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M o d e l 5363B
S e r v i c e
A2 D I G ITAL-TO - A N A L O G C O N V E R T E R B O A R D
T h e D A C board converts a B C D n u m b e r , f r o m t h e t h u m b w h e e l s w i t c h e s o r H P- I B , t o a n
a n a l o g o u t p u t v o l t a g e . T h i s d e o u t p u t I S f e d a s a reference l e v e l t o t h e a p p r o p r i a t e d i ffe r e n t i a l
a m p l i f i e r I n the probe
U6 a n d U 1 1 a re
a n d store t h e BCD n u m b e r f o r e a c ll of t h e t h ree d i g i t s I n to Voltage D r r ve r s U5, U2, and U4, The
Vo l t a g e D r i ve r s p r o v i d e o u t p uts In BCD format t o prec i s I O n resistors that d e ve l o p an a n a l og
v o l t a g e , T h e least s i g n i f i c a n t d i g i t I S s u p p l i ed b y U 4 a n d t h e m o s t S i g n i f i c a n t d i g i t i s s u p p l i ed
b y U7, Buffer U7 c o m p e n sates f o r t h e h i g h " o n " resistance of
T h e DAC Ladder i s a resi s t i ve network c o n nected to the o u t p u t s of t h e Voltage D r i ve rs . T h e
resistors of t h e o u t p u t of U 4
0 , 1 %
a n d the resistors at the o u t p u t of U 7
D A C Lad d e r I S a p p l i e d to t h e N e g a t i ve A m p l i f i e r U 3 a n d the Positive A m p l i f i e r U 1 T h e o u t p u t
of o n e o r the o t h e r a m p l i fi e r I S c o n nected to t h e n o n l n ve r t l n g i n p u t of O u t p u t A m p l i f i e r U 9 ,
depe n d i n g u p o n w h i c h F E T 1 0 6 , 0 7 , i s t u rned o n b y t h e Po l a rr ty S w i tc h c r rc u l t . A n offset
v o l t a g e , from t h e C a l i b ra t i o n D A C s , I S a p p l i e d to t h e i n ve r t i n g I n p u t o f U 9 T h e sum of these
t w o voltages i s t h e f i n a l a n a l o g o u t p u t v o l t a g e
U8 a n d associated c r rc u l t r y f o r m a p r e c i s i o n Voltage R e f e r e n c e t o e n s u re t h e acc u ra c y o f t h e
D A C . T h e e x a c t v a l u e o f R54 i s f a c t o r y s e l e c t e d t o c o m p e n s a t e t o r d i fferences I n t h e
brea k d o w n va l u e of Z e n e r d i o d e C R 5 .
8 - 6 0
T T L
to C M O S c o n v e r t e r s , Load c o m m a n d s , t h ro u g h
i
I
L S D
h a ve a t o l e r a n c e o f 1
I
M S D
U 1 0 ,
U5,
% ,
t h e resisto rs a t t h e o u t p u t of U 2 a re
%,
I
a re 0 , 0 1
T h e s c a l e d a n a l o g vo l t a g e f r o m t h e
s e q u e n t i a l l y load
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