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AD7949

Rev. B | Page 17 of 32

Input Configurations

Figure 29 shows the different methods for configuring the analog

inputs with the configuration register, CFG[12:10]. Refer to the

Configuration Register, CFG, section for more details.

The analog inputs can be configured as



Figure 29A, single-ended referenced to system ground;

CFG[12:10] = 1112.

In this configuration, all inputs (IN[7:0]) have a range of

GND to VREF.



Figure 29B, bipolar differential with a common reference

point; COM = VREF/2; CFG[12:10] = 0102.

Unipolar differential with COM connected to a ground

sense; CFG[12:10] = 1102.

In these configurations, all inputs IN[7:0] have a range of

GND to VREF.



Figure 29C, bipolar differential pairs with the negative

input channel referenced to VREF/2; CFG[12:10] = 00X2.

Unipolar differential pairs with the negative input channel

referenced to a ground sense; CFG[12:10] = 10X2.

In these configurations, the positive input channels have

the range of GND to VREF. The negative input channels are

senses referred to VREF/2 for bipolar pairs, or GND for

unipolar pairs. The positive channel is configured with

CFG[9:7]. If CFG[9:7] is even, then IN0, IN2, IN4, and IN6

are used. If CFG[9:7] is odd, then IN1, IN3, IN5, and IN7

are used (channels with parentheses). For example, for

IN0/IN1 pairs with the positive channel on IN0, CFG[9:7]

= 0002. For IN4/IN5 pairs with the positive channel on

IN5, CFG[9:7] = 1012.

Note that for the sequencer, detailed in the Channel

Sequencer section, the positive channels are always IN0,

IN2, IN4, and IN6.



Figure 29D, inputs configured in any of the preceding

combinations (showing that the AD7949 can be configured

dynamically).

GND

COM

CH0+

CH3+

CH1+

CH2+

CH4+

CH5+

CH6+

CH7+

CH0+

CH3+

CH1+

CH2+

CH4+

CH5+

CH6+

CH7+

COM–

GND

COM

IN1

IN0

IN2

IN3

IN4

IN5

IN6

IN7

IN1

IN0

IN2

IN3

IN4

IN5

IN6

IN7

IN1

IN0

IN2

IN3

IN4

IN5

IN6

IN7

IN1

IN0

IN2

IN3

IN4

IN5

IN6

IN7

A—8 CHANNELS,

SINGLE ENDED

B—8 CHANNELS,

COMMON REFERNCE

GND

COM

CH0+ (–)

CH1+ (–)

CH2+ (–)

CH3+ (–)

CH0– (+)

CH1– (+)

CH0+ (–)

CH1+ (–)

CH0– (+)

CH1– (+)

CH2– (+)

CH3– (+)

C—4 CHANNELS,

DIFFERENTIAL

GND

COM

CH2+

CH3+

CH4+

CH5+

D—COMBINATION

COM–

Figure 29. Multiplexed Analog Input Configurations

Sequencer

The AD7949 includes a channel sequencer useful for scanning

channels in a repeated fashion. Refer to the Channel Sequencer

section for further details of the sequencer operation.

Source Resistance

When the source impedance of the driving circuit is low, the

AD7949 can be driven directly. Large source impedances

significantly affect the ac performance, especially THD. The dc

performances are less sensitive to the input impedance. The

maximum source impedance depends on the amount of THD

that can be tolerated. The THD degrades as a function of the

source impedance and the maximum input frequency.

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