
AD7228A
REV. A
–5–
OP AMP SE CT ION
Each voltage-mode D/A converter output is buffered by a unity
gain noninverting CMOS amplifier. T his buffer amplifier is
tested with a 2 k
and 100 pF load but will typically drive a
2 k
and 500 pF load.
T he AD7228A can be operated single or dual supply. Operating
the part from single or dual supplies has no effect on the positive-
going settling time. However, the negative-going settling time to
voltages near 0 V in single supply will be slightly longer than the
settling time for dual supply operation. Additionally, to ensure
that the output voltage can go to 0 V in single supply, a transis-
tor on the output acts as a passive pull-down as the output volt-
age nears 0 V. As a result, the sink capability of the amplifier is
reduced as the output voltage nears 0 V in single supply. In dual
supply operation, the full sink capability of 400
μ
A at 25
°
C is
maintained over the entire output voltage range. T he single sup-
ply output sink capability is shown in Figure 4. T he negative
V
SS
also gives improved output amplifier performance allowing
an extended input reference voltage range and giving improved
slew rate at the output.
Figure 4. Single Supply Sink Current
T he output broadband noise from the amplifier is 300
μ
V
peak-to-peak. Figure 5 shows a plot of noise spectral density
versus frequency.
Figure 5. Noise Spectral Density vs. Frequency
DIGIT AL INPUT S
T he AD7228A digital inputs are compatible with either T T L or
5 V CMOS levels. All logic inputs are static-protected MOS
gates with typical input currents of less than 1 nA. Internal in-
put protection is achieved by on-chip distributed diodes.
SUPPLY CURRE NT
T he AD7228A has a maximum I
DD
specification of 22 mA and
a maximum I
SS
of 20 mA over the –55
°
C to +125
°
C tempera-
ture range. T his maximum current specification is actually de-
termined by the current at –55
°
C. Figure 6 shows a typical plot
of power supply current versus temperature.
Figure 6. Power Supply Current vs. Temperature
APPLY ING T HIS AD7228A
UNIPOLAR OUT PUT OPE RAT ION
T his is the basic mode of operation for each channel of the
AD7228A, with the output voltage having the same positive po-
larity as V
REF
. Connections for unipolar output operation are
shown in Figure 7. T he AD7228A can be operated from single
or dual supplies as outlined earlier. T he voltage at the reference
input must never be negative with respect to GND. Failure to
observe this precaution may cause parasitic transistor action and
possible device destruction. T he code table for unipolar output
operation is shown in T able II.
Figure 7. Unipolar Output Circuit