The Turbo Mode Rate (TMR) is programmed via the Sam-
pling Frequency bits of the Command Register. Due to the
increase in input capacitor sampling frequency, higher Turbo
Mode settings result in lower analog input impedance;
CALIBRATION
The ADS1212/13 offers several different types of calibra-
tion, and the particular calibration desired is programmed
via the Command Register. In the case of Background
Calibration, the calibration will repeat at regular intervals
indefinitely. For all others, the calibration is performed once
and then normal operation is resumed.
AIN Impedance (Ω) = (1MHz/fXIN)•20E6/(G•TMR)
where G is the gain setting. Because the modulator rate also
changes in direct relation to the Turbo Mode setting, higher
values result in a lower impedance for the REFIN input:
Each type of calibration is covered in detail in its respective
section. In general, calibration is recommended immediately
after power-on and whenever there is a “significant” change
in the operating environment. The amount of change which
should cause a re-calibration is dependent on the applica-
tion, effective resolution, etc. Where high accuracy is impor-
tant, re-calibration should be done on changes in tempera-
ture and power supply. In all cases, re-calibration should be
done when the gain, Turbo Mode, or data rate is changed.
REFIN Impedance (Ω) = (1MHz/fXIN)•5E6/TMR
The Turbo Mode Rate can be set to 1, 2, 4, 8, or 16. Consult
the graphs shown in the Typical Performance Curves for full
details on the performance of the ADS1212/13 operating in
different Turbo Mode Rates. Keep in mind that higher Turbo
Mode Rates result in fewer available gain settings as shown
in Table II.
After a calibration has been accomplished, the Offset Cali-
bration Register and the Full-Scale Calibration Register
contain the results of the calibration. The data in these
registers are accurate to the effective resolution of the
ADS1212/13’s mode of operation during the calibration.
Thus, these values will show a variation (or noise) equiva-
lent to a regular conversion result.
PROGRAMMABLE GAIN AMPLIFIER
The programmable gain amplifier gain setting is programmed
via the PGA Gain bits of the Command Register. Changes
in the gain setting (G) of the programmable gain amplifier
results in an increase in the input capacitor sampling fre-
quency. Thus, higher gain settings result in a lower analog
input impedance:
For those cases where this error must be reduced, it is
tempting to consider running the calibration at a slower data
rate and then increasing the converter’s data rate after the
calibration is complete. Unfortunately, this will not work as
expected. The reason is that the results calculated at the
slower data rate would not be valid for the higher data rate.
Instead, the calibration should be done repeatedly. After
each calibration, the results can be read and stored. After the
desired number of calibrations, the main controller can
compute an average and write this value into the calibration
registers. The resulting error in the calibration values will be
reduced by the square root of the number of calibrations
which were averaged.
AIN Impedance (Ω) = (1MHz/fXIN)•20E6/(G•TMR)
where TMR is the Turbo Mode Rate. Because the modulator
speed does not depend on the gain setting, the input imped-
ance seen at REFIN does not change.
The PGA can be set to gains of 1, 2, 4, 8, or 16. These gain
settings with their resulting full-scale range and typical
voltage range are shown in Table I. Keep in mind that higher
Turbo Mode Rates result in fewer available gain settings as
shown in Table II.
SOFTWARE GAIN
The calibration registers can also be used to provide system
offset and gain corrections separate from those computed by
the ADS1212/13. For example, these might be burned into
E2PROM during final product testing. On power-on, the
main controller would load these values into the calibration
registers. A further possibility is a look-up table based on the
current temperature.
The excellent performance, flexibility, and low cost of the
ADS1212/13 allow the converter to be considered for de-
signs which would not normally need a 24-bit ADC. For
example, many designs utilize a 12-bit converter and a high-
gain INA or PGA for digitizing low amplitude signals. For
some of these cases, the ADS1212/13 by itself may be a
solution, even though the maximum gain is limited to 16.
Note that the values in the calibration registers will vary from
configuration to configuration and from part to part. There is
no method of reliably computing what a particular calibration
register should be to correct for a given amount of system
error. It is possible to present the ADS1212/13 with a known
amount of error, perform a calibration, read the desired
calibration register, change the error value, perform another
calibration, read the new value and use these values to
interpolate an intermediate value.
To get around the gain limitation, the digital result can
simply be shifted up by “n” bits in the main controller—
resulting in a gain of “n” times G, where G is the gain
setting. While this type of manipulation of the output data
is obvious, it is easy to miss how much the gain can be
increased in this manner on a 24-bit converter.
For example, shifting the result up by three bits when the
ADS1212/13 is set to a gain of 16 results in an effective gain
of 128. At lower data rates, the converter can easily provide
more than 12 bits of resolution. Even higher gains are
possible. The limitation is a combination of the needed data
rate, desired noise performance, and desired linearity.
ADS1212, 1213
13
SBAS064A
BB [ BURR-BROWN CORPORATION ]
