MAX153
1Msps, µP-Compatible,
8-Bit ADC with 1µA Power-Down
The internal resistances from V
and V
may be
Input Current
REF+
REF-
asꢀlowꢀasꢀ1kΩ.ꢀSinceꢀcurrentꢀisꢀstillꢀdrawnꢀbyꢀtheꢀreferenceꢀ
inputs during power-down, reference supply current can
be reduced during shutdown by using the circuit shown
inꢀFigureꢀ7d.ꢀAꢀlogic-levelꢀn-channelꢀMOSFET,ꢀconnectedꢀ
Figureꢀ 8ꢀ showsꢀ theꢀ equivalentꢀ circuitꢀ ofꢀ theꢀ converterꢀ
input. When the conversion starts and WR is low, V is
IN
connectedꢀtoꢀ16ꢀ0.6pFꢀcapacitors.ꢀDuringꢀthisꢀacquisitionꢀ
phase, the input capacitors charge to the input voltage
through the resistance of the internal analog switches
(aboutꢀ 2kΩ).ꢀ Inꢀ addition,ꢀ aboutꢀ 12pFꢀ ofꢀ strayꢀ capaci-
tance must be charged. The input can be modeled as an
equivalentꢀRCꢀnetworkꢀ(Figureꢀ9).ꢀAsꢀsourceꢀimpedanceꢀ
increases,ꢀtheꢀcapacitorsꢀtakeꢀlongerꢀtoꢀcharge.
between V
and ground, disconnects the reference
REF-
load when the ADC enters power-down. (PWRDN = low).
TheꢀFETꢀshouldꢀhaveꢀnoꢀmoreꢀthanꢀ0.5Ωꢀofꢀon-resistanceꢀ
to maintain accuracy.
Bypassing
Theꢀtypicalꢀ22pFꢀinputꢀcapacitanceꢀallowsꢀsourceꢀresis-
tanceꢀasꢀhighꢀasꢀ2.2kΩꢀwithoutꢀsetupꢀproblems.ꢀForꢀlargerꢀ
Aꢀ 4.7µFꢀ electrolyticꢀ inꢀ parallelꢀ withꢀ aꢀ 0.1µFꢀ ceramicꢀ
capacitor should be used to bypass V ꢀtoꢀGND.ꢀTheseꢀ
DD
resistances, the acquisition time (t ) must be increased
P
capacitors should have minimal lead length.
Theꢀ referenceꢀ inputsꢀ shouldꢀ beꢀ bypassedꢀ withꢀ 0.1µFꢀ
capacitors,ꢀasꢀshownꢀinꢀFiguresꢀ7a–7c.
1
V
V
IN
IN+
10
1
GND
V
IN+
V
IN
10
MAX153
V
IN-
GND
20
12
11
+5V
V
V
V
DD
MAX153
20
12
11
0.1µF
4.7µF
+5V
V
V
V
DD
REF+
REF-
+2.5V
*
0.1µF
4.7µF
REF+
REF-
V
IN-
0.1µF
0.1µF
*CURRENT PATH MUST STILL
EXIST FROM V TO GND
IN-
Figure 7a. Power Supply as Reference
Figure 7c. Input Not Referenced to GND
20
V
IN+
V
DD
1
0.1µF
0.1µF
V
V
IN
IN+
MAX153
10
V
GND
IN-
12
11
18
MAX584
V
REF+
MAX153
20
12
11
V
V
V
+5V
DD
V
REF-
+2.5V
0.1µF
0.1µF
4.7µF
MAX584
REF+
REF-
PWRDN
PWRDN
0.1µF
N-FET
Figure 7b. External Reference, +2.5V Full Scale
Figure 7d. An n-Channel MOSFET Switches Off the Reference
Load During Power-Down
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