AD9224 Analog Devices, AD9224 Datasheet - Page 17

• Manufacturer Part Number: AD9224
• Description: 12-Bit 40 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9224

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,2 V p-p,4 V p-p,Uni (Vref) x 2,Uni 2.0V,Uni 4.0V
• Adc Architecture: Pipelined
• Pkg Type: SOP

Resistor Programmable Reference
Figure 28 shows an example of how to generate a reference
voltage other than 1.0 V or 2.0 V with the addition of two exter-
nal resistors and a bypass capacitor. Use the equation,
to determine appropriate values for R1 and R2. These resistors
should be in the 2 k to 100 k range. For the example shown,
R1 equals 2.5 k and R2 equals 5 k . From the equation
above, the resultant reference voltage on the VREF pin is 1.5 V.
This sets the input span to be 3 V p-p. To assure stability, place
a 0.1 F ceramic capacitor in parallel with R1.
The midscale voltage can be set to VREF by connecting VINB
to VREF to provide an input span of 0 to 2 VREF. Alterna-
tively, the midscale voltage can be set to 2.5 V by connecting
VINB to a low impedance 2.5 V source. For the example shown,
the valid input single-ended 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 F tantalum capacitor in parallel with a low inductance
0.1 F ceramic capacitor.
USING AN EXTERNAL REFERENCE
Using an external reference may enhance the dc performance
of the AD9224 by improving drift and accuracy. Figures 29 and
30 show examples of how to use an external reference with the
A/D. Table III is a list of suitable voltage references from Ana-
log 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.
Internal
AD589
AD1580
REF191
Internal
REV. A
Figure 28. Resistor Programmable Reference—3 V p-p
Input Span, V
10 F
4V
1V
1.00
2.0
Output
Voltage
1.235
1.225
2.048
2.5V
Table III. Suitable Voltage References
CM
VREF = 1 V
0.1 F
= 2.5 V
26
26
Drift
(ppm/ C)
10–100
50–100
5–25
R1
2.5k
R2
5k
(1 + R1/R2),
C1
0.1 F
1.5V
Initial
Accuracy
% (max)
1.4
1.2–2.8
0.08–0.8
0.1–0.5
1.4
VINA
VINB
VREF
SENSE
REFCOM
AD9224
Operating
Current
1 mA
50 A
50 A
45 A
1 mA
–17–
The AD9224 contains an internal reference buffer, A2 (see
Figure 16), that simplifies the drive requirements of an external
reference. The external reference must be able to drive about
5 k ( 20%) load. Note that the bandwidth of the reference
buffer is deliberately left small to minimize the reference noise
contribution. As a result, it is not possible to change the refer-
ence voltage rapidly in this mode.
Variable Input Span with V
Figure 29 shows an example of the AD9224 configured for an
input span of 2
reference drives the VINB pin thus setting the common-mode
voltage at 2.5 V. The input span can be independently set by a
voltage divider consisting of R1 and R2 which generates the
VREF signal. A1 buffers this resistor network and drives
VREF. Choose this op amp based on accuracy requirements. It
is essential that a minimum of a 10 F capacitor in parallel with
a 0.1 F low inductance ceramic capacitor decouple the A1’s
output to ground.
Single-Ended Input with 0 to 2
Figure 30 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 REF191, a 2.048 V external reference was selected, the
valid input range extends from 0 to 4.096 V. In this case, 1 LSB
of the AD9224 corresponds to 1 mV. It is essential that a mini-
mum of a 10 F capacitor in parallel with a 0.1 F low inductance
ceramic capacitor decouple the reference output to ground.
+5V
2.5V+VREF
2.5V–VREF
0.1 F
+5V
2.5V
0.1 F
Figure 30. Input Range = 0 V to 2
2.5V
REF
2
Figure 29. External Reference
REF
VREF centered at 2.5 V. An external 2.5 V
VREF
0V
0.1 F
22 F
10 F
CM
R1
R2
= 2.5 V
0.1 F
0.1 F
+5V
A1
VREF Range
0.1 F
+5V
VINB
SENSE
VINA
VREF
AD9224
VINA
VINB
VREF
SENSE
AD9224
VREF
AD9224