SHA1144 Datasheet, PDF (3/6 Pages) Analog Devices – HIGH RESOLUTION 14 BIT SAMPLE AND HOLD AMPLIFIER

Figure2 showsfeedbackconnectionsto the _{SHA1144 for the}
unity gainfollower mode.Output (pin _{9) is connectedto in-}
put (pin 4). Input signalis appliedto pin _3.
ANALOG
INPUT/OUTPUT
SIGNALS
P I N9
MODE
-15V ^CONTROL
Figure 2. Unity Gain Follower
Figure3 showsfeedbackconnectionsfor noninvertingop-
erationwith potentiomeuicgain.Whenthe indicatedvalues
areinstalled,gainwill be +5. As in all operationalamplifiers,
gain-bandwidthproduct is a constanrfor _a givensample-and-
hold. Effective3dB bandwidthwill be inverselyproportional
to gain.
Rl lk
Figure 3. Noninverting Operation
By using conventional operational amplifier _{feedback con-}
nections,the SHA1144 can be connectedfor use _{as an inverter,}
with various gains (as determined by the Rp/R1 _{ratio), or as a}
differential amplifier.
SHA11tl4MODE
SHAI'|'|4
1
MODECONTROL _^
INPUT
'"^ol^.l^rl^lf^*l^ry^oo
Figure 4. Aperture Uncertainty
The maximum allowableslewrate will thus equalthe quotient
of the maximum allowablevolrageuncertaintyand the 0.5ns
apertureuncertainty.For sinewaveinputs,the corresponding
maximum frequencyis expressedby:
f^{^}- o =^-
nF
C#;)
(¹
T^r)
=
3
.
1
8
x
1
0^-
6,
n
rr
-
;
F
,
where: AE ⁼ the allowablevoltageuncertainty
ErS ⁼ the sinewavemagnitude
For a systemcontaininga SHA1144and _a 14-bitA/D _with
+10V _{input signalsand an allowableinput uncertainryof}
tll?LSB (!62OpY), _{the maximum allowablesignalfrequen-}
cy will be 19.7kH2.
POWERSUPPLYAND GROUNDING _CONNECTIONS
The proper power supplyandgroundingconnectionsareshown
shownbelowin Figure5.
DATA ACQUISITION APPLICATION
Successive-approximationA/D converters can generate _zubstan-
tid linearity errors if the analog input varies _{during the period}
of conversion; even the fast 14-bit models _{available cannot}
tolerate input signal frequencies ofgreater _{than a few Hz. For}
this reason, sample-and-hold amplifiers like _{the SHA1l44 are}
connected berween the A/D and its signal source _{to hold the}
analog input constant during conversion.
When the SHAl144 is connected to an A/D, its aperture _time
uncertainty, rather than the A/D's conversion _{time, is the fac-}
tor which limits the allowable input signal frequency. _The
SHAl144, with a typical aperture delay time of _{50ns and an}
uncertainty of O.Sns,will change from _{the sample mode to the}
hold mode 50 to 50.5nsafter the ^"1" _to ^"0" _{transition of the}
mode control input. If the system timing is so arranged _{as to}
initiate the mode conuol signal _50ns early, _then switching _will
actually occur within 0.5ns of the desired time as shown
below.
TO DIGITAL
LOGIC
Figure 5. Power Supply and Grounding _Connections
The t15V power suppliesmusrbe externallybypassedasshown.
The capacitorsshouldbe tantalumtypesand shouldbe _installed
ascloseto the module pins aspossible.The analogand _digital
groundlinesshouldbe run separatelyto _{their respectivepower}
supply commonsto prevenrcouplingof digital _{switchingnoise}
to the sensitiveanalogcircuit section.
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