ADADC71
Rev. C | Page 10 of 12
GROUNDING, DECOUPLING, AND LAYOUT
CONSIDERATIONS
Many data-acquisition components have two or more ground
pins, which are not connected together within the device. These
grounds are usually referred to as the DIGITAL COMMON
(logic power return), ANALOG COMMON (analog power
return), or analog signal ground. These grounds (Pin 19 and
Pin 22) must be tied together at one point as close as possible to
the converter. Ideally, a single solid analog ground plane under
the converter would be desirable. Current flows through the
wires and etch stripes of the circuit card, and since these paths
have resistance and inductance, hundreds of millivolts can be
generated between the system analog ground point and the
ground pins of the ADADC71. Separate wide conductor stripe
ground returns should be provided for high resolution
converters to minimize noise and IR losses from the current
flow in the path from the converter to the system ground point.
In this way the ADADC71 supply currents and other digital
logic-gate return currents are not summed into the same return
path as analog signals where they would cause measurement
errors.
Each of the ADADC71’s supply terminals should be capacitively
decoupled as close to the ADADC71 as possible. A large value,
such as 1 μF, capacitor in parallel with a 0.1 μF capacitor is
usually sufficient. Analog supplies are to be bypassed to the
ANALOG COMMON (analog power return) Pin 22 and the
logic supply is bypassed to DIGITAL COMMON (logic power
return) Pin 19.
The metal cover is internally grounded with respect to the
power supplies, grounds and electrical signals. Do not
externally ground the cover.
T/H REQUIREMENTS FOR HIGH RESOLUTION
APPLICATIONS
The characteristics required for high resolution track-and-hold
amplifiers are low feedthrough, low pedestal shifts with changes
of input signal or temperature, high linearity, low temperature
coefficients, and minimal droop rate.
The aperture jitter is a result of noise within the switching
network that modulates the phase of the hold command, and is
manifested in the variations in the value of the analog input that
has been held. The aperture error which results from this jitter
is directly related to the dV/dt of the analog input.
The T/H amplifier slew rate determines the maximum
frequency tracking rate and part of the settling time when
sampling pulses and square waves. The feedthrough from input
to output while in the hold mode should be less than 1 LSB. The
amplitude of 1 LSB of the companion ADC for a given input
range will vary from 610 μV for a 14-bit ADC using a 0 V to
+10 V input range to 4.88 mV for a 12-bit ADC using a ±10 V
input range. The hold mode droop rate should produce less
than 1 LSB of droop in the output during the conversion time of
the ADC. For 610 μV/LSB, as noted in the example above, for a
50 μs 14-bit ADC, the maximum droop rate is 610 μV/50 μs or
12 μV/μs during the 50 μs conversion period.
Minimal thermal tail effects are another requirement of high
resolution applications. The self-heating errors induced by the
changing current levels in the output stages of T/H amps may
cause more than 1 LSB of error due to thermal tail effects.
The linearity error should be less than 1 LSB over the transfer
function, as set by the resolution of the ADC. The T/H
acquisition time and T/H settling time along with the
conversion time of the ADC determines the highest sampling
rate. This in turn determines the highest input signal frequency
that can be sampled at twice a cycle.
The maximum input frequency is constrained by the Nyquist
sampling theorem to be half of the maximum throughput rate.
Input frequencies higher than half the maximum throughput
rate result in under sampling or aliasing errors of the input
signal.
The pedestal shift due to input signal changes should either be
linear, to be seen as a gain error, or negligible, as with the
feedthrough specification. The temperature coefficients for drift
would be low enough such that full accuracy is maintained over
some minimum temperature range. The droop rate and
pedestal shift increases above +70
o
C (+158
o
F). For commercial
and industrial users, these shifts only appear above the highest
temperatures their equipment might expect to experience. Most
precision instrumentation is installed only in human
inhabitable work spaces or in controlled enclosures if the area
has a hostile environment. Thus, the ADADC71 used with a
sample-and-hold amplifier offers high accuracy sampling in
high precision applications.
