deviations from the rated values will be a small change in the
full-scale calibration value. This change, of course, results in
a proportional change in all code transition values (i.e., a
gain error). The specification describes the maximum change
in the full-scale calibration value from the initial value for a
change in each power supply voltage.
to obtain noise-free operation. Noise on the power supply
lines can degrade the converter’s performance. Noise and
spikes from a switching power supply are especially
troublesome.
ANALOG SIGNAL SOURCE IMPEDANCE
The signal source supplying the analog input signal to the
ADC574A will be driving into a nominal DC input imped-
ance of either 5kΩ or 10kΩ. However, the output impedance
of the driving source should be very low, such as the output
impedance provided by a wideband, fast-settling operational
amplifier. Transients in A/D input current are caused by the
changes in output current of the internal D/A converter as it
tests the various bits. The output voltage of the driving
source must remain constant while furnishing these fast
current changes. If the application requires a sample/hold,
select a sample/hold with sufficient bandwidth to preserve
the accuracy or use a separate wideband buffer amplifier to
lower the output impedance.
TEMPERATURE COEFFICIENTS
The temperature coefficients for full-scale calibration, unipo-
lar offset and bipolar offset specify the maximum change
from the +25°C value to the value at TMIN or TMAX.
QUANTIZATION UNCERTAINTY
Analog-to-digital converters have an inherent quantization
error of ±1/2LSB. This error is a fundamental property of the
quantization process and cannot be eliminated.
CODE WIDTH (QUANTUM)
Code width, or quantum, is defined as the range of analog
input values for which a given output code will occur. The
ideal code width is 1LSB.
RANGE CONNECTIONS
The ADC574A offers four standard input ranges: 0V to
+10V, 0V to +20V, ±5V, and ±10V. If a 10V input range is
required, the analog input signal should be connected to pin
13 of the converter. A signal requiring a 20V range is
connected to pin 14. In either case the other pin of the two
is left unconnected. Full-scale and offset adjustments are
described below.
INSTALLATION
LAYOUT PRECAUTIONS
Analog (pin 9) and digital (pin 15) commons are not con-
nected together internally in the ADC574A, but should be
connected together as close to the unit as possible and to an
analog common ground plane beneath the converter on the
component side of the board. In addition, a wide conductor
pattern should run directly from pin 9 to the analog supply
common, and a separate wide conductor pattern from pin 15
to the digital supply common. Analog common (pin 9)
typically carries +8mA.
To operate the converter with a 10.24V (2.5mV LSB) or
20.48V (5mV LSB) input range, insert a 120Ω, 1% metal-
film resistor in series with pin 13 for the 10.24V range, or a
240Ω, 1% metal-film resistor in series with pin 14 for the
20.48V range. Offset and gain adjustments are still per-
formed as described below. However, you must recalculate
full-scale adjustment voltages proportionately. A fixed metal-
film resistor can be used because the input impedance of the
ADC574A is trimmed to less than ±6% of the nominal
value.
If the single-point system common cannot be established
directly at the converter, pin 9 and 15 should still be
connected together at the converter; a single wide conductor
pattern then connects these two pins to the system common.
In either case, the common return of the analog input signal
should be referenced to pin 9 of the ADC. This prevents any
voltage drops that might occur in the power supply common
returns from appearing in series with the input signal.
CALIBRATION
OPTIONAL EXTERNAL FULL-SCALE
AND OFFSET ADJUSTMENTS
Coupling between analog input and digital lines should be
minimized by careful layout. For instance, if the lines must
cross, they should do so at right angles. Parallel analog and
digital lines should be separated from each other by a pattern
connected to common.
Offset and full-scale errors may be trimmed to zero using
external offset and full-scale trim potentiometers connected
to the ADC574A as shown in Figures 2 and 3 for unipolar
and bipolar operation.
If external full scale and offset potentiometers are used, the
potentiometers and associated resistors should be located as
close to the ADC574A as possible. If no trim adjustments
are used, the fixed resistors should likewise be as close as
possible.
CALIBRATION PROCEDURE —
UNIPOLAR RANGES
If adjustment of unipolar offset and full scale is not required,
replace R2 with a 50Ω, 1% metal-film resistor and connect
pin 12 to pin 9, omitting the adjustment network.
POWER SUPPLY DECOUPLING
If adjustment is required, connect the converter as shown in
Figure 2. Sweep the input through the end-point transition
voltage (0V + 1/2LSB; +1.22mV for the 10V range, +2.44mV
Logic and analog power supplies should be bypassed with
10µF tantalum-type capacitors located close to the converter
®
6
ADC574A
BB [ BURR-BROWN CORPORATION ]
