MAX1227/MAX1229/MAX1231
12-Bit 300ksps ADCs with FIFO,
Temp Sensor, Internal Reference
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Converter Operation
The MAX1227/MAX1229/MAX1231 ADCs use a fully dif-
ferential, successive-approximation register (SAR) con-
version technique and an on-chip T/H block to convert
temperature and voltage signals into a 12-bit digital
result. Both single-ended and differential configurations
are supported, with a unipolar signal range for single-
ended mode and bipolar or unipolar ranges for differ-
ential mode.
Input Bandwidth
The ADC’s input-tracking circuitry has a 1MHz small-
signal bandwidth, so it is possible to digitize high-
speed transient events and measure periodic signals
with bandwidths exceeding the ADC’s sampling rate by
using undersampling techniques. Anti-alias prefiltering
of the input signals is necessary to avoid high-frequen-
cy signals aliasing into the frequency band of interest.
Analog Input Protection
Internal ESD protection diodes clamp all pins to V
DD
and GND, allowing the inputs to swing from (GND -
0.3V) to (V
DD
+ 0.3V) without damage. However, for
accurate conversions near full scale, the inputs must
not exceed V
DD
by more than 50mV or be lower than
GND by 50mV. If an off-channel analog input voltage
exceeds the supplies, limit the input current to 2mA.
3-Wire Serial Interface
The MAX1227/MAX1229/MAX1231 feature a serial
interface compatible with SPI/QSPI and MICROWIRE
devices. For SPI/QSPI, ensure the CPU serial interface
runs in master mode so it generates the serial clock
signal. Select the SCLK frequency of 10MHz or less,
and set clock polarity (CPOL) and phase (CPHA) in the
µP control registers to the same value. The MAX1227/
MAX1229/MAX1231 operate with SCLK idling high or
low, and thus operate with CPOL = CPHA = 0 or CPOL
= CPHA = 1. Set CS low to latch input data at DIN on
the rising edge of SCLK. Output data at DOUT is
updated on the falling edge of SCLK. Bipolar true dif-
ferential results and temperature sensor results are
available in two’s complement format, while all others
are in binary.
Serial communication always begins with an 8-bit input
data byte (MSB first) loaded from DIN. Use a second
byte, immediately following the setup byte, to write to
the unipolar mode or bipolar mode registers (see
Tables 1, 3, 4, and 5). A high-to-low transition on CS ini-
tiates the data input operation. The input data byte and
the subsequent data bytes are clocked from DIN into
the serial interface on the rising edge of SCLK.
Tables 1–7 detail the register descriptions. Bits 5 and 4,
CKSEL1 and CKSEL0, respectively, control the clock
modes in the setup register (see Table 3). Choose
between four different clock modes for various ways to
start a conversion and determine whether the acquisi-
tions are internally or externally timed. Select clock
mode 00 to configure CNVST/AIN_ to act as a conver-
sion start and use it to request the programmed, inter-
nally timed conversions without tying up the serial bus.
In clock mode 01, use CNVST to request conversions
one channel at a time, controlling the sampling speed
without tying up the serial bus. Request and start inter-
nally timed conversions through the serial interface by
writing to the conversion register in the default clock
mode 10. Use clock mode 11 with SCLK up to 4.8MHz
for externally timed acquisitions to achieve sampling
rates up to 300ksps. Clock mode 11 disables scanning
and averaging. See Figures 4–7 for timing specifica-
tions and how to begin a conversion.
These devices feature an active-low, end-of-conversion
output. EOC goes low when the ADC completes the last-
requested operation and is waiting for the next input
data byte (for clock modes 00 and 10). In clock mode
01, EOC goes low after the ADC completes each
requested operation. EOC goes high when CS or CNVST
goes low. EOC is always high in clock mode 11.
Single-Ended/Differential Input
The MAX1227/MAX1229/MAX1231 use a fully differen-
tial ADC for all conversions. The analog inputs can be
configured for either differential or single-ended con-
versions by writing to the setup register (see Table 3).
Single-ended conversions are internally referenced to
GND (see Figure 3).
In differential mode, the T/H samples the difference
between two analog inputs, eliminating common-mode
DC offsets and noise. IN+ and IN- are selected from
the following pairs: AIN0/AIN1, AIN2/AIN3, AIN4/AIN5,
AIN6/AIN7, AIN8/AIN9, AIN10/AIN11, AIN12/AIN13,
and AIN14/AIN15. AIN0–AIN7 are available on the
MAX1227, MAX1229, and MAX1231. AIN8–AIN11 are
only available on the MAX1229 and MAX1231.
AIN12–AIN15 are only available on the MAX1231. See
Tables 2–5 for more details on configuring the inputs.
For the inputs that can be configured as CNVST or an
analog input, only one can be used at a time. For the
inputs that can be configured as REF- or an analog
input, the REF- configuration excludes the analog input.
