AMC1305L25 Datasheet, PDF (22/39 Pages) Texas Instruments – AMC1305x High-Precision, Reinforced Isolated Delta-Sigma Modulators

AMC1305L25, AMC1305M05, AMC1305M25
SBAS654C – JUNE 2014 – REVISED DECEMBER 2014
10 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
10.1 Application Information
10.1.1 Digital Filter Usage
The modulator generates a bit stream that is processed by a digital filter to obtain a digital word similar to a
conversion result of a conventional analog-to-digital converter (ADC). A very simple filter, built with minimal effort
and hardware, is a sinc³-type filter, as shown in Equation 2:
H (z)
¨¨©§
1  z ^OSR
1  z ^1
¸¸¹·³
(2)
This filter provides the best output performance at the lowest hardware size (count of digital gates) for a second-
order modulator. All the characterization in this document is also done with a sinc³ filter with an over-sampling
ratio (OSR) of 256 and an output word width of 16 bits.
The effective number of bits (ENOB) is often used to compare the performance of ADCs and ΔΣ modulators.
Figure 50 illustrates the ENOB of the AMC1305 with different oversampling ratios. In this document, this number
is calculated from the SNR by using Equation 3:
SNR 1.76dB  6.02dB* ENOB
(3)
16
14
12
10
8
6
4
sinc1
2
sinc2
sinc3
0
1
10
100
1000
OSR
D053
Figure 50. Measured Effective Number of Bits versus Oversampling Ratio
An example code for an implementation of a sinc³ filter in an FPGA, see the application note Combining
ADS1202 with FPGA Digital Filter for Current Measurement in Motor Control Applications (SBAA094), available
for download at www.ti.com.
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