SBAA666 February   2025 AMC0106M05 , AMC0106M25

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Design Challenges
  6. 3Design Approach
    1. 3.1 AMC0106Mxx Functionally Isolated Modulators
    2. 3.2 Circuit Design and Layout
    3. 3.3 Sinc3 Filter Design
  7. 4Test and Validation
    1. 4.1 Test Setup
    2. 4.2 Digital Interface
    3. 4.3 DC Accuracy, Noise, and Effective Number of Bits
    4. 4.4 PWM Rejection
      1. 4.4.1 DC Phase Current Measurement Over One PWM Cycle
      2. 4.4.2 AC Phase Current Measurement at 100kHz PWM
    5. 4.5 Bootstrap Supply Validation and AVDD Ripple Rejection Tests
      1. 4.5.1 LMG2100R044 Bootstrap Supply With Low Voltage-Ripple
      2. 4.5.2 Discrete Bootstrap Supply With High Voltage-Ripple
  8. 5Summary
  9. 6References

3.3 Sinc3 Filter Design

For testing, a sinc3 filter was implemented on a TMS320F28379D Real-Time Microcontroller. A sinc3 filter is a finite impulse response (FIR) filter with a constant propagation delay (group delay). The propagation delay depends on the sinc filter order, the sample clock frequency, and the oversampling ratio (OSR). For example, a sinc3 filter with an 20MHz sample clock frequency and an oversampling ratio of 64 has a propagation delay of 4.8µs and a cut-off frequency f-3dB of approximately 80kHz. The corresponding magnitude response and sampling window are shown in Figure 3-6 and Figure 3-7, respectively.

Magnitude Response, fmod = 20MHz, sinc3, OSR=64Figure 3-6 Magnitude Response, fmod = 20MHz, sinc3, OSR=64
Sinc3 OSR64 Filter CoefficientsFigure 3-7 Sinc3 OSR64 Filter Coefficients

An advantage of the delta-sigma approach is that short-circuit detection can be derived from the same modulator bit-stream by implementing a second decimation filter with a lower OSR and therefore lower latency. For example, a sinc3 filter with an OSR of 8 running at a 20MHz sample rate, has a over-current response time (settling time) of only 1.2µs, as shown in Figure 3-8.

Step Response of a Sinc3 OSR 8 Filter Figure 3-8 Step Response of a Sinc3 OSR 8 Filter

For more details on how to implement sinc filters on a C2000™ microcontroller, see Sigma Delta Filter Module (SDFM).