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AD7682/AD7689

Rev. 0 | Page 19 of 28

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 30. Multiplexed Analog Input Configuraitons

The analog inputs can be configured as

•

Figure 30A, single ended referenced to system ground;

CFG[12:10] = 1112.

•

Figure 30B, 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.

•

Figure 30C, 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]. Example: for IN0 = IN1+ and IN1 = IN1−,

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

CFG[9:7] = 0012.

•

Figure 30D, inputs configured in any of the above

combinations (showing that the AD7682/AD7689 can be

configured dynamically).

Sequencer

The AD7682/AD7689 include a channel sequencer useful for

scanning channels in a IN0 to INx 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 INx 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 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 will

begin updating again.

While operating in a sequence, the CFG 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

Only the bits for input and sequencer are highlighted.

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

with temperature sensor.

CFG

INCC

INx

REF

SEQ

As a second example, scan three paired channels without

temperature sensor and referenced to VREF/2.

CFG

INCC

INx

REF

SEQ

Source Resistance

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

AD7682/AD7689 can be driven directly. Large source

impedances significantly affect the ac performance, especially

total harmonic distortion (THD). The dc performances are less

sensitive to the input impedance. The maximum source imped-

ance 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.

DRIVER AMPLIFIER CHOICE

Although the AD7682/AD7689 are easy to drive, the driver

amplifier must meet the following requirements:

•

The noise generated by the driver amplifier must be kept as

low as possible to preserve the SNR and transition noise

performance of the AD7682/AD7689. Note that the AD7682/

AD7689 have a noise much lower than most of the other

16-bit ADCs and, therefore, can be driven by a noisier

amplifier to meet a given system noise specification. The

noise from the amplifier is filtered by the AD7682/AD7689

analog input circuit low-pass filter made by RIN and CIN or

by an external filter, if one is used. Because the typical noise

of the AD7682/AD7689 is 35 μV rms (with VREF = 5 V), the

SNR degradation due to the amplifier is

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