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
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.
DCV Range
U102A Divider
U101 Input
Amplifier Gain
ADC Input
100 mV
1 V
10 V
100 V
1000 V
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
5
Chapter 5 Theory of Operation
Internal DMM
131
Summary of Contents for 34970A
Page 1: ...Agilent 34970A Data Acquisition Switch Unit Service Guide ...
Page 34: ...32 ...
Page 35: ...2 Quick Start 2 ...
Page 49: ...3 Front Panel Overview 3 ...
Page 62: ...60 ...
Page 63: ...4 Calibration Procedures 4 ...
Page 118: ...116 ...
Page 119: ...5 Theory of Operation 5 ...
Page 159: ...6 Service 6 ...
Page 177: ...General Disassembly 4 3 2 1 6 Chapter 6 Service Disassembly 175 ...
Page 178: ...Internal DMM Disassembly 3 2 1 Chapter 6 Service Disassembly 176 ...
Page 180: ...Additional Chassis Disassembly 1 2 Chapter 6 Service Disassembly 178 ...
Page 181: ...Plug In Module Disassembly Needle nose Pliers 6 Chapter 6 Service Disassembly 179 ...
Page 182: ...180 ...
Page 183: ...7 Replaceable Parts 7 ...
Page 220: ...Chapter 7 Replaceable Parts Manufacturer s List 218 ...
Page 221: ...8 Schematics 8 ...
Page 272: ......