fXIN—The frequency of the crystal oscillator or CMOS
compatible input signal at the XIN input of the ADS1212/13.
NORMALIZED DIGITAL FILTER RESPONSE
0
–20
fMOD—The frequency or speed at which the modulator of the
ADS1212/13 is running, given by the following equation:
–40
–60
fXIN • Turbo Mode
fMOD
=
–80
128
–100
–120
–140
–160
fSAMP—The frequency or switching speed of the input
sampling capacitor. The value is given by the following
equation:
fXIN • Turbo Mode • Gain Setting
fSAMP
=
0
1
2
3
4
5
6
128
Frequency (Hz)
fDATA, tDATA—The frequency of the digital output data
produced by the ADS1212/13 or the inverse of this (the
period), respectively, fDATA is also referred to as the data rate.
FIGURE 1. Normalized Digital Filter Response.
FILTER RESPONSE
0
–20
–40
fXIN • Turbo Mode
1
fDATA
=
, tDATA =
–60
128• Decimation Ratio +1
fDATA
(
)
–80
–100
–120
–140
–160
Conversion Cycle—The term “conversion cycle” usually
refers to a discrete A/D conversion operation, such as that
performed by a successive approximation converter. As
used here, a conversion cycle refers to the tDATA time period.
However, each digital output is actually based on the modu-
lator results from the last three tDATA time periods.
0
50
100
150
200
250
300
Frequency (Hz)
FILTER RESPONSE
–40
–60
–80
–100
–120
–140
–160
DIGITAL FILTER
45
46
47
48
49
50
51
52
53
54
55
The digital filter of the ADS1212/13 computes the output
result based on the most recent results from the delta-sigma
modulator. The number of modulator results that are used
depend on the decimation ratio set in the Command Regis-
ter. At the most basic level, the digital filter can be thought
of as simply averaging the modulator results and presenting
this average as the digital output.
Frequency (Hz)
FIGURE 2. Digital Filter Response at a Data Rate of 50Hz.
FILTER RESPONSE
0
–20
–40
–60
–80
–100
–120
–140
–160
While the decimation ratio determines the number of modu-
lator results to use, the modulator runs faster at higher Turbo
Modes. These two items, together with the ADS1212/13
clock frequency, determine the output data rate:
0
50
100
150
200
250
300
Frequency (Hz)
FILTER RESPONSE
–40
–60
f
XIN • Turbo Mode
–80
fDATA
=
128• Decimation Ratio +1
(
)
–100
–120
–140
–160
Also, since the conversion result is essentially an average,
the data rate determines where the resulting notches are in
the digital filter. For example, if the output data rate is 1kHz,
then a 1kHz input frequency will average to zero during the
1ms conversion cycle. Likewise, a 2kHz input frequency
will average to zero, etc.
55
56
57
58
59
60
61
62
63
64
65
Frequency (Hz)
FIGURE 3. Digital Filter Response at a Data Rate of 60Hz.
If the effective resolution at a 50Hz or 60Hz data rate is not
adequate for the particular application, then power line fre-
quencies could still be rejected by operating the ADS1212/13
at 25/30Hz, 16.7/20Hz, 12.5/15Hz, etc. If a higher data rate
is needed, then power line frequencies must either be rejected
before conversion (with an analog notch filter) or after
conversion (with a digital notch filter running on the main
controller).
In this manner, the data rate can be used to set specific notch
frequencies in the digital filter response (see Figure 1 for the
normalized response of the digital filter). For example, if the
rejection of power line frequencies is desired, then the data
rate can simply be set to the power line frequency. Figures
2 and 3 show the digital filter response for a data rate of
50Hz and 60Hz, respectively.
ADS1212, 1213
11
SBAS064A
BB [ BURR-BROWN CORPORATION ]
