Input Amplifier - Agilent Technologies 34970A Service Manual

Chapter 5 Theory of Operation
Internal DMM

Input Amplifier

Unless otherwise noted, components in this discussion are located on
the A4 circuit assembly (34970-66504). The schematics are included
in Chapter 8 starting on page 234.
The DC Amplifier circuit is used by every measuring function except
frequency and period. Analog switch U101B selects various input
signals for measurement by the ADC. Switch U101B has three
sources which can be dynamically selected: measure customer input
(MC), measure zero input (MZ), and precharge (PRE). The MC state is
the actual input measurement. The MZ state measures internal
offset voltages which are also present in the MC measurement. The
final measurement result is computed from MC–MZ. The PRE state
is used to "precharge" internal capacitances to reduce charge
injection to the input terminal from the dynamic switching of MC and
MZ. Autozero off disables the dynamic switching of the amplifier
input. However, a new MZ value is automatically taken whenever a
new function or range is selected, even if autozero is turned off.
In the dc voltage function, ranging is accomplished through both
input relay switching (K102–K104) and solid state switching (U101).
As a result, the input to the
a full scale input on each range. The dc input amplifier is comprised
of source follower dual FET U104, amplifier U106, and associated
bias circuitry. The feedback resistors U102C and switches U101C
select non-inverting amplifier gains of x1, x10, and x100 for the dc
input amplifier circuit. Amplifier output ADIN drives the dc input to
the a-to-d converter for all measuring functions.
DCV Range U102A Divider
100 mV
1 V
10 V
100 V 1/100
1000 V 1/100
has the same nominal 10 V value for
ADC
U101 Input Amplifier Gain
Pin 5
Pin 5
Pin 5
Pin 8
Pin 8
ADC Input
x100 10 V
x10 10 V
x1 10 V
x10 10 V
x1 10 V
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