ADS7813

9

SBAS043C

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)

305

µV61µV

BINARY CODE

HEX CODE

+Full Scale –1LSB

9.999695V

4.499939V

0111 1111 1111 1111

7FFF

Midscale

0V

2.5V

0000 0000 0000 0000

0000

Midscale –1LSB

–305

µV

2.499939

µV

1111 1111 1111 1111

FFFF

–Full Scale

–10V

0.5V

10000 0000 0000 0000

8000

Q

D

S0

Q

Update of the shift

register occurs just prior

D

S1

Q

D

S2

Q

D

S14

Q

D

S15

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

W14

Q

D

W15

•••

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.

FIGURE 3. Block Diagram of the ADS7813’s Digital Inputs and Outputs.

READING DATA

The ADS7813 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 ADS7813.

Figure 4 shows when the internal shift register of the

ADS7813 is updated and how this relates to a single conver-

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

important aspect of the ADS7813: 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.

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.