MAX1136 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 10-Bit ADCs

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MAX1136 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 10-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 10-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

10ms prior to conversion or powered continuously.

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 MAX1137 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).

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 on AIN_/REF will be ignored by internal lim-

iter, which sets the highest available channel at AIN2 or

AIN10.

Internal Reference

The internal reference is 4.096V for the MAX1136/

MAX1138 and 2.048V for the MAX1137/MAX1139. 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.

Once powered up, the reference always remains on until

reconfigured. The reference should not be used to supply

current for external circuitry.

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 500â¦ 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 MAX1136âMAX1139 is bina-

ry in unipolar mode and twoâs complement in bipolar

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

bits (10). Code transitions occur halfway between suc-

cessive-integer LSB values. Figure 12 and Figure 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-

age. Use separate analog and digital PC board ground

sections with only one star point (Figure 14) connecting

the two ground systems (analog and digital). For lowest

noise operation, ensure the ground return to the star

OUTPUT CODE

111...111

111...110

FS = REF + GND

ZS = GND

FULL-SCALE

TRANSITION

100...010

100...001

100...000

011...111

011...110

011...101

1LSB = VREF

1024

000...001

000...000

(GND)

512

INPUT VOLTAGE (LSB)

Figure 12. Unipolar Transfer Function

LSB

18 ______________________________________________________________________________________

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