Datasheet

ADS1240, 1241
22
SBAS173F
www.ti.com
DEFINITION OF TERMS
An attempt has been made to be consistent with the termi-
nology used in this data sheet. In that regard, the definition
of each term is given as follows:
Analog Input Voltage—the voltage at any one analog input
relative to AGND.
Analog Input Differential Voltage—given by the following
equation: (IN+) – (IN–). Thus, a positive digital output is
produced whenever the analog input differential voltage is
positive, while a negative digital output is produced whenever
the differential is negative.
For example, when the converter is configured with a 2.5V
reference and placed in a gain setting of 1, the positive
full-scale output is produced when the analog input differen-
tial is 2.5V. The negative full-scale output is produced when
the differential is –2.5V. In each case, the actual input
voltages must remain within the AGND to AV
DD
range.
Conversion Cycle—the term
conversion cycle usually refers
to a discrete A/D conversion operation, such as that per-
formed by a successive approximation converter. As used
here, a conversion cycle refers to the t
DATA
time period.
Data Rate—The rate at which conversions are completed.
See definition for f
DATA
.
f
f
DATA
OSC
SPEED DR
=
128 2 1280 2•••
SPEED = 0, 1
DR = 0, 1, 2
f
OSC
—the frequency of the crystal oscillator or CMOS com-
patible input signal at the X
IN
input of the ADS1240 and
ADS1241.
f
MOD
—the frequency or speed at which the modulator of the
ADS1240 and ADS1241 is running. This depends on the
SPEED bit as given by the following equation:
f
f
mfactor
SAMP
OSC
=
• 8
f
f
mfactor
SAMP
OSC
=
• 4
f
f
mfactor
SAMP
OSC
=
• 2
f
f
mfactor
SAMP
OSC
=
PGA SETTING SAMPLING FREQUENCY
1, 2, 4, 8
16
32
64, 128
SPEED = 0 SPEED = 1
mfactor 128 256
pling capacitor. The value is given by one of the following
equations:
f
DATA
—the frequency of the digital output data produced by
the ADS1240 and ADS1241, f
DATA
is also referred to as the
Data Rate.
Full-Scale Range (FSR)—as with most A/D converters, the
full-scale range of the ADS1240 and ADS1241 is defined as
the input, that produces the positive full-scale digital output
minus the input, that produces the negative full-scale digital
output.
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) minus –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 has to change in order to observe a change in the
output data of one least significant bit. It is computed as
follows:
LSB Weight
Full Scale Range
N
=
−
21–
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.
5V SUPPLY ANALOG INPUT
(1)
GENERAL EQUATIONS
DIFFERENTIAL PGA OFFSET FULL-SCALE DIFFERENTIAL PGA SHIFT
GAIN SETTING FULL-SCALE RANGE INPUT VOLTAGES
(2)
RANGE RANGE INPUT VOLTAGES
(2)
RANGE
15V±2.5V ±1.25V
22.5V±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
32 156.25mV ±78.125mV ±39.0625mV
64 78.125mV ±39.0625mV ±19.531mV
128 39.0625mV ±19.531mV ±9.766mV
NOTES: (1) With a 2.5V reference. (2) Refer to electrical specification for analog input voltage range.
TABLE VI. Full-Scale Range versus PGA Setting.
2 • V
PGA
REF
±V
PGA
REF
±
•
V
PGA
REF
2
RANGE = 0
RANGE = 1
V
PGA
REF
±
•
V
PGA
REF
2
±
•
V
PGA
REF
4
f
f
mfactor
f
MOD
OSC OSC
SPEED
==
128 2•
f
SAMP
—the frequency, or switching speed, of the input sam-