AD9221/AD9223/AD9220
Variable Input Span with VCM = 2.5 V
1.5 V. This sets the input span to be 3 V p-p. To assure stabil-
ity, place a 0.1 µF ceramic capacitor in parallel with R1.
Figure 50 shows an example of the AD9221/AD9223/AD9220
configured for an input span of 2 × VREF centered at 2.5 V. An
external 2.5 V reference drives the VINB pin thus setting the
common-mode voltage at 2.5 V. The input span can be inde-
pendently set by a voltage divider consisting of R1 and R2
which generates the VREF signal. A1 buffers this resistor net-
work and drives VREF. Choose this op amp based on accuracy
requirements. It is essential that a minimum of a 10 µF capaci-
tor in parallel with a 0.1 µF low inductance ceramic capacitor
decouple the reference output to ground.
The common-mode voltage can be set to VREF by connecting
VINB to VREF to provide an input span of 0 to 2 × VREF.
Alternatively, the common-mode voltage can be set to VREF
by connecting VINB to a low impedance 2.5 V source. For
the example shown, the valid input single range for VINA is 1 V
to 4 V since VINB is set to an external, low impedance 2.5 V
source. The VREF pin should be bypassed to the REFCOM pin
with a 10 µF tantalum capacitor in parallel with a low induc-
tance 0.1 µF ceramic capacitor.
2.5V+VREF
2.5V
2.5V–VREF
4V
AD9221/
AD9223/
AD9220
VINA
VINB
VINA
1V
2.5V
REF
2.5V
VINB
+5V
0.1F
AD9220
1.5V
C1
22F
0.1F
R1
R2
VREF
R1
2.5k⍀
0.1F
10F
0.1F
A1
VREF
SENSE
R2
5k⍀
0.1F
SENSE
+5V
REFCOM
Figure 50. External Reference—VCM = 2.5 V (2.5 V on
VINB, Resistor Divider to Make VREF)
Figure 49. Resistor Programmable Reference—3 V p-p
Input Span, VCM = 2.5 V
Single-Ended Input with 0 to 2 ؋
VREF Range
Figure 51 shows an example of an external reference driving
both VINB and VREF. In this case, both the common mode
voltage and input span are directly dependent on the value of
VREF. More specifically, the common mode voltage is equal to
VREF while the input span is equal to 2 × VREF. Thus, the
valid input range extends from 0 to 2 × VREF. For example, if
the REF-191, a 2.048 external reference was selected, the valid
input range extends from 0 to 4.096 V. In this case, 1 LSB of
the AD9221/AD9223/AD9220 corresponds to 1 mV. It is es-
sential that a minimum of a 10 µF capacitor in parallel with a
0.1 µF low inductance ceramic capacitor decouple the reference
output to ground.
USING AN EXTERNAL REFERENCE
Using an external reference may enhance the dc performance of
the AD9221/AD9223/AD9220 by improving drift and accuracy.
Figures 50 through 52 show examples of how to use an external
reference with the A/D. Table III is a list of suitable voltage
references from Analog Devices. To use an external reference,
the user must disable the internal reference amplifier and drive
the VREF pin. Connecting the SENSE pin to AVDD disables
the internal reference amplifier.
Table III. Suitable Voltage References
Initial
Accuracy
% (max)
Operating
Current
(A)
VINA
VINB
AD9221/
AD9223/
AD9220
Output
Voltage
Drift
(ppm/؇C)
0V
+5V
VREF
Internal
AD589
1.00
1.235
AD1580 1.225
REF191 2.048
2.50
REF192 2.50
26
1.4
N/A
50
50
45
N/A
45
0.1F
10F
0.1F
10–100
50–100
5–25
26
5–25
10–25
3–7
1.2–2.8
0.08–0.8
0.1–0.5
1.4
0.08–0.4
0.06–0.1
0.04–0.2
VREF
0.1F
Internal
+5V
SENSE
REF43
AD780
2.50
2.50
600
1000
Figure 51. Input Range = 0 V to 2 × VREF
Low Cost/Power Reference
The AD9221/AD9223/AD9220 contains an internal reference
buffer, A2 (see Figure 35), that simplifies the drive requirements
of an external reference. The external reference must be able to
drive a ≈5 kΩ (±20%) load. Note that the bandwidth of the ref-
erence buffer is deliberately left small to minimize the reference
noise contribution. As a result, it is not possible to change the
reference voltage rapidly in this mode without the removal of
the CAPT/CAPB Decoupling Network.
The external reference circuit shown in Figure 52 uses a low
cost 1.225 V external reference (e.g., AD580 or AD1580) along
with an op amp and transistor. The 2N2222 transistor acts in
conjunction with 1/2 of an OP282 to provide a very low imped-
ance drive for VINB. The selected op amp need not be a high
speed op amp and may be selected based on cost, power and
accuracy.
–19–
REV. D
ADI [ ADI ]
