Datasheet
ADS1242, 1243
22
SBAS235H
www.ti.com
+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.
RANGE = 0
RANGE = 1
V
PGA
REF
±
•
V
PGA
REF
2
±
•
V
PGA
REF
4
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
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 GND.
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 GND to V
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
SPEED
DR
DATA
osc
SPEED DR
=
•••
=
=
128 2 1280 2
01
012
,
,,
f
OSC
—the frequency of the crystal oscillator or CMOS com-
patible input signal at the X
IN
input of the ADS1242 and
ADS1243.
f
MOD
—the frequency or speed at which the modulator of the
ADS1242 and ADS1243 is running. This depends on the
SPEED bit as given by the following equation:
f
f
mfactor
f
MOD
osc osc
SPEED
==
•128 2
f
SAMP
—the frequency, or switching speed, of the input sam-
pling capacitor. The value is given by one of the following
equations:
f
DATA
—the frequency of the digital output data produced by
the ADS1242 and ADS1243, 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 ADS1242 and ADS1243 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 ScaleRange
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.