AD9233 datasheet(18/45 Pages) AD

18 / 45 page

AD9233

Rev. A | Page 17 of 44

Single-Ended Input Configuration

Although not recommended, it is possible to operate the

AD9233 in a single-ended input configuration, as long as the

input voltage swing is within the AVDD supply. Single-ended

operation can provide adequate performance in cost-sensitive

applications. In this configuration, SFDR and distortion

performance degrade due to the large input common-mode

swing. If the source impedances on each input are matched,

there should be little effect on SNR performance. Figure 41

details a typical single-ended input configuration.

1V p-p

49.9Ω

0.1µF

10µF

0.1µF

AVDD

1kΩ

ADC

AD9233

AVDD

VIN+

VIN–

Figure 41. Single-Ended Input Configuration

VOLTAGE REFERENCE

A stable and accurate voltage reference is built into the AD9233.

The input range is adjustable by varying the reference voltage

applied to the AD9233, 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 requirements

for PCB layout of the reference.

Internal Reference Connection

A comparator within the AD9233 detects the potential at the

SENSE pin and configures the reference into four possible

states, which are 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 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 43, the

switch again 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 the AVDD pin, 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.

VREF

SENSE

0.5V

AD9233

REFT

REFB

SELECT

LOGIC

0.1µF

VIN–

VIN+

ADC

CORE

Figure 42. Internal Reference Configuration

VREF

SENSE

0.5V

AD9233

VIN–

VIN+

REFT

REFB

SELECT

LOGIC

0.1µF

ADC

CORE

Figure 43. Programmable Reference Configuration

If the internal reference of the AD9233 is used to drive multiple

converters to improve gain matching, the loading of the

reference by the other converters must be considered. Figure 44

depicts how the internal reference voltage is affected by loading.

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