Multirange, +5V, 8-Channel,

Serial 12-Bit ADCs

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17

Transfer Function

Output data coding for the MAX1270/MAX1271 is bina-

ry in unipolar mode with 1 LSB = (FS / 4096) and two’s

complement binary in bipolar mode with 1 LSB = [(2 x

| FS | ) / 4096]. Code transitions occur halfway between

successive-integer LSB values. Figures 14a and 14b

show the input/output (I/O) transfer functions for unipo-

lar and bipolar operations, respectively. For full-scale

values, refer to Table 3.

Layout, Grounding, and Bypassing

Careful PC board layout is essential for best system

performance. Use a ground plane for best perfor-

mance. To reduce crosstalk and noise injection, keep

analog and digital signals separate. Connect analog

grounds and DGND in a star configuration to AGND.

For noise-free operation, ensure the ground return from

AGND to the supply ground is low impedance and as

short as possible. Connect the logic grounds directly to

capacitors to AGND to minimize highand low-frequency

fluctuations. If the supply is excessively noisy, connect

Figure 15.

GND

DGND

DGND

AGND

+5V

+5V

SUPPLY

R* = 5

Ω

DIGITAL

CIRCUITRY

4.7

µF

0.1

µF

MAX1270

MAX1271

**

*OPTIONAL

**CONNECT AGND AND DGND WITH A GROUND PLANE OR A SHORT TRACE.

Figure 15. Power-Supply Grounding Connections

OUTPUT CODE

INPUT VOLTAGE (LSB)

0

FS

1 LSB =

FULL-SCALE

TRANSITION

12

3

11... 111

11... 110

11... 101

00... 011

00... 010

00... 001

00... 000

FS

4096

Figure 14a. Unipolar Transfer Function

OUTPUT CODE

INPUT VOLTAGE (LSB)

0

+FS - 1 LSB

1 LSB =

-FS

011... 111

011... 110

000... 001

000... 000

111... 111

100... 010

100... 001

100... 000

2|FS|

4096

Figure 14b. Bipolar Transfer Function