Filter Equation
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.
The digital filter is described by the following transfer
function:
3
π • f • N
sin
fMOD
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.
| H (f) | =
π • f
N • sin
fMOD
where N is the Decimation Ratio.
This filter has a (sin(x)/x)3 response and is referred to a sinc3
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.
EFFECTIVE RESOLUTION (BITS RMS)
DATA
RATE
(HZ)
TURBO
MODE
RATE 1
TURBO
MODE
TURBO
MODE
TURBO
MODE
TURBO
MODE
RATE 16
RATE 2
RATE 4
RATE 8
10
20
40
50
60
100
250
1000
20
19
18
17
17
15
12
21
20
20
19
19
17
14
21
21
21
20
20
19
16
12
21
21
21
21
21
19
14
21
21
21
21
20
16
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.
TABLEIV. EffectiveResolutionvsDataRateandTurboMode
Rate. (Gain setting of 1 and 1MHz clock.)
NOISE LEVEL (µVrms)
DATA
RATE
(Hz)
TURBO
MODE
RATE 1
TURBO
MODE
TURBO
MODE
TURBO
MODE
TURBO
MODE
RATE 16
RATE 2
RATE 4
RATE 8
Filter Settling
10
20
40
50
60
100
250
1000
7.6
15
30
60
60
3.8
7.6
7.6
15
15
60
3.8
3.8
3.8
7.6
7.6
15
120
1900
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).
3.8
3.8
3.8
3.8
3.8
15
3.8
3.8
3.8
3.8
7.6
120
240
1900
480
480
TABLE V. Noise Level vs Data Rate and Turbo Mode Rate.
(Gain setting of 1 and 1MHz clock.)
Significant Analog Input Change
or
ADS1213 Channel Change
The Turbo Mode feature allows trade-offs to be made
between the ADS1212/13 XIN 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
not
Valid
Data
not
Valid
Data
not
Valid
Valid
Data
Valid
Data
Valid
Data
Valid
Data
DRDY
Serial
I/O
tDATA
DATA
RATE
(Hz)
XIN CLOCK
FREQUENCY
(MHz)
TURBO
MODE
RATE
EFFECTIVE
RESOLUTION
(Bits rms)
FIGURE 4. Asynchronous ADS1212/13 Analog Input Volt-
age Step or ADS1213 Channel Change to Fully
Settled Output Data.
60
60
2
1
2
4
8
2
4
8
20
20
20
19
19
19
60
0.5
2
100
100
100
TURBO MODE
1
0.5
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
TABLE VI. Effective Resolution vs Data Rate, Clock
Frequency, and Turbo Mode Rate. (Gain set-
ting of 1.)
ADS1212, 1213
12
SBAS064A
BB [ BURR-BROWN CORPORATION ]
