Datasheet

ADS1212, 1213
12
SBAS064A
Filter Equation
The digital filter is described by the following transfer
function:
where N is the Decimation Ratio.
This filter has a (sin(x)/x)
3
response and is referred to a sinc
3
filter. For the ADS1212/13, this type of filter allows the data
rate to be changed over a very wide range (nearly four orders
of magnitude). However, the –3dB point of the filter is 0.262
times the data rate. And, as can be seen in Figures 1 and 2,
the rejection in the stopband (frequencies higher than the
first notch frequency) may only be –40dB.
These factors must be considered in the overall system
design. For example, with a 50Hz data rate, a significant
signal at 75Hz may alias back into the passband at 25Hz.
The analog front end can be designed to provide the needed
attenuation to prevent aliasing, or the system may simply
provide this inherently. Another possibility is increasing the
data rate and then post filtering with a digital filter on the
main controller.
Filter Settling
The number of modulator results used to compute each
conversion result is three times the Decimation Ratio. This
means that any step change (or any channel change for the
ADS1213) will require at least three conversions to fully
settle. However, if the change occurs asynchronously, then at
least four conversions are required to ensure complete set-
tling. For example, on the ADS1213, the fourth conversion
result after a channel change will be valid (see Figure 4).
|H(f)|=
sin
π•f • N
f
MOD






N • sin
π•f
f
MOD






3
the effective resolution of the output data at a given data rate,
but there is also an increase in power dissipation. For Turbo
Mode Rates 2 and 4, the increase is slight. For rates 8 and
16, the increase is more substantial. See the Typical Perfor-
mance Curves for more information.
In a Turbo Mode Rate of 16, the ADS1212/13 can offer 16
bits of effective resolution at a 1kHz data rate. A comparison
of effective resolution versus Turbo Mode Rates and output
data rates is shown in Table IV while Table V shows the
corresponding noise level in µVrms.
TURBO MODE
The ADS1212/13 offers a unique Turbo Mode feature which
can be used to increase the modulator sampling rate by 2, 4,
8, or 16 times normal. With the increase of modulator
sampling frequency, there can be a substantial increase in
DATA TURBO TURBO TURBO TURBO TURBO
RATE MODE MODE MODE MODE MODE
(HZ) RATE 1 RATE 2 RATE 4 RATE 8 RATE 16
10 20 21 21
20 19 20 21 21
40 18 20 21 21 21
50 17 19 20 21 21
60 17 19 20 21 21
100 15 17 19 21 21
250 12 14 16 19 20
1000 12 14 16
EFFECTIVE RESOLUTION (BITS RMS)
FIGURE 4. Asynchronous ADS1212/13 Analog Input Volt-
age Step or ADS1213 Channel Change to Fully
Settled Output Data.
DRDY
Serial
I/O
Valid
Data
Valid
Data
Valid
Data
Valid
Data
Data
not
Valid
Data
not
Valid
Data
not
Valid
Significant Analog Input Change
or
ADS1213 Channel Change
t
DATA
TABLE IV. Effective Resolution vs Data Rate and Turbo Mode
Rate. (Gain setting of 1 and 1MHz clock.)
DATA TURBO TURBO TURBO TURBO TURBO
RATE MODE MODE MODE MODE MODE
(Hz) RATE 1 RATE 2 RATE 4 RATE 8 RATE 16
10 7.6 3.8 3.8
20 15 7.6 3.8 3.8
40 30 7.6 3.8 3.8 3.8
50 60 15 7.6 3.8 3.8
60 60 15 7.6 3.8 3.8
100 240 60 15 3.8 3.8
250 1900 480 120 15 7.6
1000 1900 480 120
NOISE LEVEL (µVrms)
TABLE V. Noise Level vs Data Rate and Turbo Mode Rate.
(Gain setting of 1 and 1MHz clock.)
The Turbo Mode feature allows trade-offs to be made
between the ADS1212/13 X
IN
clock frequency, power dissi-
pation, and effective resolution. If a 0.5MHz clock is avail-
able but a 1MHz clock is needed to achieve the desired
performance, a Turbo Mode Rate of 2X will result in the
same effective resolution. Table VI provides a comparison
of effective resolution at various clock frequencies, data
rates, and Turbo Mode Rates.
DATA X
IN
CLOCK TURBO EFFECTIVE
RATE FREQUENCY MODE RESOLUTION
(Hz) (MHz) RATE (Bits rms)
60 2 2 20
60 1 4 20
60 0.5 8 20
100 2 2 19
100 1 4 19
100 0.5 8 19
TABLE VI. Effective Resolution vs Data Rate, Clock
Frequency, and Turbo Mode Rate. (Gain set-
ting of 1.)