Keysight 34401A, Service Manual

Page: 108 / 165

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 µ s 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