ADS1218_14 Datasheet, PDF (38/45 Pages) Texas Instruments – 8-Channel, 24-Bit ANALOG-TO-DIGITAL CONVERTER with FLASH Memory

ADS1218
SBAS187C – SEPTEMBER 2001 – REVISED SEPTEMBER 2005
Filter Selection—the ADS1218 uses a (sinx/x) filter
or sinc filter. There are three different sinc filters that
can be selected. A fast settling filter will settle in one
t_DATA cycle. The sinc² filter will settle in two cycles
and have lower noise. The sinc³ will achieve lowest
noise and higher number of effective bits, but
requires three cycles to settle. The ADS1218 will
operate with any one of these filters, or it can operate
in an auto mode, where it will first select the fast
settling filter after a new channel is selected for two
readings and will then switch to sinc² for one reading,
followed by sinc³ from then on.
Full-Scale Range (FSR)—as with most A/D
converters, the full-scale range of the ADS1218 is
defined as the input, which produces the positive
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.
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For example, when the converter is configured with a
2.5V reference and is placed in a gain setting of 2,
the full-scale range is: [1.25V (positive full-scale) –
(–1.25V (negative full-scale))] = 2.5V.
Least Significant Bit (LSB) Weight—this is the
theoretical amount of voltage that the differential
voltage at the analog input would have to change in
order to observe a change in the output data of one
least significant bit. It is computed as follows:
Full−Scale Range
LSB Weight +
2^N
where N is the number of bits in the digital output.
t_DATA—the inverse of f_DATA, or the period between
each data output.
GAIN
SETTING
1
2
4
8
16
34
64
128
Table 5. Full-Scale Range versus PGA Setting
5V SUPPLY ANALOG INPUT⁽¹⁾
GENERAL EQUATIONS
FULL-SCALE
RANGE
DIFFERENTIAL
INPUT
VOLTAGES ⁽²⁾
PGA OFFSET
RANGE
FULL-SCALE
RANGE
DIFFERENTIAL
INPUT
VOLTAGES ⁽²⁾
5V
2.5V
±2.5V
±1.25V
±1.25V
±0.625V
1.25V
0.625V
312.5mV
±0.625V
±312.5mV
±156.25mV
±312.5mV
±156.25mV
±78.125mV
2V_REF
PGA
" V_REF
PGA
156.25mV
±78.125mV
±39.0625mV
78.125mV
39.0625mV
±39.0625mV
±19.531mV
±19.531mV
±9.766mV
PGA SHIFT
RANGE
" V_REF
2 PGA
(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.
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