HP 3562A, Service Manual

Page: 332 / 570

C I RC U I T D E SC R I PTIONS MODE L 3562A
6-1 1 6
The anti-al ias filter is a passive low pass filter whose break frequency i s 1 00 kHz. Fol lowing
this fi lter are two non-inverting op am ps, each with a gain of 3, which feed the track and
hold circu it.
The track and hold circuit is contro l l ed by the CONY signal from the trigger board and
the TRH (track
&
hold) signal from the ADC controller. The CONY signal initiates the "hold"
and the control ler term inates it. The pu rpose of the track and hold circ u it is to hold the
voltage of the in put signal for the period of time requ i red for the value to be d igitized.
W hen tracking, the circuit acts l ike an i nverting amplifier. See figu re 6-A32a.
T/H OUT
T/H
I N
TP600
Figure 6-A32a Track
&
hold Input vs. output
Digital
TRACK
HOLD
" e conversIOn starts
w ·
en
e
controller receives a start conversion
trigger i n chan nel two generates the control signals which
coord inates the two passes and the process switches on both boards. The
T/H OUT signal is converted to a
digital word by passing the signal through an
8-b it A/D converter twice as descri bed in the fo l lowing d iscussion .
The first pass c i rcuit takes the track and hold output, divides it by fou r, and leve l sh ifts
it for the A/D converter. The controller then sets the process switch (U405) to send this
signal to the AID converter (see table 6-A32b for process switch configu ration information).
When the conversion is com plete, the 8-bit word goes to the controller which comb i nes
the word with dither (a noise signal generated i ns ide the control ler chip) and outputs a
1 3-b it word to the DAC.