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AD9224

–17–

REV. A

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 + 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 stability, place

a 0.1

µF ceramic capacitor in parallel with R1.

1.5V

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9224

2.5V

2.5k

0.1 F

Figure 28. Resistor Programmable Reference—3 V p-p

Input Span, VCM = 2.5 V

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 2.5 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.

USING AN EXTERNAL REFERENCE

Using an external reference may enhance the dc performance

of the AD9224 by improving drift and accuracy. Figures 29 and

30 show examples of how to use an external reference with the

A/D. Table III is a list of suitable voltage references from Ana-

log 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

Output

Drift

Accuracy

Operating

Voltage

(ppm/ C)

% (max)

Current

Internal

1.00

1.4

1 mA

AD589

1.235

10–100

1.2–2.8

µA

AD1580

1.225

50–100

0.08–0.8

µA

REF191

2.048

5–25

0.1–0.5

µA

Internal

2.0

1.4

1 mA

The AD9224 contains an internal reference buffer, A2 (see

Figure 16), that simplifies the drive requirements of an external

reference. The external reference must be able to drive about

5 k

Ω (±20%) load. Note that the bandwidth of the reference

buffer is deliberately left small to minimize the reference noise

contribution. As a result, it is not possible to change the refer-

ence voltage rapidly in this mode.

2.5V+VREF

2.5V–VREF

2.5V

+5V

0.1 F

22 F

VINA

VINB

VREF

SENSE

AD9224

+5V

0.1 F

2.5V

REF

Figure 29. External Reference

Variable Input Span with VCM = 2.5 V

Figure 29 shows an example of the AD9224 configured for an

input span of 2

× VREF centered at 2.5 V. An external 2.5 V

reference drives the VINB pin thus setting the common-mode

voltage at 2.5 V. The input span can be independently set by a

voltage divider consisting of R1 and R2 which generates the

VREF signal. A1 buffers this resistor network and drives

VREF. Choose this op amp based on accuracy requirements. It

is essential that a minimum of a 10

µF capacitor in parallel with

a 0.1

µF low inductance ceramic capacitor decouple the A1’s

output to ground.

Single-Ended Input with 0 to 2

VREF Range

Figure 30 shows an example of an external reference driving

both VINB and VREF. In this case, both the common-mode

voltage and input span are directly dependent on the value of

VREF. More specifically, the common-mode 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. For example, if

the REF191, a 2.048 V external reference was selected, the

valid input range extends from 0 to 4.096 V. In this case, 1 LSB

of the AD9224 corresponds to 1 mV. It is essential that a mini-

mum of a 10

µF capacitor in parallel with a 0.1 µF low inductance

ceramic capacitor decouple the reference output to ground.

REF

+5V

10 F

VINA

VINB

VREF

SENSE

AD9224

+5V

0.1 F

VREF

0.1 F

Figure 30. Input Range = 0 V to 2

× VREF

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