AD9220ARSZ datasheet(19/32 Pages) AD

19 / 32 page

REV. E

AD9221/AD9223/AD9220

–19–

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

0.1 F

VINB

2 VREF

SHORT FOR 0V TO 2V

INPUT SPAN

SENSE

SHORT FOR 0V TO 5V

INPUT SPAN

REFCOM

AD9221/

AD9223/

AD9220

Figure 21. 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 22 shows the single-ended configuration that gives the best

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 REFCOM

pin with a 10

µF tantalum capacitor in parallel with a low induc-

tance 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 19. In this case, the common-mode voltage,

VCM, is set at midsupply by connecting the transformer’s center

tap to CML of the AD9221/AD9223/AD9220. 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 single-ended input

and thus becomes VCM – VREF/2 to VCM + VREF/2.

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

3.5V

1.5V

2.5V

AD9221/

AD9223/

AD9220

Figure 22. Internal Reference—2 V p-p Input Span,

VCM = 2.5 V

Resistor Programmable Reference

Figure 23 shows an example of how to generate a reference

voltage other than 1 V or 2.5 V with the addition of two external

resistors and a bypass capacitor. Use the equation,

VREF

=× +

()

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 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 VREF 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

µF tantalum capacitor in parallel with a low inductance

0.1

µF ceramic capacitor.

1.5V

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9220

2.5V

2.5k

0.1 F

Figure 23. 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 AD9221/AD9223/AD9220 by improving drift and accuracy.

Figures 24 through 26 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 internal 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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