AMC1210 Quad Digital Filter – Overview, Design Tips, & Tricks

1. AMC1210 Quad Digital Filter – Overview, Design Tips, & Tricks Precision Data Converters Kevin Duke

2. AMC1210 - Overview

3. Analog Input + _ ∫ Comparator 1-Bit DAC Overview – What the heck does it do? • A four channel digital filter for delta-sigma modulators • Isolated current shunt & resolver applications with AMC120X • Flexible filter configuration for use with ADS120X Typical Delta-Sigma ADC Block Diagram Decimation Filter Digital Interface Serial/Parallel Bus Clocking AMC120X / ADS120X AMC1210

4. Overview – Delta Sigma Modulation

5. Overview – A Brief Look at Modulators * Devices in red are not yet released

6. Overview – Available Collateral & EVMs • AMC1210EVM – ‘Modular’ EVM with 4-channel ADS1204 on board & supporting circuitry. No TI software support. • AMC1210MB-EVM – ‘Motherboard’ EVM with 2-channel ADS1205 on board, supporting circuitry, connectors for AMC120X/ADS120X EVMs, resolver connector & software • AMC120X/ADS120X EVM – Very small DB9 connector evaluation modules featuring just the modulator and footprints for decoupling/filtering passives • MATLAB & DOS Pattern Generators for the Signal Generator • AMC1210 In Motor Control Applications Application Report

7. Overview – Pinout & Basic Connections

8. Overview – Basic Resolver Circuit

9. Overview – Basic Current Shunt Circuit

10. Overview – Register Overview • General Registers: • Control: Pin polarity, interrupt enable, depth of pattern • Pattern Generator: Shift register for pattern generator • Clock Divider: Filter enable, phase calibration, signal generator control, modulator clock frequency • Filter Registers: • Control: Modulator clocking options, sample-and-hold • Sinc Filter: Filter architecture, oversampling ratio • Integrator: Bit-shift, data-format, demodulation, oversampling ratio • Thresholds: High and Low thresholds used by the comparator • Comparator: Flag enables, comparator structure, oversampling ratio

11. Overview – Common Applications • Resolver / Motor Control: • Isolation isn’t completely necessary, ADS120X devices fit well • Filter to filter and filter to excitation synchronization is critical • What’s a resolver? • Considered the ‘true analog’ counter-part to ‘digital’ encoders • System of 3 windings; a primary or ‘excitor’ winding and two secondary windings placed 90 degrees out of phase • Current Shunts: • Isolation is important, AMC120X devices fit well • Digital comparator accommodates for alarm conditions common in current shunt monitors • General Data-Converter: • Flexible digital filter capable of fitting to a variety of applications

12. AMC1210 – Design Tips

13. Design Tips – The Sinc Filter • What is the sinc function?

14. Design Tips – The Sinc Filter • ‘Sinc Filter’ can be used in two context • The idealized low-pass filter represented by the sinc function in time and a rectangular function in frequency, so dubbed ‘sinc-in-time’ • The cascaded integrator-comb filter represented by a rectangular function in time and a sinc function in frequency, so dubbed ‘sinc-in-frequency’

15. Design Tips – The Sinc Filter • Oversampling is inherently associated with the decimation structure of a CIC filter. Increasing this oversampling ratio can yield increased resolution

16. Design Tips – Calculating Bit Shift • Only necessary for 16-bit data format as set in the integrator register, both 16 and 32 bit data formats are Binary Two’s Complement. These calculations & figures assume no integrator oversampling • First, determine the possible values output by the filter unit by examining the oversampling ratio and sinc filter structure: • Sinc1: - x to x • Sinc2: - x2 to x2 • Sinc3: - x3 to x3 • Sincfast: - 2x2 to 2x2 • Next, determine the number of bits required to represent those values, taking care to include the sign bit • Sinc1: log2(x) + 1 • Sinc1: log2(x2) + 1 • Sinc1: log2(x3) + 1 • Sincfast: log2(2x2) + 1 • Finally, apply integer truncation and the appropriate rounding then subtract 16 to calculate the shifts required

17. Design Tips – Calculating Bit Shift

18. Design Tips – Calculating Bit Shift

19. Design Tips – Calculating Bit Shift

20. Design Tips – Calculating Bit Shift

21. Design Tips – Calculating Bit Shift • Should additional filtering be applied by the integrator, the filter parameters must be included in the previous calculations

22. Design Tips – Calculating LSB Weight • Almost the same as any other data-converter • Vref/(2(bits-1) -1) • Where bits is precisely the number of bits of data recovered from the device • If this is greater than 16, the value should be truncated to 16 bits • If this is less than 16, the value may be fractional even though fractional bits cannot exist

23. Design Tips – Calculating Data Rate • Calculating data-rate from the AMC1210 is straight forward, but not explicit in the datasheet • The frequency data will be produced from the sinc filter can be expressed as: • FData_Sinc= FModulator/ SOSR • Similarly, the frequency data will be produced from the integrator filter (if active) can be expressed as: • FData_Integrator= FData_Sinc/ IOSR • The data rate equation can be simplified to: • FData= FModulator/( SOSR * IOSR ) • Sinc1,Sinc2,Sinc3, and Sincfast architectures each take the same amount of time to produce data

24. Design Tips – Resolver Applications • Resolver applications have specific timing requirements related to the filter parameters that must be met • A typical resolver application synchronizes the frequency of the carrier signal with the frequency of the motor control loop, usually between 8-20kHz • The carrier signal frequency can be defined by: • A data converter in a resolver application typically produces a conversion result once per cycle of the carrier signal

25. Design Tips – AMC1210MB-EVM Example • Resolvers come with frequency specifications related to the filtering behavior of the resolver coils • Our resolver on hand required a relatively high frequency carrier: 16kHz • Sharing a 32MHz clock source for the AMC1210 and the ADS1205 sets the ADS1205 near it’s maximum bit-rate of 16.5MHz and is an easy frequency to start from to achieve a 16kHz carrier • fCLK= 32MHz • NCDIV= 2 • NPAT= 1000 • SOSR = 125 • IOSR = 8 • N = 2

26. AMC1210 – Design Tricks

27. Design Tricks – Resolver Apps • Synchronicity is absolutely key for a successful resolver application • A synchronous sinc filter enable is possible • MFE bit in the Clock Divider Register • Individual filter enable bits in each Sinc Filter register • Resolver applications, however, also require utilizing the integrator filter • The integrator filter becomes active and starts integrating as soon as it is enabled and it sees clocks from the modulator • There is no synchronous reset for the integrator filters • Solution: • Stop the system clocks for the AMC1210 and ADS1205 until we are ready to convert • Issue a reset between acquisition blocks before writing registers

28. Design Tricks – Resolver Apps

29. Design Tricks – Resolver Apps

30. Design Tricks – Resolver Apps • Just behind synchronization in importance is minimizing zero crossing error during phase calibration • Phase calibration has a small chance to fail if the signal that phase calibration is performed against is too weak in magnitude • Solution: • Collect a brief burst of data on both sine and cosine components, then perform phase calibration on whichever signal is farthest from ground (positive or negative) • Monitor for failure during phase calibration with some time-out case, should it fail reset the AMC1210 and re-iterate through the initialization process • The good news is...once the device is up and running the position data is reliable and exhibits no phase inversion issues!

31. Design Tricks – Resolver Apps

32. Design Tricks – Resolver Apps

33. Remaining Items of Curiosity... • For any further questions don’t hesitate to make a forum post! • e2e.ti.com/support/data_converters/precision_data_converters/default.aspx