AD7908 datasheet(18/24 Pages) AD

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AD7908/AD7918/AD7928

REV. A

concern, the power-down modes should be used between

conversions or bursts of several conversions to improve power

performance. (See the Modes of Operation section.)

SUPPLY

0.1 F

10 F

AVDD

SERIAL

INTERFACE

AD7908/

AD7918/

AD7928

C/ P

0.1 F

10 F

SUPPLY

AGND

VIN0

•

VIN7

0V TO REFIN

SCLK

DOUT

DIN

VDRIVE

NOTE: ALL UNUSED INPUT CHANNELS SHOULD BE CONNECTED TO AGND

AD780

2.5V

0.1 F

REFIN

Figure 11. Typical Connection Diagram

Analog Input Selection

Any one of eight analog input channels may be selected for

conversion by programming the multiplexer with the address bits

ADD2–ADD0 in the Control Register. The channel configurations

are shown in Table II. The AD7908/AD7918/AD7928 may also be

configured to automatically cycle through a number of channels

as selected. The sequencer feature is accessed via the SEQ and

SHADOW bits in the Control Register. (See Table IV.)

The AD7908/AD7918/AD7928 can be programmed to continu-

ously convert on a selection of channels in ascending order. The

analog input channels to be converted on are selected through

programming the relevant bits in the SHADOW Register (see

Table V). The next serial transfer will then act on the sequence

programmed by executing a conversion on the lowest channel in

the selection. The next serial transfer will result in a conversion

on the next highest channel in the sequence, and so on.

It is not necessary to write to the Control Register once a

sequencer operation has been initiated. The WRITE bit must be

set to zero or the DIN line tied low to ensure the Control Register

is not accidently overwritten, or the sequence operation inter-

rupted. If the Control Register is written to at any time during

the sequence, then it must be ensured that the SEQ and SHADOW

bits are set to 1,0 to avoid interrupting the automatic conversion

sequence. This pattern will continue until such time as the

AD7908/AD7918/AD7928 is written to and the SEQ and

SHADOW bits are configured with any bit combination except

1,0. On completion of the sequence, the AD7908/AD7918/

AD7928 sequencer will return to the first selected channel in

the SHADOW Register and commence the sequence again.

Rather than selecting a particular sequence of channels, a num-

ber of consecutive channels beginning with Channel 0 may also

be programmed via the Control Register alone, without needing

to write to the SHADOW Register. This is possible if the SEQ

and SHADOW bits are set to 1,1. The channel address bits

ADD2 through ADD0 will then determine the final channel in

the consecutive sequence. The next conversion will be on Chan-

nel 0, then Channel 1, and so on until the channel selected via

the address bits ADD2 through ADD0 is reached. The cycle

will begin again on the next serial transfer, provided the WRITE

bit is set to low, or if high, that the SEQ and SHADOW bits are

set to 1,0; then the ADC will continue its preprogrammed auto-

matic sequence uninterrupted.

Regardless of which channel selection method is used, the 16-bit

word output from the AD7928 during each conversion will

always contain a leading zero, three channel address bits that

the conversion result corresponds to, followed by the 12-bit

conversion result; the AD7918 will output a leading zero, three

channel address bits that the conversion result corresponds to,

followed by the 10-bit conversion result and two trailing zeros; the

AD7908 will output a leading zero, three channel address bits that

the conversion result corresponds to, followed by the 8-bit conver-

sion result and four trailing zeros. (See the Serial Interface section.)

Digital Inputs

The digital inputs applied to the AD7908/AD7918/AD7928 are

not limited by the maximum ratings that limit the analog inputs.

Instead, the digital inputs applied can go to 7 V and are not

restricted by the AVDD + 0.3 V limit as on the analog inputs.

Another advantage of SCLK, DIN, and

CS not being restricted

by the AVDD + 0.3 V limit is the fact that power supply sequenc-

ing issues are avoided. If

CS, DIN, or SCLK are applied before

AVDD, there is no risk of latch-up as there would be on the analog

inputs if a signal greater than 0.3 V was applied prior to AVDD.

VDRIVE

The AD7908/AD7918/AD7928 also have the VDRIVE feature.

VDRIVE controls the voltage at which the serial interface operates.

VDRIVE allows the ADC to easily interface to both 3 V and 5 V

processors. For example, if the AD7908/AD7918/AD7928 were

operated with an AVDD of 5 V, the VDRIVE pin could be powered

from a 3 V supply. The AD7908/AD7918/AD7928 have better

dynamic performance with an AVDD of 5 V while still being able

to interface to 3 V processors. Care should be taken to ensure

VDRIVE does not exceed AVDD by more than 0.3 V. (See the

Absolute Maximum Ratings.)

Reference

An external reference source should be used to supply the 2.5 V

reference to the AD7908/AD7918/AD7928. Errors in the refer-

ence source will result in gain errors in the AD7908/AD7918/

AD7928 transfer function and will add to the specified full-scale

errors of the part. A capacitor of at least 0.1 µF should be placed

on the REFIN pin. Suitable reference sources for the AD7908/

AD7918/AD7928 include the AD780, REF193, AD1582,

ADR03, ADR381, ADR391, and ADR421.

If 2.5 V is applied to the REFIN pin, the analog input range can

either be 0 V to 2.5 V or 0 V to 5 V, depending on the setting of

the RANGE bit in the Control Register.

MODES OF OPERATION

The AD7908/AD7918/AD7928 have a number of different

modes of operation. These modes are designed to provide flex-

ible power management options. These options can be chosen

to optimize the power dissipation/throughput rate ratio for dif-

fering application requirements. The mode of operation of the

AD7908/AD7918/AD7928 is controlled by the power manage-

ment bits, PM1 and PM0, in the Control Register, as detailed in

Table III. When power supplies are first applied to the AD7908/

AD7918/AD7928, care should be taken to ensure that the part is

placed in the required mode of operation. (See the Powering Up

the AD7908/AD7918/AD7928 section.)

–18–

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