SHA1144_15 Datasheet, PDF (3/6 Pages) Analog Devices – HIGH RESOLUTION 14 BIT SAMPLE AND HOLD AMPLIFIER

Figure 2 shows feedback connections to the SHA1144 for the
(-
unity gain follower mode. Output (pin 9) is connected to in-
put (pin 4). Input signal is applied to pin 3.
v
ANALOG
INPUT/OUTPUT
SIGNALS
- SHA INPUT SIGNAL
-- SHA OUTPUT SIGNAL
I
+INPUT
SHA1144 MOOE _{;/"///~SAMPLE'l///~}
HOLD~~
SHA1144
~---, ^{MODE CONTROL}
INPUT
I---
50ns
I^I
Figure 2. Unity Gain Follower
Figure 3 shows feedback connections for noninverting op-
~
DESIRED
SWITCHING
POINT
( _OBSOLETE ^{eration with potentiometric gain. When the indicated values}
Figure 4. Aperture Uncertainty
are installed, gain will be +5. As in all operational amplifiers,
gain-bandwidth product is a constant for a given sample-and-
hold. Effective 3dB bandwidth will be inversely proportional
to gain.
The maximum allowable slew rate will thus equal the quotient
of the maximum allowable voltage uncertainty and the 0.5ns
aperture uncertainty. For sinewave inputs, the corresponding
maximum frequency is expressed by:
R1 1k
+15V
+INPUT
R2
4k
OUTPUT
fmax = (I1E ) (--1-) ==3.18xl08 (I1E )
EFS 2nl1t
EFS
where:
I1E = the allowable voltage uncertainty
EFS = the sinewave magnitude
For a system containing a SHA1144 and a 14-bit AID with
-15V
:tl0V input signals and an allowable input uncertainty of
:tl/2LSB (:t620IlV), the maximum allowable signal frequen-
("
Figure 3. Noninverting Operation
cy will be 19.7kHz.
By using conventional operational amplifier feedback con-
POWER SUPPLY AND GROUNDING CONNECTIONS
The proper power supply and grounding connections are shown
nections, the SHA1144 can be connected for use as an inverter, _{shown below in Figure 5.}
with various gains (as determined by the RF/RI ratio), or as a
differential amplifier.
'15V
+5V
DA TA ACQUISITION APPLICATION
Successive-approximation AID converters can generate substan-
tiallinearity errors if the analog input varies during the period
of conversion; even the fast 14-bit models available cannot
902-2
+15V COM -15V
905
COM
+5V
TO DIGITAL
LOGIC
tolerate input signal frequencies of greater than a few Hz. For
this reason, sample-and-hold amplifiers like the SHA1144 are
connected between the AID and its signal source to hold the
analog input constant during conversion.
When the SHA1144 is connected to an AID, its aperture time
uncertainty, rather than the AID's conversion time, is the fac-
tor which limits the allowable input signal frequency. The
SHA1144, with a typical aperture delay time of 50ns and an
uncertainty of 0.5ns, will change from the sample mode to the
hold mode 50 to 50.5ns after the" 1" to "0" transition of the
mode control input. If the system timing is so arranged as to
initiate the mode control signal 50ns early, then switching will
actually occur within 0.5ns of the desired time as shown
below.
10"F
35V 1+
lO"F
,+35V
7
ANALOG GND
-15V
6
+15V
SHA1144
11
DIG GND
Figure 5. Power Supply and Grounding Connections
The :t15V power supplies must be externally bypassed as shown.
The capacitors should be tantalum types and should be installed
as close to the module pins as possible. The analog and digital
ground lines should be run separately to their respective power
supply commons to prevent coupling of digital switching noise
to the sensitive analog circuit section.
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