AD9241 Analog Devices, AD9241 Datasheet - Page 16

• Manufacturer Part Number: AD9241
• Description: Complete 14-Bit, 1.25 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9241

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 1.25MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,2 V p-p,5V p-p,Uni (Vref) x 2,Uni 2.0V,Uni 5.0V
• Adc Architecture: Pipelined
• Pkg Type: QFP

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 F tantalum capacitor in parallel
with a low-inductance 0.1 F ceramic capacitor.
Single-Ended or Differential Input, 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 F tantalum capacitor in
parallel with a low inductance 0.1 F 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,
V
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 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-
CM
Figure 37. Internal Reference—5 V p-p Input Span,
V
CM
, is set at midsupply by connecting the transformers center
Figure 36. Internal Reference (2 V p-p Input Span,
V
2xVREF
CM
= 2.5 V
0V
= 1 V, or 5 V p-p Input Span, V
CM
5V
0V
– VREF/2 to V
10µF
SHORT FOR 0 TO 2V
SHORT FOR 0 TO 5V
10µF
0.1µF
INPUT SPAN
INPUT SPAN
CM
2.5V
+ VREF/2.
0.1µF
VINA
VINB
VREF
SENSE
REFCOM
CM
CM
= 2.5 V
AD9241
VINA
VINB
VREF
SENSE
REFCOM
= 2.5 V)
AD9241
–16–
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 common-mode voltage can be set to VREF by connecting
VINB to VREF to provide an input span of 0 to 2
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 F tantalum capacitor in parallel with a low induc-
tance 0.1 F ceramic capacitor.
ternal resistors and a bypass capacitor. Use the equation,
to determine appropriate values for R1 and R2. These resistors
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.
Internal
AD589
AD1580
REF191
Internal
REF192
REF43
AD780
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 k
( 20%) load. Note that the bandwidth of the reference buffer is
Figure 38. Resistor Programmable Reference (3 V p-p
Input Span, V
4V
1V
10µF
Output
Voltage
1.00
1.235
1.225
2.048
2.50
2.50
2.50
2.50
Table III. Suitable Voltage References
2.5V
CM
VREF = 1 V
0.1µF
= 2.5 V)
Drift
(ppm/ C)
26
10–100
50–100
5–25
26
5–25
10–25
3–7
R1
2.5k
R2
5k
(1 + R1/R2),
C1
0.1µF
1.5V
Initial
Accuracy
% (max)
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
VINA
VINB
VREF
SENSE
REFCOM
AD9241
Operating
Current
( A)
N/A
N/A
50
50
45
45
600
1000
VREF.
REV. 0