Rainbow Electronics MAX1273 - Manual (Page 13)

MAX1272/MAX1273
Fault-Protected, 12-Bit ADCs
with Software-Selectable Input Range
______________________________________________________________________________________ 13
External Reference
To use an external reference, disable the internal buffer
by setting the REF bit in the 8-bit control word to zero
(see Table 1), and apply a reference voltage to REF. Use
an external reference voltage ranging from 2.40V to
4.18V. External reference voltages less than 4.096V
increase the ratio of RMS noise to the LSB value (full
scale / 4096) resulting in performance degradation (loss
of effective bits—ENOB).
The REF input impedance is a minimum of 4.8k
Ω for
DC currents; therefore, the external reference must be
able to source 850µA during conversions and have an
output impedance of less than 10
Ω. Bypass REF with a
1µF capacitor to GND as close to REF as possible
(Figure 7b).
Power-Down Modes
To save power, configure the ADC for a low-current
shutdown mode by setting the PD bit in the control
byte. The MAX1272/MAX1273 features four program-
mable power-down modes: delayed standby power-
down, immediate standby power-down, delayed full
power-down, and immediate full power-down. Select
standby or full power-down by programming MODE1 in
the input control byte (Table 4). Select delayed or
immediate power-down by programming MODE0 in the
input control byte. Use the MODE0 bit to choose when
the part enters the power-down state. For example,
when MODE0 of the control byte is 0, the device
remains powered up until after the current conversion
ends (Figure 8). On the other hand, if MODE0 = 1, the
device powers down on the falling edge of the eighth
serial clock cycle and no conversion takes place
(Figure 9). In all power-down modes, the interface
remains active with the conversion results available at
DOUT. Additionally, the input overvoltage protection
remains active in all power-down modes (MAX1272).
The first high bit on DIN after CS falls (start condition)
powers up the MAX1272/MAX1273 from any software-
selected power-down condition. With external refer-
ence mode, device power-up time from full power-
down is typically 10µs. Send a control byte and allow
10µs for the device to wake up from full power-down.
The next received control byte initiates a conversion.
When in internal reference mode, full power-down
mode disables the internal reference and reference
buffer. Only the interface circuitry remains active for
reading conversion results. Send a control byte and
allow 2ms (C
REF = 1µF) for the internal reference to set-
tle and the MAX1272/MAX1273 to wake up from full
power-down mode. The next received control byte initi-
ates a conversion.
AutoShutdown™
The MAX1272/MAX1273 automatically enter standby
power-down mode after each conversion without requir-
ing any startup time on the next conversion.
Digital Interface
The MAX1272/MAX1273 feature a fully compatible
SPI/QSPI and MICROWIRE serial interface. For SPI and
QSPI, clear CPOL and CPHA in the microcontroller’s
SPI control registers.
REF
1.0 µF
MAX1272
MAX1273
C REF
Figure 7a. Internal Reference Configuration
REF OUT
GND
IN
5V
1.0 µF MAX1272
MAX1273
MAX6064
C REF
Figure 7b. External Reference Configuration
AutoShutdown is a trademark of Maxim Integrated Products, Inc.