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參數資料
| 型號： | AD9225-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Complete 12-Bit, 25 MSPS Monolithic A/D Converter |
| 中文描述： | 完整的12位，25 MSPS的單片機的A / D轉換器 |
| 文件頁數： | 16/24頁 |
| 文件大小： | 321K |
| 代理商： | AD9225-EB |
AD9225
–16–
REV. A
REFERENCE CONFIGURATIONS
The figures associated with this section on internal and external
reference operation do not show recommended matching series
resistors for VINA and VINB for the purpose of simplicity.
Please refer to section “Driving the Analog Inputs, Introduc-
tion” for a discussion of this topic. Also, the figures do not show
the decoupling network associated with the CAPT and CAPB
pins. Please refer to the section “Reference Operation” for a
discussion of the internal reference circuitry and the recom-
mended decoupling network shown in Figure 17.
USING THE INTERNAL REFERENCE
Single-Ended Input with 0 to 2
3
VREF Range
Figure 26 shows how to connect the AD9225 for a 0V to 2 V or
0 V to 4 V input range via pin strapping the SENSE pin. An
intermediate input range of 0 to 2
×
VREF can be established
using the resistor programmable configuration in Figure 28.
In either case, both the midscale voltage and input span are
directly dependent on the value of VREF. More specifically, the
midscale 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. When VINA is
≤
0 V, the digital output will be 000 Hex;
when VINA is
≥
2
×
VREF, the digital output will be FFF Hex.
Shorting the VREF pin directly to the SENSE pin places the
internal reference amplifier in unity-gain mode and the resultant
VREF output is 1 V. Therefore, the valid input range is 0 V to
2 V. However, shorting the SENSE pin directly to the REFCOM
pin configures the internal reference amplifier for a gain of 2.0
and the resultant VREF output is 2.0 V. Thus, the valid input
range becomes 0 V to 4 V. The VREF pin should be bypassed
to the REFCOM pin with a 10
μ
F tantalum capacitor in parallel
with a low-inductance 0.1
μ
F ceramic capacitor.
10
m
F
VINA
VREF
AD9225
0.1
m
F
VINB
2
3
VREF
0V
SHORT FOR 0V TO 2V
INPUT SPAN
SENSE
SHORT FOR 0V TO 4V
INPUT SPAN
REFCOM
Figure 26. Internal Reference—2 V p-p Input Span,
V
CM
= 1 V, or 4 V p-p Input Span
Figure 27 shows the single-ended configuration that gives good
dynamic performance (SINAD, SFDR). To optimize dynamic
specifications, center the common mode voltage of the analog
input at approximately by 2.5 V by connecting VINB to a low
impedance 2.5 V source. As described above, shorting the VREF
pin directly to the SENSE pin results in a 1 V reference voltage
and a 2 V p-p input span. The valid range for input signals is
1.5 V to 3.5 V. The VREF pin should be bypassed to the REF-
COM pin with a 10
μ
F tantalum capacitor in parallel with a
low-inductance 0.1
μ
F ceramic capacitor.
This reference configuration could also be used for a differential
input in which VINA and VINB are driven via a transformer as
shown in Figure 24. In this case, the common-mode voltage,
VCM , is set at midsupply by connecting the transformer’s
center tap to CML of the AD9225. VREF can be configured
for 1.0 V or 2.0 V by connecting SENSE to either VREF or
REFCOM respectively. Note that the valid input range for
each of the differential inputs is one half of the single-ended
input and thus becomes V
CM
– VREF/2 to V
CM
+ VREF/2.
1V
0.1
m
F
10
m
F
VINA
VINB
VREF
SENSE
REFCOM
AD9225
3.5V
1.5V
VCM
Figure 27. Internal Reference—2 V p-p Input Span,
V
CM
= 2.5 V
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,
VREF
= 1
V
×
(1 +
R
1/
R
2)
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
μ
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 2.5V source.
The VREF pin should be bypassed to the REFCOM pin with a
10
μ
F tantalum capacitor in parallel with a low inductance
0.1
μ
F ceramic capacitor.
1.5V
C1
0.1
m
F
10
m
F
VINA
VINB
VREF
SENSE
REFCOM
AD9225
4V
1V
2.5V
R1
2.5k
V
R2
5k
V
0.1
m
F
Figure 28. Resistor Programmable Reference +3 V p-p
Input Span, V
CM
= 2.5 V
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