ADS1208
www.ti.com
SBAS348A–MARCH 2005–REVISED MARCH 2005
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
16
14
12
10
8
256, the Sinc3 filter is
a
good choice. All
Sincfast
characterizations in this datasheet were obtained
using a Sinc3 filter with an oversampling ratio (OSR)
of 256 and an output word length of 16 bits. In a
Sinc3 filter response (shown in Figure 38 and Fig-
ure 39), the location of the first notch occurs at the
frequency of output data rate fDATA = fCLK/OSR. The
–3dB point is located at half the Nyquist frequency, or
fDATA/4. For some applications, it may be necessary
to use another filter type for better frequency re-
sponse. Device performance can be improved, for
example, by using a cascaded filter structure. The
first decimation stage can be a Sinc3 filter with a low
OSR and a second stage, high-order filter.
Sinc2
Sinc1
6
Sinc3
4
2
0
1
10
100
1000
Decimation Ratio (OSR)
Figure 40. Measured ENOB vs OSR
0
In motor control applications, a very fast response
time for overcurrent detection is required. There is a
constraint between 1µs and 5µs with 3 bits to 7 bits
of resolution. The time for full settling depends on the
filter order. Therefore, the full settling of the Sinc3
filter needs three data clocks and the Sinc2 filter
needs two data clocks. The data clock is equal to the
modulator clock divided by the OSR. For overcurrent
protection, filter types other than Sinc3 might be a
better choice. A good example is a Sinc2 filter.
Figure 41 compares the settling time of different filter
types. The Sincfast is a modified Sinc2 filter, as
shown in Equation 2:
OSR = 32
fDATA = 10MHz/32 = 312.5kHz
−
−
−
−
−
−
−
−
10
20
30
40
50
60
70
80
−
3dB: 81.9kHz
0
200
400
600
800 1000 1200 1400 1600
Frequency (kHz)
2ǒ
Ǔ
1*z*OSR
1)z*2@OSR
H(z) + ǒ
Ǔ
Figure 38. Frequency Response of Sinc3 Filter
(2)
10
9
Sinc3
30k
OSR = 32
FSR = 32768
ENOB = 9.9 Bits
8
25k
Sincfast
7
Sinc2
Settling Time =
20k
6
×
µ
s
3
1/fDATA = 9.6
5
15k
10k
5k
4
Sinc
3
2
1
0
0
0
1
2
3
4
5
6
7
8
9
10
0
5
10
15
20
25
30
35
40
µ
Settling Time ( s)
Number of Output Clocks
Figure 41. Measured ENOB vs Settling Time
Figure 39. Pulse Response of Sinc3 Filter
(fMOD = 10MHz)
For more information, see application note SBAA094,
Combining the ADS1202 with an FPGA Digital Filter
for Current Measurement in Motor Control Appli-
cations, available for download at www.ti.com.
The effective number of bits (ENOB) can be used to
compare the performance of ADCs and delta-sigma
modulators. Figure 40 shows the ENOB of the
ADS1208 with different filter types. In this datasheet,
the ENOB is calculated from the SNR:
SNR = 1.76dB + 6.02dB × ENOB
18
BB [ BURR-BROWN CORPORATION ]
