MAX11644 Datasheet, PDF (17/21 Pages) Maxim Integrated Products – 2.7V to 3.6V and 4.5V to 5.5V, Low-Power, 1-/2-Channel, 2-Wire Serial, 12-Bit ADCs

2.7V to 3.6V and 4.5V to 5.5V, Low-Power,
1-/2-Channel, 2-Wire Serial, 12-Bit ADCs
Table 6. Reference Voltage and REF Format
SEL2
SEL1
SEL0
0
0
X
0
1
X
1
0
0
1
0
1
1
1
0
1
1
1
X = Don’t care.
*Preferred configuration for internal reference.
REFERENCE
VOLTAGE
V_DD
External reference
Internal reference
Internal reference
Internal reference
Internal reference
REF
Not connected
Reference input
Not connected*
Not connected*
Reference output
Reference output
INTERNAL REFERENCE
STATE
Always off
Always off
Always off
Always on
Always off
Always on
Applications Information
Power-On Reset
The configuration and setup registers (Tables 1 and 2)
default to a single-ended, unipolar, single-channel con-
version on AIN0 using the internal clock with V_DD as the
reference. The memory contents are unknown after
power-up.
Automatic Shutdown
Automatic shutdown occurs between conversions when
the MAX11644/MAX11645 are idle. All analog circuits
participate in automatic shutdown except the internal
reference due to its prohibitively long wake-up time.
When operating in external clock mode, a STOP, not-
acknowledge, or repeated START condition must be
issued to place the devices in idle mode and benefit
from automatic shutdown. A STOP condition is not nec-
essary in internal clock mode to benefit from automatic
shutdown because power-down occurs once all con-
version results are written to memory (Figure 10). When
using an external reference or V_DD as a reference, all
analog circuitry is inactive in shutdown and supply cur-
rent is less than 0.5µA. The digital conversion results
obtained in internal clock mode are maintained in mem-
ory during shutdown and are available for access
through the serial interface at any time prior to a STOP
or a repeated START condition.
When idle, the MAX11644/MAX11645 continuously wait
for a START condition followed by their slave address
(see the Slave Address section). Upon reading a valid
address byte, the MAX11644/MAX11645 power up. The
internal reference requires 10ms to wake up, so when
using the internal reference it should be powered up
10ms prior to conversion or powered continuously.
Wake-up is invisible when using an external reference
or V_DD as the reference.
Automatic shutdown results in dramatic power savings,
particularly at slow conversion rates and with internal
clock. For example, at a conversion rate of 10ksps, the
average supply current for the MAX11645 is 60µA (typ)
and drops to 6µA (typ) at 1ksps. At 0.1ksps the aver-
age supply current is just 1µA, or a minuscule 3µW of
power consumption. See Average Supply Current vs.
Conversion Rate (External Clock) in the Typical
Operating Characteristics section).
Reference Voltage
SEL[2:0] of the setup byte (Table 1) control the refer-
ence configuration (Table 6).
Internal Reference
The internal reference is 4.096V for the MAX11644 and
2.048V for the MAX11645. When REF is configured to
be an internal reference output (SEL[2:1] = 11), decou-
ple REF to GND with a 0.1µF capacitor and a 2kΩ
series resistor (see the Typical Operating Circuit). Once
powered up, the reference always remains on until
reconfigured. The internal reference requires 10ms to
wake up and is accessed using SEL0 (Table 6). When
in shutdown, the internal reference output is in a high-
impedance state. The reference should not be used to
supply current for external circuitry. The internal refer-
ence does not require an external bypass capacitor
and works best when left unconnected (SEL1 = 0).
External Reference
The external reference can range from 1V to V_DD. For
maximum conversion accuracy, the reference must be
able to deliver up to 40µA and have an output imped-
ance of 500kΩ or less. If the reference has a higher
output impedance or is noisy, bypass it to GND as
close as possible to REF with a 0.1µF capacitor.
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