REV. B

–8–

AD7722

PIN FUNCTION DESCRIPTIONS

Mnemonic

Pin No.

Description

14

Clock Logic Power Supply Voltage for the Analog Modulator, 5 V

± 5%.

AGND1

10

Clock Logic Ground Reference for the Analog Modulator.

20, 23

Analog Power Supply Voltage, 5 V

± 5%.

AGND

9, 13, 15, 19, Ground Reference for Analog Circuitry.

21, 25, 26

39

Digital Power Supply Voltage, 5 V

± 5%.

DGND

6, 28

Ground Reference for Digital Circuitry.

REF1

22

Reference Input/Output. REF1 connects through 3 k

Ω to the output of the internal 2.5 V reference and

to the input of a buffer amplifier that drives the

Σ-∆modulator. This pin can also be overdriven with an

external reference 2.5 V.

REF2

24

Reference Input/Output. REF2 connects to the output of an internal buffer amplifier used to drive the

Σ-∆ modulator. When REF2 is used as an input, REF1 must be connected to AGND.

18

Positive Terminal of the Differential Analog Input.

UNI

7Analog Input Range Select Input.

UNI selects the analog input range for either bipolar or unipolar

operation. A logic low input selects unipolar operation. A logic high input selects bipolar operation.

CLKIN

11

Clock Input. Master clock signal for the device. The CLKIN pin interfaces the AD7722 internal

oscillator circuit to an external crystal or an external clock. A parallel resonant, fundamental-frequency,

microprocessor-grade crystal and a 1 M

Ω resistor should be connected between the CLKIN and

XTAL pins with two capacitors connected from each pin to ground. Alternatively, the CLKIN pin

can be driven with an external CMOS compatible clock. The AD7722 is specified with a clock input

frequency of 12.5 MHz.

XTAL

12

Oscillator Output. The XTAL pin connects the internal oscillator output to an external crystal.

If an external clock is used, XTAL should be left unconnected.

P/

S

8

Parallel/Serial Interface Select Input. A logic high configures the output data interface for parallel mode

operation. The serial mode operation is selected with the P/

S set to a logic low.

CAL

27

Calibration Logic Input. A logic high input for a duration of one CLKIN cycle initiates a

calibration sequence for the device gain and offset error.

RESET

17

Reset Logic Input. RESET is used to clear the offset and gain calibration registers. RESET is an

asynchronous input. RESET allows the user to set the AD7722 to an uncalibrated state if the

device had been previously calibrated. A rising edge also resets the AD7722

Σ-∆ modulator by

shorting the integrator capacitors in the modulator. In addition, RESET functions identically to

the SYNC pin described below. When operating with more than one AD7722, a RESET/SYNC

should be issued following power up to ensure the devices are synchronized. Ensure that the

supplies are settled before applying the RESET/SYNC pulse.

CS

29

Chip select is a level sensitive logic input.

CS enables the output data register for parallel mode read

operation. The

CS logic level is sensed on the rising edge of CLKIN. The output data bus is enabled

when the rising edge of CLKIN senses a logic low level on

CS if RD is also low. When CS is sensed

high, the output data bits DB15–DB0 will be high impedance. In serial mode, tie

CS to a logic low.

SYNC

30

Synchronization Logic Input. SYNC is an asynchronous input. When using more than one

AD7722 operated from a common master clock, SYNC allows each ADC’s

Σ-∆ modulator to

simultaneously sample its analog input and update its output data register. A rising edge resets

the AD7722 digital filter sequencer counter to zero. After a SYNC, conversion data is not valid until

after the digital filter settles (see Figure 7). DVAL goes low in the serial mode. When the rising

edge of CLKIN senses a logic low on SYNC (or RESET), the reset state is released; in parallel

mode,

DRDY goes high. After the reset state is released, DVAL returns high after 8192 CLKIN

cycles (128

× 64/f

of the digital filter (64 CLKIN periods), when valid data is ready to be read from the output data

register. When operating with more than one AD7722, a RESET/SYNC should be issued follow-

ing power up to ensure the devices are synchronized. Ensure that the supplies are settled before

applying the RESET/SYNC pulse.