Reference Assembly; Circuit Description; Analog-To-Digital Convertor (A/D); General - HP 3455A Operating And Service Manual

Section VILL
3455-B-4066
Figure 8-25. Simplified Voltage Reference Diagram.
8-82. REFERENCE ASSEMBLY.
8-83. General.
8-84. The reference assembly for the 3455A contains the
components and adjustments for the ohms converter refer-
ence resistance, the precision ten-to-one divider, and the
+ 10 V dc reference voltage. The reference assembly is
designed to be removed from the Multimeter for calibration
and contains all adjustments for the DCV and OHMS func-
tions.
8-85. Circuit Description.
8-86. The ohms reference circuit is an adjustable resistive
network which supplies a precise 1 kilohm or | megohm
reference. The precision ten-to-one divider is an adjustable
resistance divider used to produce the 1 volt reference volt-
age and a precise ten-to-one division for use in the opera-
tional attenuator and DC preamp feedback circuitry. Figure
8-25 shows a simplified diagram of the reference voltage cir-
cuit. The reference for this circuit is a package which con-
tains a reference diode and heater plus associated circuitry.
An operational amplifier (U2) provides the necessary gain
to supply a stable + 10 V dc output. Resistors Rg and Rp
form a voltage divider to provide the proper feedback for
THEORY OF OPERATION
the operational amplifier. These resistors are a fine-line cir-
cuit contained in an IC package and are composed of the
basic resistances plus padding resistors to match the divider
to the particular reference diode. Resistance Rg also
includes a potentiometer which is used as the "'fine"' adjust-
ment for calibrating the + 10 V dc output. The circuit is
returned to the - 15 volt supply to reduce ground currents.
8-87. ANALOG—TO-DIGITAL CONVERTER (A/D).
8-88. General.
8-89. The 3455A Multimeter uses a multi-slope integration
technique to convert analog input signals to digital informa-
tion. This method permits relatively high speed, high accur-
acy measurements. The following explanation of the A/D
Converter operation uses the integrator output waveform
pictured in Figure 8-26. The waveform shown is for a nega-
tive input voltage. For positive inputs the integrator output
would range between 0 and - 10 volts. This waveform can
be divided into three major portions: the integration period
{time T), the run-down period (time T2) and the auto-zera
period (time T3). During time T1, the input voltage is inte-
grated and the most significant digits of the output reading
are determined. During time T2 the input voltage is re-
moved and the charge remaining on the integrator capacitor
is used to determine the least significant digits of the
output reading. During time T3, the integrator is reset to
approximately O volts and readied for the next reading. At
time Tg, the input voltage from the DC Preamp is applied
to the A/D converter and causes the integrator capacitor to
charge (period tc}). The rate at which the integrator capac-
itor charges depends upon the amplitude of the input volt-
age applied (see Figure 8-27). If the voltage at the output of
the integrator reaches plus or minus 10 volts the 10 V com-
parator is enabled and interrupts the inguard controller.
The controller switches in a reference current opposite in
polarity amount of time (period td) and causes the inte-
grator to discharge. At the end of period td, the reference
voltage is removed allowing the integrator to again charge
(period tc). This charge, discharge sequence may be
repeated throughout integration period T|.
8-90. The period during which the digital counters are
"counting" occurs during the td cycles. The total number
of "counts" is therefore dependent upon the number of td
1f fox ts.
Te 13
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Figure 8-26. Integrator Output Waveform.
8-16
Model 3455A