AD9246 datasheet(18/45 Pages) AD

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AD9246

Rev. A | Page 17 of 44

AD9246

0.1µF

2V p-p

VIN+

VIN–

CML

0.1µF

25Ω

Figure 40. Differential Double Balun Input Configuration

AD9246

AD8352

0Ω

0.1µF

VIN+

VIN–

CML

0.1µF

8, 13

VCC

200Ω

ANALOG INPUT

Figure 41. Differential Input Configuration Using the AD8352

Table 9. 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 43)

2 × VREF

Internal Fixed Reference

AGND to 0.2 V

1.0

2.0

VOLTAGE REFERENCE

A stable and accurate voltage reference is built into the AD9246.

The input range is adjustable by varying the reference voltage

applied to the AD9246, 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 following sections. The Reference

Decoupling section describes the best practices and require-

ments for PCB layout of the reference.

Internal Reference Connection

A comparator within the AD9246 detects the potential at the

SENSE pin and configures the reference into four possible

states, as summarized in Table 9. If SENSE is grounded, the

reference amplifier switch is connected to the internal resistor

divider (see Figure 42), setting VREF to 1 V.

Connecting the SENSE pin to VREF switches the reference

amplifier input 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 43, the switch sets to the SENSE pin. This puts the

reference amplifier in a noninverting mode with the VREF

output defined as

⎟

⎠

⎞

⎜

⎝

⎛ +

VREF

If the SENSE pin is connected to AVDD, the reference amplifier

is disabled, and an external reference voltage can be applied to

the VREF pin (see the External Reference Operation section).

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

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

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