AD9689 datasheet(31/134 Pages) AD

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Data Sheet

AD9689

Rev. A | Page 31 of 134

Dither

The AD9689 has internal on-chip dither circuitry that improves

the ADC linearity and SFDR, particularly at smaller signal levels.

A known but random amount of white noise is injected into the

input of the AD9689. This dither improves the small signal

linearity within the ADC transfer function and is precisely

subtracted out digitally. The dither is turned on by default and

does not reduce the ADC input dynamic range. The data sheet

specifications and limits are obtained with the dither turned on.

The dither is on by default. It is not recommended to turn it off.

Absolute Maximum Input Swing

The absolute maximum input swing allowed at the inputs of the

AD9689 is 5.8 V p-p differential. Signals operating near or at

this level can cause permanent damage to the ADC. See Table 6

for more information.

VOLTAGE REFERENCE

A stable and accurate 0.5 V voltage reference is built into the

AD9689. This internal 0.5 V reference sets the full-scale input

range of the ADC. The full-scale input range can be adjusted via

the ADC input full-scale control register (Register 0x1910). For

more information on adjusting the input swing, see Table 46 and

Table 53. Figure 85 shows the block diagram of the internal 0.5 V

reference controls.

The SPI Register 0x18A6 enables the user to either use this

internal 0.5 V reference, or to provide an external 0.5 V

reference. When using an external voltage reference, provide a

0.5 V reference. The full-scale adjustment is made using the SPI,

irrespective of the reference voltage. For more information on

adjusting the full-scale level of the AD9689, refer to the Memory

Map section.

The SPI writes required to use the external voltage reference, in

order, are as follows:

1. Set Register 0x18E3 to 0x00 to turn off the VCM export.

2. Set Register 0x18E6 to 0x00 to turn off the temperature

diode export.

3. Set Register 0x18A6 to 0x01 to turn on the external voltage

reference.

The use of an external reference may be necessary, in some

applications, to enhance the gain accuracy of the ADC or to

improve thermal drift characteristics. Figure 84 shows the

typical drift characteristics of the internal 0.5 V reference.

0.5060

0.5055

0.5050

0.5045

0.5040

0.5035

0.5030

–10

110

130

JUNCTION TEMPERATURE (°C)

Figure 84. Typical Reference Voltage (VREF) Drift

The external reference must be a stable 0.5 V reference. The

ADR130 is a sufficient option for providing the 0.5 V reference.

Figure 86 shows how the ADR130 can be used to provide the

external 0.5 V reference to the AD9689. The dashed lines show

unused blocks within the AD9689 while using the ADR130 to

provide the external reference.

ADC

CORE

INPUT FULL-SCALE

RANGE ADJUST

SPI REGISTER

(0x1910)

VREF PIN

CONTROL SPI

(0x18A6)

VIN+A/VIN+B

VIN–A/VIN–B

VREF

INTERNAL

0.5V

REFERENCE

GENERATOR

INPUT

FULL-SCALE

ADJUST

Figure 85. Internal Reference Configuration and Controls

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