ADS1243IPWT datasheet(22/25 Pages) TI

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ADS1242, 1243

SBAS235B

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+5V SUPPLY ANALOG INPUT(1)

GENERAL EQUATIONS

DIFFERENTIAL

PGA OFFSET

FULL-SCALE

DIFFERENTIAL

PGA SHIFT

GAIN SETTING

FULL-SCALE RANGE

INPUT VOLTAGES(2)

RANGE

INPUT VOLTAGES(2)

RANGE

15V

±2.5V

±1.25V

22.5V

±1.25V

±0.625V

1.25V

±0.625V

±312.5mV

0.625V

±312.5mV

±156.25mV

312.5mV

±156.25mV

±78.125mV

156.25mV

±78.125mV

±39.0625mV

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

PGA

REF

•

PGA

REF

•

PGA

REF

mfactor

SAMP

OSC

• 8

mfactor

SAMP

OSC

• 4

mfactor

SAMP

OSC

• 2

mfactor

SAMP

OSC

PGA SETTING

SAMPLING FREQUENCY

1, 2, 4, 8

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 VDD 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 tDATA time period.

Data Rate—The rate at which conversions are completed.

See definition for fDATA.

SPEED

DATA

osc

SPEED

••

•

128 2

1280 2

01 2

fOSC—the frequency of the crystal oscillator or CMOS com-

patible input signal at the XIN input of the ADS1242 and

ADS1243.

fMOD—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:

mfactor

MOD

osc

SPEED

•

128 2

fSAMP—the frequency, or switching speed, of the input sam-

pling capacitor. The value is given by one of the following

equations:

fDATA—the frequency of the digital output data produced by

the ADS1242 and ADS1243, fDATA 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

−

–

where N is the number of bits in the digital output.

tDATA—the inverse of fDATA, or the period between each data

output.

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