AD9640 datasheet(29/54 Pages) AD

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AD9640

Rev. B | Page 27 of 52

VOLTAGE REFERENCE

A stable and accurate voltage reference is built into the AD9640.

The input range can be adjusted by varying the reference voltage

applied to the AD9640, using either the internal reference or an

externally applied reference voltage. The input span of the ADC

tracks reference voltage changes linearly. The various reference

modes are summarized in the next few sections. The Reference

Decoupling section describes the best practices PCB layout of

the reference.

Internal Reference Connection

A comparator within the AD9640 detects the potential at the

SENSE pin and configures the reference into four possible

modes, which are summarized in Table 14. If SENSE is grounded,

the reference amplifier switch is connected to the internal

resistor divider (see Figure 51), setting VREF to 1 V. Connecting

the SENSE pin to VREF switches the reference amplifier output

to the SENSE pin, completing the loop and providing a 0.5 V

reference output. If a resistor divider is connected external to

the chip, as shown in Figure 52, the switch again sets to the

SENSE pin. This puts the reference amplifier in a noninverting

mode with the VREF output defined as

⎟

⎠

⎞

⎜

⎝

⎛ +

VREF

The input range of the ADC always equals twice the voltage at

the reference pin for either an internal or an external reference.

VREF

SENSE

0.5V

AD9640

SELECT

LOGIC

0.1µF

1.0µF

VIN–A/VIN–B

VIN+A/VIN+B

ADC

CORE

Figure 51. Internal Reference Configuration

0.5V

AD9640

SELECT

LOGIC

VIN–A/VIN–B

VIN+A/VIN+B

ADC

CORE

VREF

SENSE

0.1µF

1.0µF

Figure 52. Programmable Reference Configuration

If the internal reference of the AD9640 is used to drive multiple

converters to improve gain matching, the loading of the reference

by the other converters must be considered. Figure 53 shows

how the internal reference voltage is affected by loading.

–1.25

LOAD CURRENT (mA)

–0.25

–0.50

–0.75

–1.00

0.5

1.0

1.5

VREF = 0.5V

VREF = 1V

Figure 53. VREF Accuracy vs. Load

External Reference Operation

The use of an external reference may be necessary to enhance

the gain accuracy of the ADC or improve thermal drift charac-

teristics. Figure 54 shows the typical drift characteristics of the

internal reference in 1 V mode.

Table 14. Reference Configuration Summary

Selected Mode

SENSE Voltage

Resulting VREF (V)

Resulting Differential Span (V p-p)

External Reference

AVDD

N/A

2 × External Reference

Internal Fixed Reference

VREF

0.5

1.0

Programmable Reference

0.2 V to VREF

⎟

⎠

⎞

⎜

⎝

⎛ +

(see Figure 52)

2 × VREF

Internal Fixed Reference

AGND to 0.2 V

1.0

2.0

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