AD7699 datasheet(19/32 Pages) AD

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Data Sheet

AD7699

Rev. F | Page 19 of 32

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 REFERENCE

GND

COM

CH0+ (–)

CH1+ (–)

CH2+ (–)

CH3+ (–)

CH0– (+)

CH1– (+)

CH0+ (–)

CH1+ (–)

CH0– (+)

CH1– (+)

CH2– (+)

CH3– (+)

C—4 CHANNELS,

DIFFERENTIAL (GND SENSE)

GND

COM

CH2+

CH3+

CH4+

CH5+

D—COMBINATION

COM–

Figure 33. Multiplexed Analog Input Configurations

The AD7699 analog inputs can be configured in either a

unipolar single-ended or pseudo differential mode, which

means that the positive input pin of the AD7699 can accept

signal between 0 V and VREF, and its negative input (or COM)

pin must be always referenced to either a ground or a fixed dc

voltage, VREF/2, as follows:

•

Configuration A in Figure 33: single-ended referenced to

system ground; CFG[12:10] = 1112.

•

Configuration B in Figure 33: 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.

•

Configuration C in Figure 33: bipolar differential pairs

with INx− referenced to VREF/2; CFG[12:10] = 00X2.

Unipolar differential pairs with INx− referenced to a

ground sense; CFG[12:10] = 10X2. In this configuration,

the INx+ is identified by the channel in CFG[9:7]. For

example, for IN0 = IN1+ and IN1 = IN1−, CFG[9:7] =

0002; for IN1 = IN1+ and IN0 = IN1−, CFG[9:7] = 0012.

•

Configuration D in Figure 33: inputs configured in any of

the above combinations (showing that the AD7699 can be

configured dynamically).

Sequencer

The AD7699 includes a channel sequencer useful for scanning

channels in a IN0 to IN[7:0] fashion. Channels are scanned as

singles or pairs, with or without the temperature sensor, after

the last channel is sequenced.

The sequencer starts with IN0 and finishes with IN[7:0] set in

CFG[9:7]. For paired channels, the channels are paired

depending on the last channel set in CFG[9:7]. Note that the

channel pairs are always paired as IN (even) = INx+ and IN

(odd) = INx− regardless of CFG[7].

To enable the sequencer, CFG[2:1] are written to for initializing

the sequencer. After CFG[13:0] are updated, DIN must be held

low while reading data out (at least for Bit 13), or the CFG register

begins updating again.

While operating in a sequence, the CFG register can be changed

by writing 012 to CFG[2:1]. However, if changing CFG11 (paired

or single channel) or CFG[9:7] (last channel in sequence), the

sequence reinitializes and converts IN0 (or IN1) after CFG is

updated.

Examples

Bit[13], Bits[6:3], and Bit 0 are configured for the input and

sequencer.

As a first example, scan all IN[7:0] referenced to COM = GND

with the temperature sensor.

CFG

INCC

INx

REF

SEQ

As a second example, scan three paired channels without the

temperature sensor and referenced to VREF/2.

CFG

INCC

INx

REF

SEQ

X means don’t care.

Source Resistance

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

AD7699 can be driven directly. Large source impedances signifi-

cantly affect the ac performance, especially total harmonic

distortion (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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