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

2. AMC Overview

3. 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 + _
Analog Input ∫
1-Bit DAC
Comparator
Decimation
Filter
Digital
Interface
Serial/Parallel Bus
Clocking
AMC120X / ADS120X
AMC1210

4. Overview – Delta Sigma Modulation

5. Overview – A Brief Look at Modulators
Device Name
Resolution (More later...)
Input Range
Channels
Sample Rate
Reference?
Isolation?
ADS1201 24
Vref 1
1kSPS
Int / Ext No
ADS1202 16
+/- 320mV
40kSPS
Internal
ADS1203
ADS1204
+/- 250mV 4
ADS1205
+/- 2.5V 2
ADS1208
100mV
ADS1209
AMC1203
Int
Yes
AMC1204
78kSPS
AMC1201 ?
AMC1204B
78kSPS?
AMC1304
Family
* 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!
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