Technical data

A-to-D Converter
The analog-to-digital converter (ADC) is used to change dc voltages into
digital information (schematic shown on page 9-12). The circuitry
consists of an integrator amplifier (
U402 and U420), current steering
switch
U411, resistor network U102E, voltage reference U403, ADC
controller U501, and residue ADC U500.
The
ADC method used by the 34401A is called multislope III. It is based
on patented Agilent
ADC technology. Multislope III is a charge
balancing continuously integrating analog-to-digital converter. The
ADC
charge balancing algorithm is always running, even when the
multimeter is not triggered. The input voltage continuously forces
charge onto the integrator capacitors
C400 and C401 through U102E–R16.
Switches
U411A and U411B steer fixed positive or negative reference
currents onto the integrator capacitor to cancel, or balance, the
accumulated input charge. The level shifted (
R403 and R406) output of
the integrator is checked every 2.66 µs by the
U501 COMP input. Logic
state machines in
U501 control the U411 current steering to continuously
seek an approximate 2.5 V level on the integrator amplifier output,
FLASH. If the ADC input voltage ADIN is between ±15 V, the integrator
output (
FLASH) will remain within the 0 V to 5 V range of the U500 on
chip
ADC. An input greater than +15 V may cause the integrator output
(
U402–6) to saturate at about –18 V. An input less than –15 V may cause
U402 to saturate with an output of about +18 V. The U500 ADC input
(
FLASH) is clamped to 0 V or 5 V by R405 and CR403 to protect U500.
The integrator amplifier is formed by
U402 and U420. Resistors R420 and
R421 affect the amplifier stability. Amplifier oscillation may occur if
their values are incorrect. Amplifier
U420 improves the offset voltage
characteristics of integrator amplifier
U402.
5
Chapter 5 Theory of Operation
A-to-D Converter
101