MAX1236_09 Datasheet, PDF(18/22 Page) Maxim Integrated Products – 2.7V to 3.6V and 4.5V to 5.5V, Low-Power, 4-/12-Channel, 2-Wire Serial, 12-Bit ADCs

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MAX1236_09 Datasheet, PDF (18/22 Pages) Maxim Integrated Products – 2.7V to 3.6V and 4.5V to 5.5V, Low-Power, 4-/12-Channel, 2-Wire Serial, 12-Bit ADCs

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

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

Table 6. Reference Voltage and AIN_/REF Format

SEL2

SEL1

SEL0

REFERENCE VOLTAGE

VDD

External Reference

Internal Reference

AIN_/REF

Analog Input

Reference Input

Analog Input

Reference Output

INTERNAL REFERENCE

STATE

Always Off

Always On

Always Off

Always On

Wake-up is invisible when using an external reference

or VDD as the 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 60ÂµA (typ) and

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

ply current is just 1ÂµA, or a minuscule 3ÂµW 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 0.1ÂµF capacitor

and a 2kÎ© 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

The external reference can range from 1V to VDD. For

maximum conversion accuracy, the reference must be

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

ance of 500kÎ© 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 0.1ÂµF capacitor.

Transfer Functions

Output data coding for the MAX1236âMAX1239 is bina-

ry in unipolar mode and twoâs complement in bipolar

mode with 1 LSB = (VREF / 2N) where âNâ is the number

of bits (12). Code transitions occur halfway between

successive-integer LSB values. Figures 12 and 13

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

lar 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-

OUTPUT CODE

11 . . . 111

11 . . . 110

11 . . . 101

MAX1236â

FULL-SCALE MAX1239

TRANSITION

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

012 3

INPUT VOLTAGE (LSB)

Figure 12. Unipolar Transfer Function

FS = VREF

ZS = GND

1 LSB = VREF

4096

FS - 3/2 LSB

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