ADS1216 datasheet(13/28 Pages) TI

13 / 28 page

ADS1216

SBAS171B

CALIBRATION

The offset and gain errors in the ADS1216, or the complete

system, can be reduced with calibration. Internal calibration of

the ADS1216 is called self calibration. This is handled with

three commands. One command does both offset and gain

calibration. There is also a gain calibration command and an

offset calibration command. Each calibration process takes

seven tDATA periods to complete. Therefore, it takes 14 tDATA

periods to complete both an offset and gain calibration.

For system calibration, the appropriate signal must be

applied to the inputs. The system offset command requires a

“zero” differential input signal. It then computes an offset that

will nullify offset in the system. The system gain command

requires a positive “full-scale” differential input signal. It then

computes a value to nullify gain errors in the system. Each of

these calibrations will take seven tDATA periods to complete.

Calibration should be performed after power on, a change in

temperature, or a change of the PGA. The RANGE bit (ACR

bit 2) must be zero during calibration. For operation with a

reference voltage greater than (AVDD – 1.5) Volts, the buffer

must also be turned off during calibration. Calibration will

remove the effects of the ODAC, therefore, changes to the

ODAC register must be done after calibration, otherwise the

calibration will remove the effects of the offset.

At the completion of calibration the DRDY signal goes low

which indicates the calibration is finished and valid data is

available.

DIGITAL FILTER

The Digital Filter can use either the fast settling, sinc2, or sinc3

filter, as shown in Figure 2. In addition, the Auto mode

changes the sinc filter after the input channel or PGA is

changed. When switching to a new channel, it will use the fast

FIGURE 3. Filter Frequency Responses.

FIGURE 2. Filter Step Responses.

SETTLING TIME

FILTER

(Conversion Cycles)

Sinc3

3(1)

Sinc2

2(1)

Fast

1(1)

NOTE: (1) With Synchronized Channel Changes.

CONVERSION CYCLE

Discard

Fast

Sinc2

Sinc3

AUTO MODE FILTER SELECTION

FILTER SETTLING TIME

Adjustable Digital Filter

Data Out

Modulator

Output

Fast Settling

Sinc2

Sinc3

SINC3 FILTER RESPONSE

(–3dB = 0.262 • f

DATA = 15.76Hz)

Frequency (Hz)

–20

–40

–60

–80

–100

–120

120

150

180

210

240

270

300

SINC2 FILTER RESPONSE

(–3dB = 0.318 • f

DATA = 19.11Hz)

Frequency (Hz)

–20

–40

–60

–80

–100

–120

120

150

180

210

240

270

300

FAST SETTLING FILTER RESPONSE

(–3dB = 0.469 • f

DATA = 28.125Hz)

Frequency (Hz)

–20

–40

–60

–80

–100

–120

NOTE: f

DATA = 60Hz.

120

150

180

210

240

270

300

settling filter, for the next two conversions the first of which

should be discarded. It will then use the sinc2 followed by the

sinc3 filter to improve noise performance. This combines the

low-noise advantage of the sinc3 filter with the quick response

of the fast settling time filter. The frequency response of each

filter is shown in Figure 3.

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