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AD7923

Data Sheet

Rev. D | Page 16 of 24

TYPICAL CONNECTION DIAGRAM

Figure 19 shows a typical connection diagram for the AD7923.

In this setup the AGND pin is connected to the analog ground

plane of the system. In Figure 19, REFIN is connected to a

decoupled 2.5 V supply from a reference source, the AD780, to

provide an analog input range of 0 V to 2.5 V (if the range bit is

1) or 0 V to 5 V (if the range bit is 0). Although the AD7923 is

connected to AVDD of 5 V, the serial interface is connected to a

3 V microprocessor. The VDRIVE pin of the AD7923 is connected

to the same 3 V supply of the microprocessor to allow a 3 V

logic interface (see the Digital Inputs section). The conversion

result is output in a 16-bit word. This 16-bit data stream

consists of two leading 0s, two address bits indicating which

channel the conversion result corresponds to, followed by the

12 bits of conversion data. For applications where power

consumption is 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.

SERIAL

INTERFACE

AD780

2.5V

AD7923

0.1

µF

µC/µP

0.1

µF

SUPPLY

0.1

µF

AGND

AVDD

•

0V TO REFIN

SCLK

DOUT

DIN

VDRIVE

REFIN

NOTES

1. ALL UNUSED INPUT CHANNELS MUST BE CONNECTED TO AGND.

VIN0

VIN3

Figure 19. Typical Connection Diagram

Analog Input Selection

Any one of four analog input channels can be selected for

conversion by programming the multiplexer with Address Bits

ADD1 and ADD0 in the control register. The channel

configurations are shown in Table 7.

The AD7923 can also be configured to automatically cycle

through selected channels. The sequencer feature is accessed via

the SEQ1 and SEQ0 bits in the control register (see Table 9).

The AD7923 can be programmed to continuously convert on a

number of consecutive channels in ascending order from

Channel 0 to a selected final channel as determined by Channel

Address Bits ADD1 and ADD0. This is possible if the SEQ1 and

SEQ0 bits are set to 1, 1. The next serial transfer then acts on

the sequence programmed by executing a conversion on

Channel 0. The next serial transfer results in a conversion on

Channel 1, and so on, until the channel selected via Address

Bits ADD1 and ADD0 is reached. It is not necessary to write to

the control register again once a sequencer operation has been

initiated. The write bit must be set to 0 or the DIN line must be

set low to ensure that the control register is not accidentally

overwritten or the sequence operation is interrupted. If the

control register is written to at any time during the sequence,

the user must ensure that the SEQ1 and SEQ0 bits are set to 1, 0

to avoid interrupting the automatic conversion sequence. This

pattern continues until the AD7923 is written to and the SEQ1

and SEQ0 bits are configured with any bit combination except

1, 0, resulting in the termination of the sequence. If uninter-

rupted, however (write bit = 0, or write bit = 1 and SEQ1 and

SEQ0 bits are set to 1, 0), then upon completion of the

sequence, the AD7923 sequencer returns to Channel 0 and

commences the sequence again.

Regardless of which channel selection method is used, the

16-bit word output from the AD7923 during each conversion

always contains two leading 0s, and two channel address bits

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

conversion result. (see the Serial Interface section).

Digital Inputs

The digital inputs applied to the AD7923 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 that possible power supply

sequencing issues are avoided. If CS, DIN, or SCLK are applied

before AVDD, there is no risk of latchup as there would be on the

analog inputs if a signal greater than 0.3 V were applied prior to

AVDD.

VDRIVE

The AD7923 also has 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 AD7923 were operated with an AVDD of 5 V, the

VDRIVE pin could be powered from a 3 V supply. The AD7923

has a larger dynamic range with an AVDD of 5 V while still being

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

ensure that VDRIVE does not exceed AVDD by more than 0.3 V

(see the Absolute Maximum Ratings section).

Reference

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

reference to the AD7923. Errors in the reference source result in

gain errors in the AD7923 transfer function and 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 AD7923 include the AD780, REF 192, and the

AD1582.

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