Datasheet

MAX153 1Msps, µP-Compatible,
8-Bit ADC with 1µA Power-Down
www.maximintegrated.com
Maxim Integrated │ 10
The internal resistances from V
REF+
and V
REF-
may be
aslowas1kΩ.Sincecurrentisstilldrawnbythereference
inputs during power-down, reference supply current can
be reduced during shutdown by using the circuit shown
inFigure7d.Alogic-leveln-channelMOSFET,connected
between V
REF-
and ground, disconnects the reference
load when the ADC enters power-down. (PWRDN = low).
TheFETshouldhavenomorethan0.5Ωofon-resistance
to maintain accuracy.
Bypassing
A 4.7µF electrolytic in parallel with a 0.1µF ceramic
capacitor should be used to bypass V
DD
toGND.These
capacitors should have minimal lead length.
The reference inputs should be bypassed with 0.1µF
capacitors,asshowninFigures7a–7c.
Input Current
Figure 8 shows the equivalent circuit of the converter
input. When the conversion starts and WR is low, V
IN
is
connectedto160.6pFcapacitors.Duringthisacquisition
phase, the input capacitors charge to the input voltage
through the resistance of the internal analog switches
(about 2kΩ). In addition, about 12pF of stray capaci-
tance must be charged. The input can be modeled as an
equivalentRCnetwork(Figure9).Assourceimpedance
increases,thecapacitorstakelongertocharge.
Thetypical22pFinputcapacitanceallowssourceresis-
tanceashighas2.2kΩwithoutsetupproblems.Forlarger
resistances, the acquisition time (t
P
) must be increased
Figure 7a. Power Supply as Reference
Figure 7b. External Reference, +2.5V Full Scale
Figure 7c. Input Not Referenced to GND
Figure 7d. An n-Channel MOSFET Switches Off the Reference
Load During Power-Down
GNDV
IN-
10
V
IN
V
IN+
1
V
DD
V
REF+
V
REF-
20
12
4.7µF0.1µF
+5V
11
MAX153
0.1µF
+2.5V
20
4.7µF0.1µF
+5V
GNDV
IN-
10
V
IN
V
IN+
1
V
DD
V
REF+
V
REF-
12
11
MAX153
MAX584
+2.5V
V
IN
V
IN+
V
IN-
1
GND
V
DD
V
REF+
V
REF-
10
20
12
4.7µF0.1µF
0.1µF
+5V
11
MAX153
0.1µF
*
*CURRENT PATH MUST STILL
EXIST FROM V
IN-
TO GND
MAX584
V
DD
V
IN+
PWRDN
20
V
REF+
V
REF-
PWRDN
12
11
0.1µF
0.1µF
18
MAX153
0.1µF
N-FET