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AD7949

Rev. A | Page 18 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 29. Multiplexed Analog Input Configuraitons

The analog inputs can be configured as

•

Figure 29A, single ended referenced to system ground;

CFG[12:10] = 1112.

•

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.

•

Figure 29C, 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 29D, inputs configured in any of the above combin-

ations (showing that the AD7949 can be configured

dynamically).

Sequencer

The AD7949 includes 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] is 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

AD7949 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 impedance depends on

the amount of THD that can be tolerated.

DRIVER AMPLIFIER CHOICE

Although the AD7949 is 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 AD7949. Note that the AD7949 has a

noise much lower than most of the other 14-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 AD7949 analog input circuit

low-pass filter made by RIN and CIN or by an external filter,

if one is used.

•

For ac applications, the driver should have a THD

performance commensurate with the AD7949. Figure 17

shows THD vs. frequency for the AD7949.

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