Datasheet

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SBAS312B − JANUARY 2005 − REVISED AUGUST 2007
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18
The effective number of bits (ENOB) can be used to
compare the performance of ADCs and delta-sigma
modulators. Figure 35 shows the ENOB of the
ADS1205 with different filter types. In this data sheet,
the ENOB is calculated from the SNR:
SNR = 1.76dB + 6.02dB
×
ENOB
1 10 100 1000
OSR
ENOB (Bits)
16
14
12
10
8
6
4
2
0
Sincfast
Sinc3
Sinc
Sinc2
Figure 35. Measured ENOB vs OSR
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
resolution. The time for full settling is dependent on the
filter order. Therefore, the full settling of the Sinc
3
filter
needs three data clocks and the Sinc
2
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 Sinc
3
might be a better choice. A
simple example is a Sinc
2
filter. The Sincfast is a
modified Sinc
2
filter:
H(z) +
ǒ
1 * z
−OSR
1 * z
−1
Ǔ
2
ǒ
1 ) z
−2 OSR
Ǔ
Figure 36 compares the settling time of different filter
types operating with a 10MHz modulator clock.
02 68410
Settling Time (
µ
s)
ENOB (Bits)
10
9
8
7
6
5
4
3
2
1
0
Sincfast
Sinc3
Sinc
Sinc2
Figure 36. Measured ENOB vs Settling Time
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.
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