SP574B/674B/1674B/774B

12–Bit Sampling A/D Converters

© Copyright 2000 Sipex Corporation

7

2

28

STS

DGND

12/8

CS

A0

R/C

CE

THREE–STATE BUFFERS AND CONTROL

NIBBLE A

NIBBLE B

NIBBLE C

27

26

25

24

23

22

21 20

19 18

17 16

OUTPUT BITS

MSB

LSB

3

4

5

6

CONTROL

LOGIC

OSCILLATOR

12–BITS

12–BITS

SAMPLE/HOLD

CDAC

LSB

MSB

OFFSET/GAIN

TRIM NETWORK

REF

REF

AMP

1

15

VLOGIC

+5V

711

VCC

VEE

9

AGND

+15V

N.C.

8

10

VREF

OUT

VREF

IN

R2

100

Ω

R1

100K

Ω

100K

Ω

-15V

+15V

100

Ω

10V

IN

20V

IN

BIP

OFF

13

14

12

ANALOG

INPUTS

0TO10V

0TO20V

12–BIT SAR

COMP

STROBE

10µF

0.1µF

10µF

0.1µF

+

+

Figure 2. Unipolar Input Connections

signals between ground traces and cross digital

lines at right angles only.

Grounding Considerations

Any ground path from the analog and digital

ground should be as low resistance as possible to

accommodate the ground currents present with

this device.

The analog ground current is approximately

6mA DC while the digital ground is 3mA DC.

The analog and digital common pins should be

tied together as close to the package as possible

to guarantee best performance. The code–de-

pendent currents flow through the V

V

digital common pins.

Power Supplies

The supply voltages for the SPx74B must be kept

as quiet as possible from noise pickup and also

regulated from transients or drops. Because the

part has 12–bit accuracy, voltage spikes on the

supply lines can cause several LSB deviations on

the output. Switching power supply noise can be a

problem. Careful filtering and shielding should be

employed to prevent the noise from being picked

up by the converter.

Capacitor bypass pairs are needed from each sup-

ply pin to its respective ground to filter noise and

counter the problems caused by the variations in

supply current. A 10

µF tantalum and a 0.1µF

ceramic type in parallel between V

digital common (pin15), and V

analog common (pin 9) is sufficient. V

ated internally so pin 11 may be grounded or

connected to a negative supply if the SPx74B is

being used to upgrade an already existing design.

CALIBRATION AND CONNECTION

PROCEDURES

Unipolar

The calibration procedure consists of adjusting the

converter’s most negative output to its ideal value for

offsetadjustment,andthenadjustingthemostpositive

output to its ideal value for gain adjustment.

Starting with offset adjustment and referring to

Figure 2, the midpoint of the first LSB increment

should be positioned at the origin to get an output

+1.22mV for the 10V range and +2.44mV for the

20V range should be applied to the SPx74B.

Adjust the offset potentiometer R

tion flickers between 0000 0000 0000 and 0000

0000 0001.