AD9244-40PCB datasheet(21/36 Pages) AD

21 / 36 page

AD9244

Rev. C | Page 21 of 36

Using an External Reference

To use an external reference, the internal reference must be dis-

abled by connecting the SENSE pin to AVDD. The AD9244

contains an internal reference buffer, A2 (see Figure 48), that

simplifies the drive requirements of an external reference. The

external reference must be able to drive a 5 kΩ (±20%) load.

The bandwidth of the reference is deliberately left small to

minimize the reference noise contribution. As a result, it is not

possible to drive VREF externally with high frequencies.

Figure 51 shows an example of an external reference driving

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

voltage and input span are directly dependent on the value of

VREF. Both the input span and the center of the input span are

equal to the external VREF. Thus, the valid input range extends

from (VREF + VREF/2) to (VREF − VREF/2). For example, if

the Precision Reference Part REF191, a 2.048 V external refer-

ence, is used, the input span is 2.048 V. In this case, 1 LSB of the

AD9244 corresponds to 0.125 mV.

It is essential that a minimum of a 10 μF capacitor, in parallel

with a 0.1 μF low inductance ceramic capacitor, decouple the

reference output to AGND.

AVDD

REFT

REFB

VREF

SENSE

AD9244

VIN+

VIN–

20pF

0.1

μF

0.1

μF

0.1

μF

0.1

μF

0.1

μF

VREF + VREF/2

VREF – VREF/2

VREF

Figure 51. Using an External Reference

Digital Outputs

Table 10 details the relationship among the ADC input, OTR,

and digital output format.

Data Format Select (DFS)

The AD9244 can be programmed for straight binary or twos

complement data on the digital outputs. Connect the DFS pin to

AGND for straight binary and to AVDD for twos complement.

Digital Output Driver Considerations

The AD9244 output drivers can be configured to interface with

5 V or 3.3 V logic families by setting DRVDD to 5 V or 3.3 V,

respectively. The output drivers are sized to provide sufficient

output current to drive a wide variety of logic families.

However, large drive currents tend to cause glitches on the

supplies and can affect converter performance. Applications

requiring the ADC to drive large capacitive loads or large

fanouts can require external buffers or latches.

DIGITAL INPUTS AND OUTPUTS

Table 10. Output Data Format

Input (V)

Condition (V)

Binary Output Mode

Twos Complement Mode

OTR

VIN+ – VIN−

< –VREF/2 − 0.5 LSB

00 0000 0000 0000

10 0000 0000 0000

VIN+ – VIN−

= −VREF/2

00 0000 0000 0000

10 0000 0000 0000

VIN+ – VIN−

= 0

10 0000 0000 0000

00 0000 0000 0000

VIN+ – VIN−

= +VREF/2 − 1.0 LSB

11 1111 1111 1111

01 1111 1111 1111

VIN+ – VIN−

> +VREF/2 − 0.5 LSB

11 1111 1111 1111

01 1111 1111 1111

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