ADS800

13

SBAS035B

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set halfway between the two references. This feature can be

used to adjust the gain error, improve gain drift, or to change

the full-scale input range of the ADS800. Changing the full-

scale range to a lower value has the benefit of easing the

swing requirements of external input drive amplifiers. The

external references can vary as long as the value of the

external top reference (REFTEXT) is less than or equal to

+3.4V, the value of the external bottom reference (REFBEXT)

is greater than or equal to +1.1V, and the difference between

the external references are greater than or equal to 1.5V.

For the differential configuration, the full-scale input

range will be set to the external reference values that are

selected. For the single-ended mode, the input range is

2 • (REFTEXT – REFBEXT), with the common-mode being

centered at (REFTEXT + REFBEXT)/2. Refer to the typical

characteristics for “Expected Performance vs Full-Scale In-

put Range”.

The circuit in Figure 10 works completely on a single +5V

supply. As a reference element, it uses the micro-power

reference REF1004-2.5, which is set to a quiescent current

of 0.1mA. Amplifier A2 is configured as a follower to buffer the

+1.25V generated from the resistor divider. To provide the

necessary current drive, a pull-down resistor, RP, is added.

Amplifier A1 is configured as an adjustable gain stage, with

a range of approximately 1 to 1.32. The pull-up resistor

again relieves the op amp from providing the full current

drive. The value of the pull-up/down resistors is not critical

and can be varied to optimize power consumption. The

need for pull-up, pull-down resistors depends only on the

drive capability of the selected drive amplifiers and thus can

be omitted.

FIGURE 9. ADS800 Interface Schematic with AC-Coupling and External Buffers.

GND

LSB

MSB

GND

11

12

13

14

15

16

17

18

9

8

7

6

5

4

3

2

1

19

+V

S

CLK

+V

S

OE

MSBI

+V

S

REFB

CM

REFT

+V

S

GND

IN

IN

GND

15

16

17

18

19

20

21

22

23

24

25

26

27

28

14

13

12

11

10

9

8

7

6

5

4

3

2

1

ADS800

0.1

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

R

1

50

Ω

R

2

50

Ω

Ext

Clk

AC Input

Signal

11

12

13

14

15

16

17

18

9

8

7

6

5

4

3

2

Dir

G+

1

19

Dir

G+

–541

–541

Mini-Circuits

T T1-6-KK81

or equivalent

22pF

22pF

(1)

NOTE: (1) All capacitors should be located as close to the pins as the manufacturing

process will allow. Ceramic X7R surface-mount capacitors or equivalent are recommended.

+5V