ADS1216 datasheet(24/28 Pages) TI

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ADS1216

SBAS171B

PGA SETTING

SAMPLING FREQUENCY

1, 2, 4, 8

64, 128

mfactor

SAMP

OSC

mfactor

SAMP

OSC

• 2

mfactor

SAMP

OSC

• 4

mfactor

SAMP

OSC

• 8

BITS rms

BIPOLAR Vrms

UNIPOLAR Vrms

298nV

149nV

1.19

µV

597nV

4.77

µV2.39µV

19.1

µV9.55µV

76.4

µV

38.2

µV

505

µV

152.7

µV

1.22mV

610

µV

SPEED = 0

SPEED = 1

mfactor

128

256

Effective Resolution—the effective resolution of the

ADS1216 in a particular configuration can be expressed in

two different units: bits rms (referenced to output) and Vrms

(referenced to input). Computed directly from the converter’s

output data, each is a statistical calculation. The conversion

from one to the other is shown below.

“Effective number of bits” (ENOB) or “effective resolution”

is commonly used to define the usable resolution of the

A/D converter. It is calculated from empirical data taken

directly from the device. It is typically determined by apply-

ing a fixed known signal source to the analog input and

computing the standard deviation of the data sample set. The

rms noise defines the

±σ interval about the sample mean

(which implies that 95% of the data values fall within this

range) and the peak-to-peak noise defines the

±3σ interval

about the sample mean (which implies that 99.6% of the data

values fall within this range).

The data from the A/D converter is output as codes, which

then can be easily converted to other units, such as ppm or

volts. The equations and table below show the relationship

between bits or codes, ppm, and volts.

Decimation Ratio

mfactor Decimation Ratio

DATA

MOD

OSC









•









602

•

.•

PGA

REF

















PGA

REF

















602

.•

mfactor

MOD

OSC

ENOB

ppm

–

log (

)

602

Filter Selection—the ADS1216 uses a (sinx /x) filter or sinc

filter. Actually there are three different sinc filters that can

be selected. A fast settling filter will settle in one tDATA

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 ADS1216 will operate with any one of these filters, or

it can operate in an auto mode, where it will select the fast

settling filter after a new channel is selected and will then

switch to sinc2 followed by sinc3. This allows fast settling

response and still achieves low noise after the necessary

number of tDATA cycles.

fOSC—the frequency of the crystal oscillator or CMOS

compatible input signal at the XIN input of the ADS1216.

fMOD—the frequency or speed at which the modulator of the

ADS1216 is running. This depends on the SPEED bit as

given by the following equation:

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

sampling capacitor. The value is given by one of the follow-

ing equations:

fDATA—the frequency of the digital output data produced by

the ADS1216, 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 ADS1216 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 as

shown in Table V.

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 (nega-

tive 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:

LSB Weight

Full Scale Range

−

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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