AD9241AS View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'AD9241AS' PDF : 25 Pages View PDF
AD9241
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 2.5
and the resultant VREF output is 2.5 V. Thus, the valid input
range becomes 0 V to 5 V. The VREF pin should be bypassed
to the REFCOM pin with a 10 µF tantalum capacitor in parallel
with a low-inductance 0.1 µF ceramic capacitor.
2xVREF
0V
10µF
0.1µF
SHORT FOR 0 TO 2V
INPUT SPAN
SHORT FOR 0 TO 5V
INPUT SPAN
VINA
VINB
VREF
AD9241
SENSE
REFCOM
Figure 36. Internal Reference (2 V p-p Input Span,
VCM = 1 V, or 5 V p-p Input Span, VCM = 2.5 V)
Single-Ended or Differential Input, VCM = 2.5 V
Figure 37 shows the single-ended configuration that gives the
best SINAD performance. To optimize dynamic specifications,
center the common-mode voltage of the analog input at
approximately 2.5 V by connecting VINB to VREF, a low-
impedance 2.5 V source. As described above, shorting the
SENSE pin directly to the REFCOM pin results in a 2.5 V
reference voltage and a 5 V p-p input span. The valid range
for input signals is 0 V to 5 V. The VREF pin should be by-
passed to the REFCOM pin with a 10 µF tantalum capacitor in
parallel with a low inductance 0.1 µF ceramic capacitor.
This reference configuration could also be used for a differential
input wherein VINA and VINB are driven via a transformer as
shown in Figure 29. In this case, the common-mode voltage,
VCM, is set at midsupply by connecting the transformers center
tap to CML of the AD9241. VREF can be configured for 1 V or
2.5 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 VCM – VREF/2 to VCM + VREF/2.
5V
0V
10µF
2.5V
0.1µF
VINA
VINB
AD9241
VREF
SENSE
REFCOM
Figure 37. Internal Reference—5 V p-p Input Span,
VCM = 2.5 V
Resistor Programmable Reference
Figure 38 shows an example of how to generate a reference
voltage other than 1 V or 2.5 V with the addition of two ex-
ternal resistors and a bypass capacitor. Use the equation,
VREF = 1 V × (1 + R1/R2),
to determine appropriate values for R1 and R2. These resistors
should be in the 2 kΩ to 100 kΩ range. For the example shown,
R1 equals 2.5 kΩ and R2 equals 5 kΩ. 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 µF ceramic capacitor in parallel with R1.
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 2.5 V
by connecting VINB to a low impedance 2.5 V source. For
the example shown, the valid input signal 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.
4V
1V
2.5V
10µF
0.1µF
R1
2.5kΩ
R2
5kΩ
VINA
1.5V
C1
0.1µF
VINB
VREF
AD9241
SENSE
REFCOM
Figure 38. Resistor Programmable Reference (3 V p-p
Input Span, VCM = 2.5 V)
USING AN EXTERNAL REFERENCE
Using an external reference may enhance the dc performance
of the AD9241 by improving drift and accuracy. Figures 39
through 41 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 inter-
nal reference amplifier.
Table III. Suitable Voltage References
Output
Voltage
Drift
(ppm/؇C)
Initial
Accuracy
% (max)
Operating
Current
(A)
Internal
AD589
AD1580
REF191
Internal
REF192
REF43
AD780
1.00
1.235
1.225
2.048
2.50
2.50
2.50
2.50
26
10–100
50–100
5–25
26
5–25
10–25
3–7
1.4
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
N/A
50
50
45
N/A
45
600
1000
The AD9241 contains an internal reference buffer, A2 (see
Figure 26), 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 reference buffer is
–16–
REV. 0
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