Simpson 460 4 Series - Page 40

Theory of Operation
5.3.3. The A/D Converter circuit provides two basic full range
sensitivities: 200mV and 2V. This feature simplifies the attenu-
ator design. No attenuation is required on the 200mV and 2V
ranges. The same attenuator ratio is used on the 20V and 200V
ranges. A separate attenuator tap is provided for the 1000 volt
range.
5.3.4 AC Voltage Measurements
5.3.5 The basic AC voltage measurement circuit is shown in
Figure 5-2(b).
5.3.6 The AC voltage being measured is connected to the V-9
and COMMON jacks, attenuated according to the range selected,
and applied to an Input Amplifier. The output of the amplifier
is converted into DC by an AC to DC Converter, and the resulting
DC voltage is measured by the A/D Converter.
5.3.7 The attenuator is not used on the 200mV and 2V ranges.
On the higher ranges, the attenuator is frequency-compensated to
provide accurate readings over a wide frequency range.
5.3.8 The Input Amplifier achieves a high input impedance and
presents a low output impedance to the converter.
5.3.9 The AC to DC Converter uses a half-wave operational
amplifier/rectifier circuit which provides two basic full range
sensitivities: 200mV and 2V. It is average-responding, but its
calibration (gain) is based on the rms value of a sine wave.
5.3.10 The output of the Operational Amplifier has two rectifying
diodes and two feedback resistors, R1 and R2. These components
drive a summing resistor R3. The junction of the summing
resistor and feedback resistors is connected to the amplifier input
to provide negative feedback. With a sine-wave input signal, the
positive half cycles through the other diode. The positive half
5-4