AD9243EB datasheet(17/25 Pages) AD

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AD9243

REV. A

–16–

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.

10 F

VINA

VREF

AD9243

0.1 F

VINB

2xVREF

SHORT FOR 0V TO 2V

INPUT SPAN

SENSE

SHORT FOR 0V TO 5V

INPUT SPAN

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 by 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 in which 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 AD9243. 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.

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9243

2.5V

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 exter-

nal 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 k

Ω to 100 kΩ range. For the example shown,

R1 equals 2.5 k

Ω 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 stabil-

ity, 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 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.

1.5V

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9243

2.5V

2.5k

0.1 F

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 AD9243 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

Initial

Operating

Output

Drift

Accuracy

Current

Voltage

(ppm/ C)

% (max)

( A)

Internal

1.00

1.4

N/A

AD589

1.235

10–100

1.2–2.8

AD1580

1.225

50–100

0.08–0.8

REF191

2.048

5–25

0.1–0.5

Internal

2.50

1.4

N/A

REF192

2.50

5–25

0.08–0.4

REF43

2.50

10–25

0.06–0.1

600

AD780

2.50

3–7

0.04–0.2

1000

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