Datasheet

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SBAS312B − JANUARY 2005 − REVISED AUGUST 2007
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17
FILTER USAGE
The modulator generates only a bitstream, which does
not output a digital word like an A/D converter. In order
to output a digital word equivalent to the analog input
voltage, the bitstream must be processed by a digital
filter.
A very simple filter, built with minimal effort and
hardware, is the Sinc
3
filter:
H(z) +
ǒ
1 * z
−OSR
1 * z
−1
Ǔ
3
This filter provides the best output performance at the
lowest hardware size (for example, a count of digital
gates). For oversampling ratios in the range of 16 to
256, this is a good choice. All the characterizations in
the data sheet are also done using a Sinc
3
filter with an
oversampling ratio of OSR = 256 and an output word
length of 16 bits.
In a Sinc
3
filter response (shown in Figure 33 and
Figure 34), the location of the first notch occurs at the
frequency of output data rate f
DATA
= f
CLK
/OSR. The
–3dB point is located at half the Nyquist frequency or
f
DATA
/4. For some applications, it may be necessary to
use another filter type for better frequency response.
This performance can be improved, for example, by a
cascaded filter structure. The first decimation stage can
be a Sinc
3
filter with a low OSR and the second stage
a high-order filter.
For more information, see application note SBAA094,
Combining the ADS1202 with an FPGA Digital Filter for
Current Measurement in Motor Control Applications,
available for download at www.ti.com.
0
−
10
−
20
−
30
−
40
−
50
−
60
−
70
−
80
Gain (dB)
Frequency (kHz)
0 200 400 600 800 1000 1200 1400 1600
OSR = 32
f
DATA = 10MHz/32 = 312.5kHz
−
3dB: 81.9kHz
Figure 33. Frequency Response of Sinc
3
Filter
30k
25k
20k
15k
10k
5k
0
Output Code
Number of Output Clocks
0 5 10 15 20 25 30 35 40
OSR = 32
FSR = 32768
ENOB = 9.9 Bits
Settling Time =
3
×
1/f
DATA =9.6
µ
s
Figure 34. Pulse Response of Sinc
3
Filter
(f
MOD
= 10MHz)
(2)