HP 3457A Service Manual page 179

Dc IN +V
rer
25 24
naz $ RAL 23
static 3 F 99.10% parr
D
PROTECTION
a
20
ec00e f S
'ogte Lator ————# ras.
+ | "rec S
eo Lever a i
E SHIFT
1
* es
l |e.
Rae s
Eta Rag i 18
swiToes | 10x |
T i Tat
et static F 00K
A PROTECTION ~ sas BE tans 18
p gp
C00
p:—2 Looe LatoH #o5 | far Pras
: legge. 112.5 18
el ae Vee
ous LeveL Gore | jy
SHIFT
T
RAS
+ Af.
pare 14. 706K
eo oe
| L
* 1? ALE -
MP IN |
Lato ™ "\ cMP OUT
oRIVER
to eawiniin
zie re 7 *] *]
Viewwos Vorarrat +5V Vier +15V 34520 F.12.2
Figure 8-2. A/D Hybrid (U511)
8-33. A/D Analog
The A/D Analog section consists of the A/D Integrator, Slope Amplifier, DA Compensation and Analog
Auto-Zero Loop. In addition to the A/D Hybrid (U5I1), this is all the analog circuitry required to
make the A/D Converter operate. Each section is discussed below.
A/D Integrator (U512 and associated circuitry)
The function of the integrator is to accumulate charge from the unknown input current signal so it can
later be nulled out by using currents of known value. The A/D Integrator consists of a high speed
amplifier with a capacitor (C511) in its feedback loop from output to (-) input. In this instrument, the
amplifier is a composite of Q514, U5I2 and the associated bias components. Transistor Q514 functions
as a low input current, low current noise, input gain stage for integrator amplifier, U5S12.
HP 3457A Multimeter
8-14