ADS7812

9

SBAS042A

www.ti.com

BINARY TWO’S COMPLEMENT

DIGITAL OUTPUT

TABLE III. Ideal Input Voltage and Corresponding Digital Output for Two Common Input Ranges.

DESCRIPTION

ANALOG INPUT

Full-Scale Range

±10V

0.5V to 4.5V

Least Significant Bit (LSB)

4.88mV

0.98mV

BINARY CODE

HEX CODE

+Full Scale –1LSB

9.99512V

4.49902V

0111 1111 1111

7FF

Midscale

0V

2.5V

0000 0000 0000

000

Midscale –1LSB

–4.88mV

2.49902 V

1111 1111 1111

FFF

–Full Scale

–10V

0.5V

1000 0000 0000

800

FIGURE 3. Block Diagram of the ADS7812’s Digital Inputs and Outputs.

READING DATA

The ADS7812’s digital output is in Binary Two’s Comple-

ment (BTC) format. Table III shows the relationship be-

tween the digital output word and the analog input voltage

under ideal conditions.

Figure 3 shows the relationship between the various digital

inputs, digital outputs, and internal logic of the ADS7812.

Figure 4 shows when the internal shift register of the

ADS7812 is updated and how this relates to a single conver-

sion cycle. Together, these two figures point out a very

important aspect of the ADS7812: the conversion result is

not available until after the conversion is complete. The

implications of this are discussed in the following sections.

FIGURE 4. Timing of the Shift Register Update.

Q

D

S0

Q

Update of the shift

register occurs just prior

D

S1

Q

D

S2

Q

D

S10

Q

D

S11

Q

Shift Register

Working Register

Converter Core

D

SOUT

Q

D

W0

Q

Each flip-flop in the

working register is

latched as the

conversion proceeds

D

W1

Q

D

W2

Q

D

W10

Q

D

W11

•••

Delay

DATA

BUSY

DATACLK

CDAC

Control Logic

Clock

REF

EXT/INT

CONV

CS

NOTE: (1) If EXT/INT is HIGH (external clock), DATACLK is HIGH, and CS is LOW

during this time, the shift register will not be updated and the conversion result will be lost.

CONV

t

t

25

BUSY

NOTE: Update of the internal shift register occurs in the

shaded region. If EXT/INT is HIGH, then DATACLK

must be LOW or CS must be HIGH during this time.