+5V, Low-Power, 12-Bit Serial ADCs

_______________________________________________________________________________________

7

PIN

DIP

WIDE SO

NAME

FUNCTION

11

Supply voltage, +5V ±5%

2

3

AIN

36

SHDN

Three-level shutdown input. Pulling

SHDN low shuts the MAX187/MAX189

down to 10µA (max) supply current. Both MAX187 and MAX189 are fully opera-

tional with either

SHDN high or floating. For the MAX187, pulling SHDN high

enables the internal reference, and letting

SHDN float disables the internal

reference and allows for the use of an external reference.

4

8

REF

Reference voltage—sets analog voltage range and functions as a 4.096V output

for the MAX187 with enabled internal reference. REF also serves as a +2.5V to

and MAX189. Bypass with 4.7µF if internal reference is used, and with 0.1µF if an

external reference is applied.

5

—

GND

Analog and digital ground

—

10

AGND

Analog ground

—

11

DGND

Digital ground

6

12

DOUT

Serial data output. Data changes state at SCLK’s falling edge.

715

CS

Active-low chip select initiates conversions on the falling edge. When

CS is high,

DOUT is high impedance.

8

16

SCLK

Serial clock input. Clocks data out with rates up to 5MHz.

—

2,4,5,7,9,13,14

N.C.

Not internally connected. Connect to AGND for best noise performance.

_______________________________________________________________________Pin Description

_______________Detailed Description

Converter Operation

The MAX187/MAX189 use input track/hold (T/H) and

successive approximation register (SAR) circuitry to

convert an analog input signal to a digital 12-bit output.

No external hold capacitor is needed for the T/H.

Figures 3a and 3b show the MAX187/MAX189 in their

simplest configuration. The MAX187/MAX189 convert

T/H acquisition time. The MAX187’s internal reference

is trimmed to 4.096V, while the MAX189 requires an

external reference. Both devices accept external refer-

requires only three digital lines, SCLK,

CS, and DOUT,

and provides easy interface to microprocessors (

µPs).

Both converters have two modes: normal and shut-

down. Pulling

SHDN low shuts the device down and

reduces supply current to below 10µA, while pulling

SHDN high or leaving it floating puts the device into the

operational mode. A conversion is initiated by

CS

falling. The conversion result is available at DOUT in

unipolar serial format. A high bit, signaling the end of

conversion (EOC), followed by the data bits (MSB first),

make up the serial data stream.

The MAX187 operates in one of two states: (1) internal

reference and (2) external reference. Select internal

reference operation by forcing

SHDN high, and external

reference operation by floating

SHDN.

Analog Input

Figure 4 illustrates the sampling architecture of the

ADC’s analog comparator. The full-scale input voltage

depends on the voltage at REF.

REFERENCE

ZERO

FULL

SCALE

SCALE

Internal Reference

0V

+4.096V

(MAX187 only)

External Reference

0V

For specified accuracy, the external reference voltage