A20 Board Component Numbering Structure; Input Switching And "Range And Function Control Logic"; 100 V And 1000 V Ranges; Fet Switching Technique - HP 3456A Operating And Service Manual

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each having its own numbering sequence. Table 8-1
summarizes this numbering s!ructure. Note that the 600
series numbers are used for calibration (Cal) com­
ponents and that the numbering scheme deviates in this
instance due to the different Cat components being used
in four of the A20 circuits. Locating these components
together, behind a removabte front panel section per­
mits a complete instrument calibration white the instru­
ment is rack mounted.
Table 8·1. A20 Board Component Numbering Structura.
Componenl lII u mben
Series
Input Switching
1 0 0
Series
Ohm's Current Source
200
Series
Input Amplifier
300
Series
AID Converter
400
Series
Reference
500
Calibration
Series
600
Series
Range and Funclion Control Logic
700
8-30.
Input Switching and "Range and Function Con­
trol Logic".
All of the reed relay and Fet switching con­
trol signals for the Input Switching circuits, except for
HAF, LMES, and LGND, come from the Range and
Function Control Logic (see Figure 8-4 Range and
Function Control Switching). Logic Signals HAF,
LMES, and LGND come directly from the Inguard
microprocessor A30-U J3. A30-U J 3 also provides the
logic input for the Range and Function Control Logic.
8-31. 100
1000
V and
V Ranges.
WI
1000
switch QI09
'11 b
e ena
bl d b th
e
100 V
and
V ranges (see Figure 8-8 Simplified
ror
Switching).
Schema
tic
Inpul
(9.9 M and 99.975 k fine-line resistors) in this
divider
path at
tenuates the input
--
otection for these ranges is provided by the 2.55
limit pr
k ohm
resistor (RIOt) in series with the
(see Schematic 1 , Inpul Switching).
divider
I
U I '
8,4
e
l .a
ir
i
C
u
me
Relay KlO2 and Fet
TO
y
e con
t
I 1
ogle
f
or
The
100:1
voltage
10
the desired levels. Current-
-
10 M ohm
1
,
:
RANGE
AND
F U N C T I O N
:
CONTROL
:
L O G I C
,
,
:
L I N E S
Figure 8-4. Ranltl and Function Control SWltchmg.
8-32. 100
t
mY,
low thermal relay) and Fet switch Q l 16 are enabled for
the 100 mV to l O V ranges. Current-limit protection for
these ranges is provided by four 27 k ohm resistors in
series with K 103.
8-33.

Fet Switching Technique.

bias the Fet switches on or off have open collector tran­
sistor outputs. These outputs are connected via 100 k
ohm pull-up resistors to the output levels of either Gate
Bias Amplifier #1 or #2, except for UI04b and U lO4c
(see Figure 8-5 Comparator/Fet-Switch circuit). With
the + 2 volt reference applied to its negative input ter­
minal, a low logic input signal will cause the com­
parator's output to be driven to
switch will be off (refer to Table 8-2). With a high logic
input signal, the comparator's output transistor is turn­
ed off and its output will therefore rise to the pull-up
voltage level (usually the input signal level). The
N-channel Fet switch will be closed with this pull-up
voltage applied to its gate.
8-34, The pull-up resistors for U I04b and c are con­
nected to ground, thus their outputs will rise to ground
when they are turned off. Note that in some cases the
+ 2 volt reference is applied to the positive terminal of
the comparator rather than the negative terminal. Com­
parator operation will then be exactly opposite of that
just described. Table 8-2 lists all of the normal com­
parator input levels, comparator outputs, and resultant
Fet switch state. The l O k ohm resistors connected
gates of Ihe Fet switches along with the stray
capacitance act as low pass filters to prevent pick up and
rectification of stray AC signals.
Table 8-2 Comparator Operation
th
e
." . rtlm.
Com.,.rltor:, Input
Pot. Term.
O V
+ 2 V
+ 2 v
+ 5 V
+ 2 V
+ 2 v
O V
+ 5 V
- -
HA'
L M E S
L G N D
[RELAYS AND F E T S l
: 0 :
: 0 :
: 0 :
:
. _ ,
2 D
D E C O D E D L I N E S
Model 3456A
10
V, and
V Ranges.
Relay KlO3 (a
'
The co
mparators that
-
1 8 V and the Fet
COmPlrltor'. Outjlul
- 1 8 V
--
-
Pull-Up Voltage
Pull-Up Voltage
- 1 8 V
-
I N P U T
S\. J J T C H J N G
--
HSi-�-.
10
o
the
Fit Switch
Open
Closed
Closed
Open
o

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