–13–

REV. D

AD7701

AGND

AD7701

10k

Ω

10k

Ω

0.1

µF

0.1

µF

DGND

0.1

µF

REF

10V

± 1V

AD707

0.1

µF

Figure 17. Single Supply Operation

SLEEP MODE

The low power standby mode is initiated by taking the SLEEP

input low, which shuts down all analog and digital circuits and

reduces power consumption to 10

µW. The calibration coeffi-

cients are still retained in memory, but as the converter has been

quiescent, it is necessary to wait for the filter settling time

(507,904 cycles) before accessing the output data.

DIGITAL INTERFACE

The AD7701’s serial communications port allows easy inter-

facing to industry-standard microprocessors. Three different

modes of operations are available, optimized for different types

of interface.

SYNCHRONOUS SELF-CLOCKING MODE (SSC)

The SSC mode (MODE pin high) allows easy interfacing to

serial-parallel conversion circuits in systems with parallel data

communication. This mode allows interfacing to 74XX299

Universal Shift registers without any additional decoding. The

SSC mode can also be used with microprocessors such as the

68HC11 and 68HC05, which allow an external device to clock

their serial port.

Figure 18 shows the timing diagram for the SSC mode. Data is

clocked out by an internally generated serial clock. The AD7701

divides each sampling interval into sixteen distinct periods.

Eight periods of 64 clock pulses are for analog settling and eight

periods of 64 clock pulses are for digital computation. The

status of CS is polled at the beginning of each digital computation

period. If it is low at any of these times then SCLK will become

active and the data word currently in the output register will be

transmitted, MSB first. After the LSB has been transmitted

DRDY

goes high and SDATA goes three-state. If CS, having

been brought low, is taken high again at any time during data

transmission, SDATA and SCLK will go three-state after the

current bit finishes. If CS is subsequently brought low,

transmission will resume with the next bit during the sub-

sequent digital computation period. If transmission has not been

initiated and completed by the time the next data word is

available, DRDY will go high for four clock cycles then low

again as the new word is loaded into the output register.

A more detailed diagram of the data transmission in the SSC

mode is shown in Figure 19. Data bits change on the falling

edge of SCLK and are valid on the rising edge of SCLK.

ANALOG SETTLING

DIGITAL COMPUTATION

SCLK (O)

SDATA (O)

HI-Z

HI-Z

HI-Z

HI-Z

MSB

LSB

DRDY (O)

DIGITAL COMPUTATION

CS POLLED

CS (I)

INTERNAL

STATUS

72 CLKIN CYCLES

64 CLKIN CYCLES

64 CLKIN CYCLES

1024 CLKIN CYCLES

Figure 18. Timing Diagram for SSC Data Transmission Mode