Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- ABSOLUTE MAXIMUM RATINGS
- ELECTRICAL CHARACTERISTICS: AVDD = 5V
- ELECTRICAL CHARACTERISTICS: AVDD = 3V
- DIGITAL CHARACTERISTICS: TMIN to TMAX, DVDD = 2.7V to 5.25V
- FLASH CHARACTERISTICS: TMIN to TMAX, DVDD = 2.7V to 5.25V, unless otherwise specified.
- PIN CONFIGURATION
- TIMING SPECIFICATIONS
- TIMING SPECIFICATION TABLE
- TIMING SPECIFICATION TABLE
- TYPICAL CHARACTERISTICS
- OVERVIEW
- INPUT MULTIPLEXER
- TEMPERATURE SENSOR
- BURNOUT CURRENT SOURCES
- INPUT BUFFER
- IDAC1 AND IDAC2
- PGA
- PGA OFFSET DAC
- MODULATOR
- VOLTAGE REFERENCE INPUT
- ON-CHIP VOLTAGE REFERENCE
- VRCAP PIN
- CLOCK GENERATOR
- CALIBRATION
- DIGITAL FILTER
- DIGITAL I/O INTERFACE
- SERIAL PERIPHERAL INTERFACE
- DATA READY
- DSYNC OPERATION
- MEMORY
- REGISTER BANK
- RAM
- FLASH
- REGISTER MAP
- COMMAND DEFINITIONS
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LSB Weight
Full−Scale Range
2
N
2V
REF
PGA
V
REF
PGA
V
REF
2PGA
ADS1218
SBAS187C – SEPTEMBER 2001 – REVISED SEPTEMBER 2005
Filter Selection—the ADS1218 uses a (sinx/x) filter For example, when the converter is configured with a
or sinc filter. There are three different sinc filters that 2.5V reference and is placed in a gain setting of 2,
can be selected. A fast settling filter will settle in one the full-scale range is: [1.25V (positive full-scale) –
t
DATA
cycle. The sinc
2
filter will settle in two cycles (–1.25V (negative full-scale))] = 2.5V.
and have lower noise. The sinc
3
will achieve lowest
Least Significant Bit (LSB) Weight—this is the
noise and higher number of effective bits, but
theoretical amount of voltage that the differential
requires three cycles to settle. The ADS1218 will
voltage at the analog input would have to change in
operate with any one of these filters, or it can operate
order to observe a change in the output data of one
in an auto mode, where it will first select the fast
least significant bit. It is computed as follows:
settling filter after a new channel is selected for two
readings and will then switch to sinc
2
for one reading,
followed by sinc
3
from then on.
where N is the number of bits in the digital output.
Full-Scale Range (FSR)—as with most A/D
converters, the full-scale range of the ADS1218 is
t
DATA
—the inverse of f
DATA
, or the period between
defined as the input, which produces the positive
each data output.
full-scale digital output minus the input, which
produces the negative full-scale digital output. The
full-scale range changes with gain setting; see
Table 5 .
Table 5. Full-Scale Range versus PGA Setting
5V SUPPLY ANALOG INPUT
(1)
GENERAL EQUATIONS
GAIN FULL-SCALE DIFFERENTIAL PGA OFFSET FULL-SCALE DIFFERENTIAL PGA SHIFT
SETTING RANGE INPUT RANGE RANGE INPUT RANGE
VOLTAGES
(2)
VOLTAGES
(2)
1 5V ±2.5V ±1.25V
2 2.5V ±1.25V ±0.625V
4 1.25V ±0.625V ±312.5mV
8 0.625V ±312.5mV ±156.25mV
16 312.5mV ±156.25mV ±78.125mV
34 156.25mV ±78.125mV ±39.0625mV
64 78.125mV ±39.0625mV ±19.531mV
128 39.0625mV ±19.531mV ±9.766mV
(1) With a 2.5V reference.
(2) The ADS1218 allows common-mode voltage as long as the absolute input voltage on A
IN+
or A
IN–
does not go below AGND or above
AV
DD
.
38