SHA4_15 Datasheet, PDF (3/4 Pages) Analog Devices – SAMPLE AND HOLD AMPLIFIERS

GENERAL DESCRIPTION
The SHA IA is a fast sample-and-hold module with low droop rate and
overall accuracy compatible with 12-bit A/D conversion systems opera-
ting to %LSB accuracy. When in the "sample" mode, the module appears
as a fast amplifier with 51ls settling time to 0.01 %, 1nA input current,
251lV IC drift, and unity gain with :t10V at :t20mA output current
capability. When in the "hold" mode, the droop rate is 501lV/ms max,
so the SHA IA will hold an input signal to 0.01% of full scale (20V pop)
for 40ms, sufficient for 12-bit A/D conversion.
SAMPLE TO HOLD CHARACTERISTICS
Of prime importance in selecting Sample-and-Hold amplifiers is the
transition characteristics when the module is commanded into hold by
the digital control line. A finite delay will occur between initiation of
the hold command, and actual disconnection of the hold capacitor from
OBSOLETE the input buffer amplifier. In the SHA lA, this delay time is 40ns maxi-
mum. The uncertainty, or jitter over which this delay time will vary
from cycle to cycle, as the module is repeatedly comma,nded into hold,
is :t5ns. In most systems, the jitter specification is the limiting factor on
overall system speed for a given accuracy, since fixed delays can be re-
moved by adjusting the system timing. The 5ns jitter specification
means the SHA IA can track a signal slewing up to 0.2V Ills, and
"capture" that signal to within a 1mV accuracy for A/D conversion.
OVERALL ACCURACY
The SHA IA is guaranteed to have an overall throughput non linearity
of 2mV max over a :t10V input range, or 1mV max over 0 to +10V
inputs. This specification combines the effects of common mode errors.
gain non linearity and sample to hold offset nonlinearity. It is no longer
necessary to guess at the combined effects of individual errors since the
SHA IA specification guarantees that its total non-linearity errors are
sufficiently low to insure %LSB accuracy in 12-bit systems.
GROUNDING
Many data acquisition systems suffer from digital ground induced
noise appearing in the analog system. To counteract this problem, the
SHA IA has three separate ground systems. The digital ground is actual~
Iy one side of a differential amplifier, with the Sample/Hold digital
control input being the other input of this amplifier. This effectively
prevents digital ground noise from being impressed into the analog
signal channel. The power ground and analog input/output grounds are
also separate, so that power supply ground noise is reduced by the re-
jection coefficients of the amplifiers, normally well over 9OdB. Ground
connection instructions are given in Figure 1.
- ^iNPUT
ANALOG I
SIGNAL
-=
ANALOG
OUTPUT
A DC path must exist between
the Analog, Digital, and power
supply grounds. Multiple grounds
on signal and power return lines
should be avoided. If possible,
only one external ground should
. ^{exist on the Analog ground system}
ANALOG iN
GARNOALOG
CONTROL
iN
. ^{DIG GRD}
OIGITAL
CONTROL
SIGNAL
-15
-15
ANALDG
OUT
ANALOG
GRD
PWR
GRD
.15
CO., I POWER
SUPPLY
0'5 =
-.. ANALOG
W DEVICES
Fig. 1 SHA fA Connections and Grounding
CO98c-1 0-1 /80
PRINTED IN U.S.A.
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