HP 3457A Service Manual page 194

The attenuated signal is connected to amplifier A3Q307, U302 by FET switches Q301-Q303. The signal
for the 30mV and 300mV ranges is routed through FET switch A3Q303. The signal for the 3V and 30V
ranges is routed through switch A3Q302 and the signal for the 300V range is routed through switch
A3Q301.
Diodes A3CR307/CR311 and CR308/CR310 provide over voltage protection on the 30mV and 300mV
ranges. Diodes CR306, CR309, CR310 and CR305, CR312, CR311 provide over voltage protection on
the 3V and 30V ranges.
8-58. Amplifier Circuit
The AC Converter amplifier is a two stage amplifier whose purpose is to provide a 2.7 volt full seale
input voltage to the RMS Converter for all full scale AC inputs. The active elements of the amplifier
are FET A3Q307 and operational amplifiers A3U302 and A3U304. FET A3Q307 provides high input
impedance to isolate the amplifier from the attenuators. Overall circuit gain, from the input of U302
to the output of U304 is X100 for the 30mV and 3V ranges and X10 for the 300mV, 30V and 300V
ranges. To achieve an overall gain of 100, A3U302 amplifies the signal by 10, as determined by fee
back resistors A3R328/R329. Analog switch A3U303 (pins 16- 1) selects the output of amplifier
A3U302 as the input to U304. U304 amplifies the signal by 10. as determined by resistors
A3R332/R333, Therefore, the overall gain is 100. To achieve an overall gain of 10, analog switch
A3U303 pins 16- 1 open and 3 - 4 close. This selects an input signal for U304 from the junction of
resistors A3R328/R329. This signal level is equivalent to the signal level at the input of U302 so the
X10 gain of U302 has been nullified and the overall circuit gain is the X10 gain of U304. The output
of U304 is connected to the input of RMS converter A3U305.
8-59. RMS Converter
The True RMS Converter output is a positive DC voltage with its value equal to the true rms value of
the input. The RMS converter is a high performance integrated circuit which uses an implicit computa-
tional method to convert the AC signal to an equivalent DC voltage. The converter contains an ab-
solute value converter (full-wave rectifier), a squarer-divider circuit, a low pass filter (integrator) and a
precision current mirror.
AC measurement speed is limited by the RMS converter filter circuit. The circuit has been designed to
allow two measurement speeds; AC Slow (ACS) and AC Fast (ACF) that are set by the command
ACBAND. The AC Fast function permits faster reading rates but with reduced accuracy at low
frequencies. AC Slow provides the greatest accuracy at low frequencies but at a reduced reading rate.
This occurs because the filter reduces low frequency ripple and thus sets the lower cutoff point of our
frequency range. The filter speed of the RMS converter is determined by capacitors A3C318 and
A3C319. During AC Slow operation, switch A3U306 (pin 6) is closed to connect capacitor A3C319
in parallel with A3C318. When using the AC Fast function, switch A3U306 (pins 5- 6) opens to
remove A3C319 from the circuit.
The output of the RMS converter is connected, through switches A3U306 (pins | - 16) and A2U403
(pins 5 -6), to pin 40 of input hybrid A2ZUI10L. From this point on the measurement process is the same
as a DC voltage measurement.
A3U309 is a comparator that will sense if U305 is outputting an over voltage level and shut down
U305 at pin 5.
HP 3457A Multimeter
8-29