SHA1144_15 Datasheet, PDF (4/6 Pages) Analog Devices – HIGH RESOLUTION 14 BIT SAMPLE AND HOLD AMPLIFIER

OPERATION WITH AN AID CONVERTER
Figure 6 below shows the appropriate connections between
the SHA1144 and a successive approximation AID converter
in block diagram form.
OPERATION WITH AN AID AND MULTIPLEXER
The subsystem of Figure 9 may also be connected to a multi-
plexer like the Harris HI508A as shown below.
INPUT
SHA1144
INPUT
OUTPUT
MODE
CONTROL
INPUT
TO
DIGITAL
CONVERTER
DIGITAL
OUTPUT
""°0 E} STATUS
CONVERT
COMMAND
: CHANNEL"NPUT
I
I
I
I
I^I
I
I
CHANNELS INPUT
HI50SA
OUTPUT
SHAl144
INPUT
OUTPUT
MODE
CONTROL
A/D
CONVERTER
INPUT
ITAYtJS STATUS
DIGITAL
OUTPUT
!
CONVERT J1...
COMMAND
Figure 6. SHA 1144 and AID Connections
CONVERT
COMMAND
INPUT
The resulting timing sequence at the start of conversion is illus-
trated in Figure 7,
Figure 9. AID, SHA, and MPX Connections
SHA1144
INPUT
OUTPUT
SIGNALS
/II _{SHA1144 MODE}
OCONVERT
COMMAND
STATUS OUTPUT!
B_{-S-jjr ~N~~~ CONTROL}
~"""'/~I--------
-----
--n
I
~ '/ SAMPLE """"""""""""'\;'
II
I
1
0
1
I
f
0
--- INPUT
OUTPUT
H'O'LD~""""""""""""""""'\;
AP:R~RE
SWITCHING TRANSIENT
SETTLING ~ 1",
DELA y ~ 50n'
O _{Figure 7. AID and SHA Timing at Start of Conversion}
L _{n Note that the leading edgeof the convert command pulsecauses}
E_{---, the converter's STATUS output to go to Logic "0" which in}
TE turn switches the SHA1144 from sample to hold. As discussed
The leading edge of the convert command pulse sets the
STATUS output to Logic "0" thereby switching the SHA1144
to "hold"; the corresponding change to Logic" 1" of the
STATUS output increments the binary counter and changes
the multiplexer address. Since the SHA1144's aperture time is
small with respect to the multiplexer switching time, it will
have switched to the hold mode before the multiplexer actu-
ally changes channels. The multiplexer switching transients
will settle out long before the SHA returns to "sample" at the
end of conversion. The timing sequence described above is
illustrated in Figure 10.
CONVERT
COMMAND
A;Eug;:t'NPUT
MUL TlPLEXER
CHANNEL
----Il
n
n
n
I CH" +8V
CH2, -10V
I
CH 3, +3V
I
r
CH4, OV
£1
previously, the typical SHA1144 actually changes modes 50
+10V
to 50.5ns after the "1" to "0" transition of the mode control
input. This mode switching causes a transient on the output
terminal which decays to within 0.003% of the final value in
MULTIPLEXER
OUTPUT!
SHA INPUT
R=
OV
-10V
f
\to
approximately Ills, Once the transient has settled, the convert
command input is returned to Logic "0" and the conversion
SHA1'44 MODE
proceeds. As shown in Figure 8, the STATUS signal returns
to Logic "1" and the SHA1144 returns to the sample mode
at the end of conversion. Within 611s,it will have acquired the
input signal to 0.003% accuracy and a new conversion cycle
may be started.
SHA OUTPUT/
AID INPUT
CHANNEL BEING
DIGITIZED
.:'HW !~"~'E:'
-^l -^f - ^{"'"~'E:';"'" ~,~"'~ ~".}
CHANNEL 1
CHANNEL 2
CHANNEL 3
-- INPUT
SHA1144
INPUT!
I
- OUTPUT
Figure 10. AID, SHA, and MPX Timing
OUTPUT
SIGNALS
SHA1144 MODE
1
1
I
0::HO' LD \:W/',/'////,
I '"
SAMPL ^E' //'//////ffi
This method of sequencing the multiplexer may be altered to
permit random addressing or addressing in a preset pattern.
'n.
:d
~
STATUSOUTPUT! 1
MODECONTROL
INPUT
0
L
_I ACQUISITION
[I
TIME~6"s
f
The timing of the multiplexer address changes may also be
altered but consideration should be given to the effects of
feed through in the SHA1144. Feedthrough is the coupling of
analog input signals to the output terminal while the SHA is
~
Figure 8. AID and SHA Timing at End of Conversion
in "hold". Large multiplexer switching transients occuring
during AID conversion may introduce an error.
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