ad386 Analog Devices, Inc., ad386 Datasheet - Page 6

• Manufacturer Part Number: ad386
• Description: True 16-bi Track-and-hold Amplifie
• Manufacturer: Analog Devices, Inc.

Settling Time: the time required in track mode for the OUtput to
Nonlinearity: the degree to which a plot of output versus input
deviates from the straight line defined by the end points. It is
usually specified as a percentage of full scale.
leakage currents.
Feedthrough: the fraction of inpUt signal variation which appears
at the output in hold mode as a result of capacitive coupling.
DielectricAbsorption: the tendency of charges within a capacitor
voltage of an open circuit capacitor after a large rapid change.
Acquisition Time: the time required after entering track mode for
the voltage on the hold capacitor to settle to within a specified
fraction of full scale. This is usually specified for a full-scale
step change in oUtput voltage.
reach its final value within a specified fraction of full scale fol-
lowing a step change in the input voltage.
Pedestal: a step change in the output voltage which occurs when
Hold Mode Settling Time: the time required for the pedestal to
reach its final value to within a specified fraction of full scale.
Droop: the change in the held output voltage resulting from
to redistribUte themselves over time, resulting in "creep" in the
Aperture Delay: the time required by the internal switch(es) to
Aperture Jitter: the uncertainty in Aperture Delay caused by
mum input frequency which can be sampled without error.
switching from track mode to hold mode.
TERMINOLOGY
disconnect the hold capacitor from the input, which produces an
effective delay in the sample timing.
internal noise and the variation of switching thresholds with sig-
nallevel. The error caused by aperture jitter depends on the
rate of change of the input and as such determines the maxi-
AD386
ANALOG DEVICESfAX-ON-DEMAND HOTLINE
COMMAND
TRACK
ANALOG
OUTPUT
ANALOG
INPUT
-FS
+FS
+FS
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TRANSIENT
AMPLITUDE
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Figure 13. T/H Characteristic Features
ACCURACY
Page
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II
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II
I HOLD
7
APERTURE
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HOlD MODE SETTLING TIME
DELAY
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THEORY OF OPERATION
The architecture of the AD386 differs from that usually encoun-
capacitor in a conventional TIH (Figure 14) is always connected
switch A is open and switch B is dosed. Since the summing
lows the input. The switches change state in hold mode which
disconnects the capacitOr from the input and holds the oUtput
voltage constant. The clamping action of switch A reduces the
variations across switch B, improving feedthrough performance.
tered in inverting Track-and-Hold (TIH) circuits. The hold
from the amplifier's output to its inverting inpuI. In track mode
junction is a virtual ground, the voltage across the capacitor fol-
sition times. A small resistOr creates greater demands on the cir-
cuit which drives the TIH, while a small capacitor leads to
increased pedestal and droop. In addition, the parallel combina-
tion of the feedback resistor and the hold capacitor acts as a low
pass filter and constrains both bandwidth and acquisition time.
This circuit forces several tradeoffs. The hold capacitor's charg-
ing current is limited by the input resistor. Either the resistor or
the capacitor, or both, must be made small to obtain fast acqui~
T/H
V'N
Figure 14. Conventional
TO SPECIFIED
R
- -
ACQUISITION
FEEDTHROUGH
DROOP
ABSORPTION
A
+ DIELECTRIC
- - -J - -
ACCURACY
TIME
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Inverting Integrator T/H
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