Agilent 34970A/34972A Data Acquisition / Switch Unit

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 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 4
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 ADC has the same nominal 10 V value for 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.
5
DCV Range U102 Divider U101 Input Amplifier Gain ADC Input
100 mV
1 V
10 V
100 V
1000V
1/100
1/100
Pin 5
Pin 5
Pin 5
Pin 8
Pin 8
x100
x10
x1
x10
x1
10 V
10 V
10 V
10 V
10 V
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