltage V_i can be calculated 

geometrically from the triangle as:

                                                        V_i = V_ref (T1/T2)                                  (6.1)

6.1.4 Voltage-to-frequency conversion digital voltmeter

In this instrument, the unknown voltage signal is fed via a range switch and an amplifier into a converter circuit whose output is in the form of a train of voltage pulses at a frequency proportional to the magnitude of the input signal. The main advantage of this type of DVM is its ability to reject a.c. noise.

6.1.5 Digital multimeter

This is an extension of the DVM. It can measure both a.c. and d.c. voltages over a number of ranges through inclusion within it of a set of switchable amplifiers and attenuators. It is widely used in circuit test applications as an alternative to the analogue multimeter, and includes protection circuits that prevent damage if high voltages are applied to the wrong range.