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8 - 6 8 .
I n s h o rt , a s i m p l i f i e d way of t h i n k i n g of t h e c a l i b ra t i o n process i s t h a t t h e state ma c h i n e
A 1 U 1 1 . 25 , 2 8 , 30 · t u r n s o n t h e a p p r o p e ri ate p r o b e , resets t h e R eference D A C A 2 s o t h a t i t ' s
o u t p u t l e v e l i s O .OO V . o u tp u ts -7 5 mV v i a t h e STA R T C a l i b ra t i o n D A C A 1 U 3 , U 4 to t h e p r o b e ,
c h e c k s f o r a STA R T T R I G G E R . i n c re m e nts t h e D A C o u t p u t b y -r0 .6 m V , c h ec k s f o r a t r i g g e r,
e t c . T h i s process i s re peated u n t i l a START T R I G G E R i s rece i ved , or a ti m e r A 1 U 1 4 A o n t h e
m o th e rb o a rd w h i c h w a s t r i g g e re d e a r l y i n t h e C a l i b ra t i o n R o u t i n e , state 1 70 s i g n a l s t h a t t h e
c a l i b ra t i o n p rocess h a s t a k e n too l o n g . T h e t i m e rs o u t p u t i s > 2 2 0 m s l o n g , a n d i s u s e d t o te l l
t h e state m a c h i n e t h a t s o m et h i n g i s n o t a l l o w i n g t h e p r o b e s t o c a l i b rate.
8-6 9 .
O n ce the p r o c e ss has bee n c o m p l eted f o r the STA R T c h a n n e l p r o b e , t h e state ma c h i n e
l ocates a n d e n a b le s t h e S T O P p ro b e a n d c a l i b rates i t i n t h e s a m e way. T h e S T O P c h a n n e l
Ca l i b ra t i o n D A C c o n s i sts of A 1 U 1 / U 6 w h i l e t h e a ss o c i a t e d c o m p o n e n ts a re J F ETs A6/A7 U 2 ,
a n d t r a n s i s t o r a r rays A 1 U 1 3/ 1 7 . T h e c a l i b ra t i o n ro u t i n e i s e x a c t l y t h e s a m e u s i n g t h e STO P
c h a n n e l c o m p o n e n t s .
8-7 0 .
I f t h e c a l i b ra t i o n p r o c e ss i s n o t c o m pl eted beca use o f t h e state m a c h i n e n o t rec e i v i n g a
t ri g g e r f o r s o m e rea so n , i n e i t h e r t h e STA R T or S T O P c h a n n e l s , t h e state m a c h i n e w i l l e x i t t h e
Ca l i b ra t i o n R o u t i n e a n d g o to t h e E r r o r R o u t i n e . T h i s ca u s e s t h e state ma c h i n e to l o o p a n d
b l i n k t h e L E D s o n a n d off .
8-7 1 .
T h e J F ET pa i rs of U 1 a n d U 2 i n t h e p r o b e a sse m b l i e s a re m a t c h e d o r s o rted at t h e
m a n u fa c t u re r p e r H P s p ec i f i c a t i o n s . T h i s a ss u res a c l o s e n e s s i n spec i f i c a t i o n s between a l l o f
t h e STA R T a n d S T O P i n p u t ' i n v e rt i n g J F E T s .
8-7 2 .
H ow e v e r , t h e re i s sti l l a d i ff e r e n c e i n t h e V g s , rati n g s of t h e two J F ETs w i th i n a s i n g l e
A6/A7 U 1 o r U 2 p a c k a g e . T h i s m e a n s t h a t t h e t r i g g e r v o l t a g e of t h e i n ve rt i ng J F ET a n d t h a t of
t h e n o n i n v e rt i n g J F ET w i l l be off by w h ateve r t h e d i ffe r e n c e i s betw een t h e i r re spec t i ve V g s
rat i n g s . A l s o , a s m a l l offset e xi t s between t h e p l u s a n d m i n u s t e rm i n a l s o f t h e c o m p a rators
w h i c h se n se a t r i g g e r s i t u a t i o n . And f i n a l l y , the c o m m o n - b a se l oa d of t h e c u r r e n t so u rc e of
Q 1 /C R 1 or Q 2/ C R 2 adds to t h i s v o l ta g e d i sc re p e n c y .
8-73.
T h e s u m o f a l l o f t h e s e v o l t a g e d i ffe re n c e s i s t e r m e d t h e tota l offset v o l ta g e . If a l l o t h e r
e r rors i n t h e t r i g g e ri n g syste m we re z e r o , t h i s v o l ta g e wo u l d b e e xa c t l y t h e d i ffe re n c e between
t h e v o l ta g e rea d i n g f ro m the t h u m b w h e e l s w i t c h e s and voltage at w h i c h t h e i n s t r u m e n t p r o b e
tri g g e rs . S i n c e t h i s tota l offset v o l t a g e h e re a f te r re fe rred to a s s i m p l y t h e offset v o l t a g e c a n
b e r e l a t i v e l y h i g h w h e n c o m pa red to t h e i n p u t v o l ta g e , i t m u st s o m e h o w be ca l i b rated o u t of
th e c i rc u i t .
8-74.
T h e f o l l ow i n g ,
Fig u re 8 -4 ,
b a s i c a l l y a d i ffe re n t i a l a m p l i f i e r of l o w c o m m o n - m o d e g a i n , w h i c h pe rm i ts o p e ra t i o n ove r a
�
1 OV ra n g e .
8-7 5 .
To ze ro o u t t h e effe c t o f t h e offset v o l ta g e , t w o Ca l i b ra t i o n D A Cs a re u s e d w h i c h f i n d
a n d s t o re t h e offset va l u e se l e c ted STA R T a n d S T O P p r o b e s o n e D A C f o r eac h . T h i s process
g ets t h e t r i g g e r e r ror d u e to offset v o l t a g e d o w n f ro m a s m u c h a s 75 mV to .6 m V .
8-7 6 .
T h e i m p o r t a n c e of t h e c a l i b ra t i o n c i rc u i t c a n n o w b e see n . I t effe c t i ve l y re d u c e s t h e
tri g g e r e rr o r d ue to off set v o l t a g e to a n e g l i g i b l e l e ve l , p r o d u c i n g a m o re a c c u rate
m e a s u re m e n t .
' T h e R e f e re n ce D A C
A 2
I S te m p o ra r i l y reset d u r i n g t h e C a l i b r at i o n R o u t i n e S t a t e 1 66 . w h i c h p u t s I t S
o u t p u t v o l t a g e a t 0 O O V T h e re f o r e t h e o u t p u t of t h e Refe re n c e D A C a t p i n 6 of A 2 U 9 i s t h e o u t p u t of t h e
C a l i b r at i o n D A C d u r i n g t h e c a l i b ra t i o n c y c l e
i s a s i m p l i f i ed d i a g ra m of t h e i n p u t c i rc u i t o f t h e 5363 6 . I t i s
M od e l 5363 6
S e r v i c e
8-1 7
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