Wake-up is invisible when using an external reference

Automatic shutdown results in dramatic power savings,

particularly at slow conversion rates and with internal

clock. For example, at a conversion rate of 10ksps, the

average supply current for the MAX1237 is 60µA (typ) and

drops to 6µA (typ) at 1ksps. At 0.1ksps the average sup-

ply current is just 1µA, or a minuscule 3µW of power con-

sumption, see Average Supply Current vs. Conversion

Rate in the

Typical Operating Characteristics section.

Reference Voltage

SEL[2:0] of the setup byte (Table 1) control the reference

and the AIN_/REF configuration (Table 6). When

AIN_/REF is configured to be a reference input or refer-

ence output (SEL1 = 1), differential conversions on

AIN_/REF appear as if AIN_/REF is connected to GND

(see Note 2 and Table 4). Single-ended conversion in

scan mode AIN_/REF is ignored by the internal limiter,

which sets the highest available channel at AIN2 or

AIN10.

Internal Reference

The internal reference is 4.096V for the MAX1236/

MAX1238 and 2.048V for the MAX1237/MAX1239. SEL1 of

the setup byte controls whether AIN_/REF is used for an

analog input or a reference (Table 6). When AIN_/REF is

configured to be an internal reference output (SEL[2:1] =

11), decouple AIN_/REF to GND with a 0.1µF capacitor

and a 2kΩ series resistor (see the Typical Operating

Circuit ). Once powered up, the reference always remains

on until reconfigured. The internal reference requires 10ms

to wake up and is accessed using SEL0 (Table 6). When

in shutdown, the internal reference output is in a high-

impedance state. The reference should not be used to

supply current for external circuitry. The internal reference

does not require an external bypass capacitor and works

best when not connected to the pin (SEL1 = 0).

External Reference

maximum conversion accuracy, the reference must be

able to deliver up to 40µA and have an output imped-

ance of 500Ω or less. If the reference has a higher out-

put impedance or is noisy, bypass it to GND as close to

AIN_/REF as possible with a 0.1µF capacitor.

Transfer Functions

Output data coding for the MAX1236–MAX1239 is bina-

ry in unipolar mode and two’s complement in bipolar

bits (12). Code transitions occur halfway between suc-

cessive-integer LSB values. Figures 12 and 13 show

the input/output (I/O) transfer functions for unipolar and

bipolar operations, respectively.

Layout, Grounding, and Bypassing

Only use PC boards. Wire-wrap configurations are not

recommended since the layout should ensure proper

separation of analog and digital traces. Do not run ana-

log and digital lines parallel to each other, and do not

layout digital signal paths underneath the ADC pack-

2.7V to 3.6V and 4.5V to 5.5V, Low-Power,

4-/12-Channel, 2-Wire Serial, 12-Bit ADCs

18

______________________________________________________________________________________

SEL2

SEL1

SEL0

REFERENCE VOLTAGE

AIN_/REF

INTERNAL REFERENCE

STATE

00

X

Analog Input

Always Off

0

1

X

External Reference

Reference Input

Always Off

1

0

0

Internal Reference

Analog Input

Always Off

1

0

1

Internal Reference

Analog Input

Always On

1

1

0

Internal Reference

Reference Output

Always Off

1

1

1

Internal Reference

Reference Output

Always On

Table 6. Reference Voltage and AIN_/REF Format

OUTPUT CODE

FULL-SCALE

TRANSITION

11 . . . 111

11 . . . 110

11 . . . 101

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

12

3

0

FS

FS - 3/2 LSB

ZS = GND

INPUT VOLTAGE (LSB)

MAX1236–

MAX1239

1 LSB =

4096

Figure 12. Unipolar Transfer Function