AD7682BCPZ datasheet(22/35 Pages) AD

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AD7682/AD7689

Data Sheet

Rev. H | Page 22 of 35

Input Configurations

Figure 32 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 shown in

•

Figure 32 (A), 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 32 (B), 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 this configuration, all inputs IN[7:0]

have a range of GND to VREF.

•

Figure 32 (C), 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 a sense

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, as

indicated by the channels with parentheses in Figure 32 (C).

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 32 (D), inputs configured in any of the preceding

combinations (showing that the AD7682/AD7689 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 32. Multiplexed Analog Input Configurations

Sequencer

The AD7682/AD7689 include a channel sequencer useful for

scanning channels in a repeated fashion. Refer to the Channel

Sequencer section for further details on the sequencer

operation.

Source Resistance

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

AD7682/AD7689 can be driven directly. Large source imped-

ances 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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