MAX1115 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – Single-Supply, Low-Power, Serial 8-Bit ADCs

Single-Supply, Low-Power, Serial 8-Bit ADCs
OUTPUT CODE
11111111
11111110
11111101
FULL-SCALE
TRANSITION
SYSTEM POWER SUPPLIES
GND
+3V/+5V
FS = V_REFIN + V_IN-
1LSB = V_REFIN
256
00000011
00000010
00000001
00000000
012 3
INPUT VOLTAGE (LSB)
FS
FS - 1/2 LSB
Figure 7. Input/Output Transfer Function
active conversion process interrupt the current conver-
sion and cause the input multiplexer to switch to V_DD/2.
To reinitiate a conversion on CH0, it is necessary to allow
for a conversion to be complete and all of the data to be
read out. Once a conversion has been completed, the
MAX1115/MAX1116 goes into Autoshutdown mode
(typically <1µA) until the next conversion is initiated.
Applications Information
Power-On Reset
When power is first applied, the MAX1115/MAX1116
are in AutoShutdown (typically <1µA). A conversion
can be started by toggling CNVST high to low.
Powering up the MAX1115/MAX1116 with CNVST low
does not start a conversion.
AutoShutdown and Supply Current
Requirements
The MAX1115/MAX1116 are designed to automatically
shutdown once a conversion is complete, without any
external control. An input sample and conversion
process typically takes 5µs to complete, during which
time the supply current to the analog sections of the
device are fully on. All analog circuitry is shutdown after
a conversion completes, which results in a supply cur-
rent of <1µA (see Shutdown Current vs. Supply Voltage
plot in the Typical Operating Characteristics section).
The digital conversion result is maintained in a static
register and is available for access through the serial
interface at any time.
1µF
10Ω
0.1µF
GND IN-
MAX1115
MAX1116
V_DD
DGND V_DD
DIGITAL
CIRCUITRY
Figure 8. Power-Supply Connections
The power consumption consequence of this architec-
ture is dramatic when relatively slow conversion rates
are needed. For example, at a conversion rate of
10ksps, the average supply current for the MAX1115 is
15µA, while at 1ksps it drops to 15µA. At 0.1ksps it is
just 0.3µA, or a miniscule 1µW of power consumption
(see Average Supply Current vs. Conversion Rate plot
in the Typical Operating Characteristics sections).
Transfer Function
Figure 7 depicts the input/output transfer function.
Output coding is binary with a +2.048V reference,
1LSB = 8mV(VREF/256).
Layout, Grounding, and Bypassing
For best performance, board layout should ensure that
digital and analog signal lines are separated from each
other. Do not run analog and digital (especially clock)
lines parallel to one another or run digital lines under-
neath the ADC package.
Figure 8 shows the recommended system-ground con-
nections. A single-point analog ground (star-ground
point) should be established at the ADC ground.
Connect all analog grounds to the star-ground. The
ground-return to the power supply for the star ground
should be low impedance and as short as possible for
noise-free operation.
High-frequency noise in the V_DD power supply can
affect the comparator in the ADC. Bypass the supply to
the star ground with a 0.1µF capacitor close to the V_DD
pin of the MAX1115/MAX1116. Minimize capacitor lead
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