M
+5V, Low-Power, 12-Bit S erial ADCs
_______________________________________________________________________________________
7
PIN
DIP
WIDE SO
NAME
FUNCTION
1
2
1
3
V
DD
AIN
Supply voltage, +5V ±5%
Sampling analog input, 0V to V
REF
range
3
6
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
V
input for a precision reference for both MAX187 (disabled internal reference)
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.
7
15
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 Desc ription
_______________Detailed Desc ription
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
input signals in the 0V to V
range in 10μs, including
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-
ence voltages from +2.5V to V
DD
. The serial interface
requires only three digital lines, SCLK,
CS,
and DOUT,
and provides easy interface to microprocessors (
μ
Ps).
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
SCALE
FULL
SCALE
Internal Reference
(MAX187 only)
External Reference
0V
+4.096V
0V
V
REF
For specified accuracy, the external reference voltage
range spans from +2.5V to V
DD
.