Analog-To-Digital Ia/Di Converter; Input Amplifier Gains; Gain Switching; Traditional Dual-Slope Converter - HP 3456A Operating And Service Manual

Model 3456A
87 dB. A selectable integrator circuit (consisting of
C303. R327. R326. R325. R324. Q302. Q303. Q304. and
U308) assures a high DC gain with a constant band­
width when the feedback path (gain) is changed. Selec­
table gains of I , 2, 10, and 1 00 are provided by the four
individual gain-setting feedback paths between U308
and Q310 and the four selectable resistance choices bet­
ween U307 and U308. With full-scale inputs, the output
of the Input Amplifier is 10 volts. Table 8-3 summarizes
the amplifier's gain for all functions and ranges.
Table 8·] Input Amplifier Geins
Flllluio.
100
DCV
DCV
DCV
100
OCV
1 000
DCV
ACV All Ranges
100
Ohms
l K , 10K
Ohms
Ohms
, OOK· I G
" ' 00:1
vOltage divider in Input Swilching
ranges.
H A I , HA2, HAlO, and HAIOO

8-44. Gain Switching.

logic commands (5 volt logic) from the Range and Func­
tion Control Logic circuits select the feedback path and
I ,
also the resistance between U307 and U308 for the
respective 2, 10. and 100 gain configurations. Com­
parators U304a, U304c, U304d, and U304b control the
I
Fet switches in the feedback paths for the respective
gains of 100. 10, 2, and
ching theory). Note that Fet switches Q302, Q303, and
Q304 are not controlled by comparators but are con­
trolled directly by the logic signals from the Range and
Function Control Logic circuits. Thin-film resistors
(U200 fineline resistor package) with .010/0 tolerance are
used in the feedback paths for the gains of 100, ID, and
2. Gain of 2 is used for Ohms measurements only.
8-45. Gale Bias Amplifier 112 (GRII2).
gain amplifier whose output is a buffered version of the
input to the Input Amplifier. The output of GB#2 is fed,
for signal conditioning purposes, to three of the four
Fet switches in the gain selecting feedback paths for the
Input Amplifier and also to Fet switch QI05 in the Input
Switching circuit. GB#2 supplies a gate bias voltage for
these Fet switches to make the gate-to-source voltage
equal to zero during the time the switches are on. TP304
is isolated from the output of GB2 by R3l9 to prevent
circuit loading when an oscilloscope is used to monitor
this lest point.
8-46. +2V, + 18V, and + JOV Supplementary VoUage
The 2V source supplies a reference voltage
Sources. +
for the Input Amplifier comparator stages (U304 and
.
11 .... G.i"
'00
mV
, v ' 0
'0 v ,
\ o } "
V
V
,
'00
ohms
ohms
' 0
ohms
2
circuits for these
(see Section 8-33 for Fet swit­
GB#2 is a unity
+
U301). The 18V source is used by the Gate Bias
Amplifier #1 circuit in the Input Switching section in ad­
dition to Gate Bias Amplifier #2 and Op Amp U308 in
the Input Amplifier. If the
below approximately 14.4V. the CR3021
+
serves as a back-up voltage source to protect those cir­
+
cuits using 1 8 volts. The output from the regulated
+
30V source is used by the input voltage divider in the
+
l8V source and by Q3 1 l /31O and U307.
8-47. Low O hm s Offset Circuit. The Low Ohms Offset
circuit (connected to pin 1 2 of U2(0) is used for ohms
measurement only-refer to the Ohms Measurement
section for theory of operation.
8·48. ANALOG·TO·DlGITAL IA/DI CONVERTER.
8·49. General.
11
8-50. The 3456A Digital Vohmeter uses a technique
called Multi-Slope to convert analog input signals to
digital information. This technique is called Multi-Slope
11 because it has many of the features of the original
multi-slope technique (-hp- 3455A Digital Voltmeter) .
The Multi-Slope 1 1 method provides high speed and
high accuracy analog to digital conversion.
8-51. Tn.ditional Dual-Slope
understand Multi-Slope 1 1 , look first al the traditional
dual-slope technique (Figure 8-9). This technique can be
divided into two separate operations: runup and run­
down. During runup the input voltage is integrated by
charging a capacilOT for a defined lime period. After
runup, the integrator's output voltage is proportional
the input voltage. Rundown measures this voltage by
applying a reference of opposite polarity and noting the
time required to discharge the capacitor.
Figure 8·9. DUII·Slope Conversion.
8-52. Dual-Slope is slow because the final measurement
is not known until zero crossing. Multi-Slope 11 is faster
primarily because of the rundown technique used. Also,
Multi-Slope 11 is more accurate because of its unique
runup method.
Service
+ 18V source fails or drops
+
15V circuit
Conl/erter. To beller
10
8-9