HP 3562A Service Manual page 329

MOD E L 3562A
A3 2, A34 ANALOG-ta-DIGITAL CONVERTERS
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(Refer to ADC Board Block Diagram on f i g u re 6-A32) The ADC board converts an alog
d ata from the i n put board i nto 1 3-b it serial data word s . The main parts of the ADC are
two variable atten u ators and thei r accom panyi ng am p l ifiers, an anti-al ias fi lter, a track
and hold c i rc u it, an 8-bit A/D co nverter (to avoid confu sion, the board is referred to as
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ADC" and the part is cal led
Other c i rcu its on the board i n c l ude f i rst and second pass c i rcu its and the p rocess switc h,
a dc offset D/A converter, an over range/half range c i rcu it, a m i ssed sample c i rcu it, a
d i agnostic tester, and sl ave/m aster sel ection c i rcu its.
The c i rcu its in the d ata path may be d ivided i nto analog and digita l . The s ignal is analog
u ntil it passes throu gh the A/D converter. T h i s discussion is d ivided i nto ana log and d igital
sections.
Analog
The signal from the i n put board ente rs the ADC board th rough ) 200 and a bal u n and goes
to attenuator #2 (#1 is on the i n put board). This atten u ator is variab le in 2 dB ste ps from
o
dB to 1 2 d B. It can also ground the i nput of the next a m p l ifier (for c a l i b ration pu rposes).
The configu ration i n formation for th is atte n u ator and atten u ator #3 come from seria l-to­
parallel (S/P) sh ift register U203. T h is i nformation comes on the board from the input board
and is shifted th rough U603, U203, and U202. The companion i nput board m ust be i nstalled
for the ADC board to receive configu ration i nfo rmation .
The signal from atten u ator #2 goes to a m p l ifier #3. T h i s am pl ifier i s a non-inverti ng
op amp c i rc u it with a gain of 3 . The s ignal from this am p l if i e r goes to atte nu ator #3. T h i s
atte nuator is identical to atten u ator # 2 except that i n stead o f selecti ng a g rou nded i n put
for the fol lowing a m p l ifier it has another step of atten u ation, giv i ng it a range of 0 d B
to 1 4 d B i n 2 d B steps.
The purpose of all the a m p l ifiers and variab le atten u ators up to this po i nt is to ass u re
that the strength of the s i g n a l to the anti-al ias f i l ter does not over d rive it. Refer to tab le
6-A32a for i nformation conce m i ng atten u ator configu ration for a given range seiection.
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A/D converter"), a D/A converter (DAC), and a contro l ler.
C I RC U I T D E SC R I PT I O N S
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