MAX1117 Datasheet, PDF (12/14 Pages) Maxim Integrated Products – Single-Supply, Low-Power, 2-Channel, Serial 8-Bit ADCs

Single-Supply, Low-Power,
2-Channel, Serial 8-Bit ADCs
OUTPUT CODE
11111111
11111110
11111101
FULL-SCALE
TRANSITION
SYSTEM POWER SUPPLIES
GND
+3V/+5V
FS = V_REF
1LSB = V_REF
256
00000011
00000010
00000001
00000000
012 3
INPUT VOLTAGE (LSB)
FS
FS - 1 1/2 LSB
Figure 7. Input/Output Transfer Function
Applications Information
Power-On Reset
When power is first applied, the MAX1117/MAX1118/
MAX1119 are in AutoShutdown state (<1µA typ). A conver-
sion can be started by toggling CNVST high to low. Powering
up the MAX1117/MAX1118/MAX1119 with CNVST low will
not start a conversion. Conversions initiated prior to the
external reference settling (MAX1118) will result in errors.
Thus, it is necessary to allow the external reference to stabi-
lize prior to initiating a conversion.
AutoShutDown and Supply Current
Requirements
The MAX1117/MAX1118/MAX1119 are designed to
automatically shutdown once a conversion is complete
without any external control. An input sample and con-
version process will typically take 5µs to complete, dur-
ing which time the supply current to the analog
sections of the device is fully on. All analog circuitry is
shutdown after a conversion completes, which results
in a supply current of <1µA (see Shutdown Current vs.
Supply Voltage Plot in the Typical Operating
Characteristics). The digital conversion result is main-
tained in a static register and is available for access
through the serial interface at any time.
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 MAX1117 is
1µF
10Ω*
0.1µF
GND
V_DD
DGND V_DD
MAX1117
MAX1118
MAX1119
DIGITAL
CIRCUITRY
*OPTIONAL
Figure 8. Power-Supply Connections
15µA, while at 1ksps it drops to 1.5µA and 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).
External Voltage Reference (MAX1118)
Connect an external reference between +1V and V_DD
at the REF pin. The DC input impedance at REF is
extremely high, consisting of leakage current only
(10nA typ). During a conversion, the reference must be
able to deliver up to 20µA average load current and
have an output impedance of 100Ω or less. If the refer-
ence has higher output impedance or is noisy, bypass
it close to the REF pin with a 10nF or larger capacitor.
Transfer Function
Figure 7 depicts the input/output transfer function.
Output coding is binary with a +2.048V reference 1LSB
= 8mV (V_REF/256).
Layout, Grounding, Bypassing
For best performance, the 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
underneath 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
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