AD6684 datasheet(25/107 Pages) AD

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Data Sheet

AD6684

Rev. 0 | Page 25 of 107

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

100

150

200

250

300

350

400

450

500

550

600

BUFFER CURRENT SETTING (µA)

Figure 59. AVDD3 Power vs. Buffer Current Setting

In certain high frequency applications, the SFDR can be

improved by reducing the full-scale setting.

Table 9 shows the recommended buffer current settings for the

different analog input frequency ranges.

Table 9. SFDR Optimization for Input Frequencies

Nyquist Zone

Input Buffer Current Control

Setting, Register 0x1A4C and

First, Second, and Third

Nyquist

240 (Register 0x1A4C, Bits[5:0] =

Fourth Nyquist

400 (Register 0x1A4C, Bits[5:0] =

Absolute Maximum Input Swing

The absolute maximum input swing allowed at the inputs of the

AD6684 is 4.3 V p-p differential. Signals operating near or at

this level can cause permanent damage to the ADC.

VOLTAGE REFERENCE

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

AD6684. This internal 0.5 V reference is used to set the full-

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

adjusted via the ADC function register (Register 0x1910). For

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

Figure 60 shows the block diagram of the internal 0.5 V

reference controls.

ADC

CORE

FULL-SCALE

VOLTAGE

ADJUST

VREF PIN

CONTROL SPI

(0x18A6)

VREF

VIN–A/

VIN–B

VIN+A/

VIN+B

INTERNAL

VREF

GENERATOR

INPUT FULL-SCALE

RANGE ADJUST

SPI REGISTER

(0x1910)

Figure 60. Internal Reference Configuration and 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 AD6684, refer to the

Memory Map section.

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

order, are as follows:

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

Set Register 0x18E6 to 0x00 to turn off temperature diode

export.

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.

The external reference has to be a stable 0.5 V reference. The

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

Figure 61 shows how the ADR130 can be used to provide the

external 0.5 V reference to the AD6684. The grayed out areas

show unused blocks within the AD6684 while using the

ADR130 to provide the external reference.

FULL-SCALE

VOLTAGE

ADJUST

VREF

0.1µF

VOUT 4

SET 5

NC 6

VIN

GND

ADR130

0.1µF

INPUT

VREF PIN AND

FULL-SCALE

VOLTAGE

CONTROL

INTERNAL

VREF

GENERATOR

Figure 61. External Reference Using ADR130

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