HP 3562A Service Manual page 333

MOD E L 3562A
The DAC co nverts the f i rst pass word back to a voltage w h i c h goes to the second pass
c i rc u it (U406). T h i s c i rc u it com pares the converted s i g n a l to the original i n put signal and
p rod u ces a d ifference vo ltage. The control ler sets the the process switch to send th i s
d i ffere nce voltage t o the A/D converte r where i t is converted . When the conversion i s
complete, the contro l l e r i nputs the res u lt, scales i t down and adds i t to the f i rst pass word
to obta i n a 1 3-bit data word. The contro l ler sends the converted data to the d igital fi lter
in a serial format after the d igital fi lter sends a req uest for the data (D R E Q L).
Between each of the passes, the control l e r sets the process switc h to send a .34 volt d c
s ignal to the AID co nverter t o reset i t . T h i s "th i rd pass" s hows i n figure 6-A32b a s the
. high portion of the signal. The f i rst pass appears as the s ignal fol lowing a rou ghly s i nusoidal
p atte rn and the second pass appears as a no isy signal cente red with i n the f i rst pass s i n e
wave.
Figure 6-A32b
Process switch output; TP40S
Table 6-A3 2b Process switch configuration Information
F i rst Pass Circuit
.34 V d c Reference
Second Pass C ircu it
6.2 V reference voltage
The ADC board uses the 6.2 voltage refe rence generated by the DAC . T h is red u ces the
error due to tem pe ratu re variations. The vo l tage reference is used by the A/D converter,
the track and hold c i rc u it, and the over range/half range c i rc u it. S i nce the A/D converter's
gain is contro l led by this refe ren ce, the adj ustment for the voltage refe rence is also the
AID converter's gain adj ustment.
1
x
o
o o
C I RC U I T D E SC R I PT I O N S
6-1 1 7