Datasheet

MAX187/MAX189
+5V, Low-Power, 12-Bit Serial ADCs
8Maxim Integrated
Figure 3a. MAX187 Operational Diagram
Figure 4. Equivalent Input Circuit
Figure 3b. MAX189 Operational Diagram
Track/Hold
In track mode, the analog signal is acquired and stored
in the internal hold capacitor. In hold mode, the T/H
switch opens and maintains a constant input to the
ADC’s SAR section.
During acquisition, the analog input AIN charges capaci-
tor C
HOLD
. Bringing CS low ends the acquisition interval.
At this instant, the T/H switches the input side of C
HOLD
to GND. The retained charge on C
HOLD
represents a
sample of the input, unbalancing the node ZERO at the
comparator’s input.
In hold mode, the capacitive DAC adjusts during the
remainder of the conversion cycle to restore node ZERO
to 0V within the limits of a 12-bit resolution. This action
is equivalent to transferring a charge from C
HOLD
to the
binary-weighted capacitive DAC, which in turn forms a
digital representation of the analog input signal. At the
conversion’s end, the input side of C
HOLD
switches back
to AIN, and C
HOLD
charges to the input signal again.
The time required for the T/H to acquire an input sig-
nal is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
the acquisition time lengthens and more time must be
allowed between conversions. Acquisition time is calcu-
lated by:
t
ACQ
= 9 (R
S
+ R
IN
) 16pF
where R
IN
= 5kI, R
S
= the source impedance of the
input signal, and t
ACQ
is never less than 1.5Fs. Source
impedances below 5kI do not significantly affect the AC
performance of the ADC.
Input Bandwidth
The ADCs’ input tracking circuitry has a 4.5MHz small-
signal bandwidth, and an 8V/Fs slew rate. It is possible
to digitize high-speed transient events and measure
periodic signals with bandwidths exceeding the ADC’s
sampling rate by using undersampling techniques. To
avoid aliasing of unwanted high-frequency signals into
the frequency band of interest, an anti-alias filter is rec-
ommended. See the MAX274/MAX275 continuous-time
filters data sheet.
ON
OFF
SHUTDOWN
INPUT
ANALOG INPUT
0 TO +5V
+5V
1
2
3
4
V
DD
AIN
SHDN
REF
8
7
6
5
SCLK
DOUT
GND
SERIAL
INTERFACE
4.7µF
4.7µF
0.1µF
CS
MAX187
AIN
TRACK INPUT
HOLD
C
PACKAGE
GND
TRACK
HOLD
R
IN
5kI
C
HOLD
16pF
-+
C
SWITCH
COMPARATOR
ZERO
REF
12-BIT CAPACITIVE DAC
AT THE SAMPLING INSTANT,
THE INPUT SWITCHES FROM
AIN TO GND.
ON
OFF
SHUTDOWN
INPUT
ANALOG INPUT
0 TO +5V
REFERENCE
INPUT
+5V
1
2
3
4
V
DD
AIN
SHDN
REF
8
7
6
5
SCLK
DOUT
GND
SERIAL
INTERFACE
0.1µF
4.7µF
0.1µF
CS
MAX189