THEORY OF OPERATION
The ADS1212 and ADS1213 are precision, high dynamic
range, self-calibrating, 24-bit, delta-sigma A/D converters
capable of achieving very high resolution digital results.
Each contains a programmable gain amplifier (PGA); a
second-order delta-sigma modulator; a programmable digi-
tal filter; a microcontroller including the Instruction, Com-
mand and Calibration registers; a serial interface; a clock
generator circuit; and an internal 2.5V reference. The
ADS1213 includes a 4-channel input multiplexer.
The output data rate of the ADS1212/13 can be varied from
less than 1Hz to as much as 6.25kHz, trading off lower
resolution results for higher data rates. In addition, the data
rate determines the first null of the digital filter and sets the
–3dB point of the input bandwidth (see the Digital Filter
section). Changing the data rate of the ADS1212/13 does not
result in a change in the sampling rate of the input capacitor.
The data rate effectively sets the number of samples which
are used by the digital filter to obtain each conversion result.
A lower data rate results in higher resolution, lower input
bandwidth, and different notch frequencies than a higher
data rate. It does not result in any change in input impedance
or modulator frequency, or any appreciable change in power
consumption.
In order to provide low system noise, common-mode rejec-
tion of 100dB and excellent power supply rejection, the
design topology is based on a fully differential switched
capacitor architecture. Turbo Mode, a unique feature of the
ADS1212/13, can be used to boost the sampling rate of the
input capacitor, which is normally 7.8kHz with a 1MHz
clock. By programming the Command Register, the sam-
pling rate can be increased to 15.6kHz, 31.2kHz, 62.5kHz,
or 125kHz. Each increase in sample rate results in an
increase in performance when maintaining the same output
data rate.
The ADS1212/13 also includes complete on-board calibra-
tion that can correct for internal offset and gain errors or
limited external system errors. Internal calibration can be
run when needed, or automatically and continuously in the
background. System calibration can be run as needed and the
appropriate input voltages must be provided to the ADS1212/
13. For this reason, there is no continuous system calibration
mode. The calibration registers are fully readable and writ-
able. This feature allows for switching between various
configurations—different data rates, Turbo Mode Rates, and
gain settings—without re-calibrating.
The programmable gain amplifier (PGA) of the ADS1212/
13 can be set to a gain of 1, 2, 4, 8 or 16—substantially
increasing the dynamic range of the converter and simplify-
ing the interface to the more common transducers (see Table
I). This gain is implemented by increasing the number of
samples taken by the input capacitor from 7.8kHz for a gain
of 1 to 125kHz for a gain of 16. Since the Turbo Mode and
PGA functions are both implemented by varying the sam-
pling frequency of the input capacitor, the combination of
PGA gain and Turbo Mode Rate is limited to 16 (see Table
II). For example, when using a Turbo Mode Rate of 8
(62.5kHz at 1MHz), the maximum PGA gain setting is 2.
The various settings, rates, modes, and registers of the
ADS1212/13 are read or written via a synchronous serial
interface. This interface can operate in either a self-clocked
mode (Master Mode) or an externally clocked mode (Slave
Mode). In the Master Mode, the serial clock (SCLK) fre-
quency is one-quarter of the ADS1212/13 XIN clock fre-
quency.
The high resolution and flexibility of the ADS1212/13 allow
these converters to fill a wide variety of A/D conversion
tasks. In order to ensure that a particular configuration will
meet the design goals, there are several important items
which must be considered. These include (but are certainly
not limited to) the needed resolution, required linearity,
desired input bandwidth, power consumption goal, and sen-
sor output voltage.
ANALOG
INPUT(1)
ANALOG INPUT
UTILIZING VBIAS
(1,2)
FULL-
EXAMPLE
VOLTAGE
RANGE(3)
(V)
FULL-
SCALE
RANGE
(V)
EXAMPLE
SCALE
RANGE
(V)
VOLTAGE
RANGE(3)
(V)
GAIN
SETTING
1
2
4
8
16
10
5
2.5
0 to 5
40
20
10
5
±10
±5
±2.5
±1.25
±0.625
1.25 to 3.75
1.88 to 3.13
2.19 to 2.81
2.34 to 2.66
The remainder of this data sheet discusses the operation of
the ADS1212/13 in detail. In order to allow for easier
comparison of different configurations, “effective resolu-
tion” is used as the figure of merit for most tables and
graphs. For example, Table III shows a comparison between
data rate (and –3dB input bandwidth) versus PGA setting at
a Turbo Mode Rate of 1 and a clock rate of 1MHz. See the
Definition of Terms section for a definition of effective
resolution.
1.25
0.625
2.5
NOTE: (1) With a 2.5V reference, such as the internal reference. (2) This
example utilizes the circuit in Figure 12. Other input ranges are possible. (3)
The ADS1212/13 allows common-mode voltage as long as the absolute
input voltage on AINP or AINN does not go below AGND or above AVDD
.
TABLE I. Full-Scale Range vs PGA Setting.
TURBO MODE RATE
AVAILABLE PGA SETTINGS
1
2
4
1, 2, 4, 8, 16
1, 2, 4, 8
1, 2, 4
1, 2
8
16
1
TABLE II. Available PGA Settings vs Turbo Mode Rate.
ADS1212, 1213
9
SBAS064A
BB [ BURR-BROWN CORPORATION ]
