AD7908 datasheet(16/24 Pages) AD

16 / 24 page

AD7908/AD7918/AD7928

REV. A

POWER-ON

DUMMY CONVERSION

DIN = ALL 1s

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A2–A0 FOR CONVERSION.

SEQ = 1 SHADOW = 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

A2–A0 IN THE CONTROL REGISTER

WRITE BIT = 0

CONTINUOUSLY CONVERTS ON THE SELECTED

SEQUENCE OF CHANNELS BUT WILL ALLOW

RANGE, CODING, AND SO ON, TO CHANGE IN THE

CONTROL REGISTER WITHOUT INTERRUPTING

THE SEQUENCE, PROVIDED SEQ = 1

SHADOW = 0

WRITE BIT = 1,

SEQ = 1,

SHADOW = 0

Figure 4. SEQ Bit = 1, SHADOW Bit = 1 Flowchart

CIRCUIT INFORMATION

The AD7908/AD7918/AD7928 are high speed, 8-channel, 8-bit,

10-bit, and 12-bit, single supply, A/D converters, respectively.

The parts 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 AD7908/AD7918/

AD7928 are capable of throughput rates of 1 MSPS when provided

with a 20 MHz clock.

The AD7908/AD7918/AD7928 provide the user with an on-chip

track-and-hold, A/D converter, and a serial interface housed in

a 20-lead TSSOP package. The AD7908/AD7918/AD7928 each

have eight 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 AD7908/AD7918/AD7928 is 0 V to REFIN or 0 V

to 2

REFIN, depending on the status of Bit 1 in the Control

REFIN range, the part must be oper-

ated from a 4.75 V to 5.25 V supply.

The AD7908/AD7918/AD7928 provide flexible power management

options to allow the user to achieve the best power performance

for a given throughput rate. These options are selected by pro-

gramming the power management bits, PM1 and PM0, in the

Control Register.

CONVERTER OPERATION

The AD7908/AD7918/AD7928 are 8-, 10-, and 12-bit succes-

sive approximation analog-to-digital converters based around a

capacitive DAC, respectively. The AD7908/AD7918/AD7928

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

to 2

REFIN. Figures 5 and 6 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 5 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

VIN7

AGND

SW1

SW2

COMPARATOR

CONTROL

LOGIC

CAPACITIVE

DAC

Figure 5. ADC Acquisition Phase

When the ADC starts a conversion (see Figure 6), SW2 will

open and SW1 will move to position B, causing the comparator

to become unbalanced. The Control Logic and the Capacitive

DAC are used to add and subtract fixed amounts of charge from

the sampling capacitor to bring the comparator back into a

balanced condition. When the comparator is rebalanced, the

conversion is complete. The Control Logic generates the ADC

output code. Figures 8 and 9 show the ADC transfer functions.

VIN0

VIN7

AGND

SW1

SW2

COMPARATOR

CONTROL

LOGIC

CAPACITIVE

DAC

Figure 6. ADC Conversion Phase

Analog Input

Figure 7 shows an equivalent circuit of the analog input structure

of the AD7908/AD7918/AD7928. The two diodes D1 and D2

provide ESD protection for the analog inputs. Care must be taken

to ensure that the analog input signal never exceeds the supply

rails by more than 300 mV. This will cause these diodes to

become forward biased and start conducting current into the

substrate. 10 mA is the maximum current these diodes can conduct

without causing irreversible damage to the part. The capacitor

C1 in Figure 7 is typically about 4 pF and can primarily be attrib-

uted to pin capacitance. The resistor R1 is a lumped component

made up of the on resistance of the track-and-hold switch and

also includes the on resistance of the input multiplexer. The

total resistance is typically about 400

. The capacitor C2 is the

ADC sampling capacitor and has a capacitance of 30 pF typically.

For ac applications, removing high frequency components from

the analog input signal is recommended by use of an RC low-

pass filter on the relevant analog input pin. In applications where

harmonic distortion and signal-to-noise ratio are critical, the analog

input should be driven from a low impedance source. Large

source impedances will significantly affect the ac performance of

the ADC. This may necessitate the use of an input buffer ampli-

fier. The choice of the op amp will be a function of the particular

application.

–16–

AD7908 Datasheet(PDF) 16 Page - Analog Devices

AD7908 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL