AD7923 datasheet(11/20 Pages) AD

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AD7923

–11–

REV. 0

Figure 2 reflects the traditional operation of a multichannel ADC,

where each serial transfer selects the next channel for conversion.

In this mode of operation the Sequencer function is not used.

Figure 3 shows how to program the AD7923 to continuously

convert on a sequence of consecutive channels from Channel 0

to a selected final channel. To exit this mode of operation and

revert back to the traditional mode of operation of a multichannel

ADC (as outlined in Figure 2), ensure that the WRITE bit = 1

and SEQ1 = SEQ0 = 0 on the next serial transfer.

POWER-ON

DUMMY CONVERSION

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A1, A0 FOR CONVERSION.

SEQ1 = 0, SEQ0 = x

DOUT: CONVERSION RESULT FROM PREVIOUSLY

SELECTED CHANNEL A1, A0

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT A1, A0 FOR CONVERSION.

SEQ1 = 0, SEQ0 = x

WRITE BIT = 1,

SEQ1 = 0,

SEQ0 = x

Figure 2. SEQ1 Bit = 0, SEQ0 Bit = x Flowchart

POWER-ON

DUMMY CONVERSION

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A1, A0 FOR CONVERSION.

SEQ1 = 1, SEQ0 = 1

DOUT: CONVERSION RESULT FROM CHANNEL 0

CONTINUOUSLY CONVERTS ON A CONSECUTIVE

SEQUENCE OF CHANNELS FROM CHANNEL 0 UP

TO AND INCLUDING THE PREVIOUSLY SELECTED

A1, A0 IN THE CONTROL REGISTER

WRITE BIT = 0

CONTINUOUSLY CONVERTS ON THE SELECTED

SEQUENCE OF CHANNELS BUT WILL ALLOW

RANGE, CODING, ETC., TO CHANGE IN THE CON-

TROL REGISTER WITHOUT INTERRUPTING THE

SEQUENCE, PROVIDED SEQ = 1, SEQ0 = 0

WRITE BIT = 1,

SEQ1 = 1,

SEQ0 = 0

Figure 3. SEQ1 Bit = 1, SEQ0 Bit = 1 Flowchart

CIRCUIT INFORMATION

The AD7923 is high speed, 4-channel, 12-bit, single-supply

A/D converter. The part can be operated from a 2.7 V to 5.25 V

supply. When operated from either a 5 V or 3 V supply, the

AD7923 is capable of throughput rates of 200 kSPS. The con-

version time may be as short as 800 ns when provided with a

20 MHz clock.

The AD7923 provides the user with an on-chip, track-and-hold

A/D converter, and with a serial interface housed in a 16-lead

TSSOP package. The AD7923 has four single-ended input

channels with a channel sequencer, allowing the user to select a

channel sequence through which the ADC can cycle with each

consecutive

CS falling edge. The serial clock input accesses data

from the part, controls the transfer of data written to the ADC,

and provides the clock source for the successive approximation

A/D converter. The analog input range for the AD7923 is 0 V

to REFIN or 0 V to 2

¥ REFIN, depending on the status of Bit 1

in the Control Register. For the 0 to 2

¥ REFIN range, the part

must be operated from a 4.75 V to 5.25 V supply.

The AD7923 provides flexible power management options to

allow the user to achieve the best power performance for a given

throughput rate. These options are selected by programming the

Power Management bits, PM1 and PM0, in the Control Register.

CONVERTER OPERATION

The AD7923 is a 12-bit successive approximation analog-to-

digital converter based around a capacitive DAC. The AD7923

can convert analog input signals in the range 0 V to REFIN or 0 V

to 2

¥ REFIN. Figures 4 and 5 show simplified schematics of the

ADC. The ADC is comprised of Control Logic, SAR, and a

capacitive DAC, which are used to add and subtract fixed amounts

of charge from the sampling capacitor to bring the comparator

back into a balanced condition. Figure 4 shows the ADC during

its acquisition phase. SW2 is closed and SW1 is in position A. The

comparator is held in a balanced condition and the sampling

capacitor acquires the signal on the selected VIN channel.

VIN0

VIN3

AGND

SW1

SW2

COMPARATOR

CONTROL

LOGIC

CAPACITIVE

DAC

Figure 4. ADC Acquisition Phase

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