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

Precision Data Converters

Kevin Duke

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AMC1210 - Overview

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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

ComparatorDecimation

FilterDigital

Interface

Clocking

Serial/Parallel Bus

AMC120X / ADS120X AMC1210

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Overview – Delta Sigma Modulation

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Overview – A Brief Look at Modulators

Device NameResolution

(More later...)Input Range Channels Sample Rate Reference? Isolation?

ADS1201 24 Vref 1 1kSPS Int / Ext No

ADS1202 16 +/- 320mV 1 40kSPS Internal No

ADS1203 16 +/- 320mV 1 40kSPS Internal No

ADS1204 16 +/- 250mV 4 40kSPS Int / Ext No

ADS1205 16 +/- 2.5V 2 40kSPS Int / Ext No

ADS1208 16 100mV 1 40kSPS Int / Ext No

ADS1209 16 +/- 2.5V 2 40kSPS Int / Ext No

AMC1203 16 +/- 320mV 1 40kSPS Int Yes

AMC1204 16 +/- 320mV 1 78kSPS Int Yes

AMC1201 16 +/- 250mV 1 ? Int Yes

AMC1204B 16 +/- 250mV 1 78kSPS? Int Yes

AMC1304 16 Family 1 ? Int / Ext Yes

* Devices in red are not yet released

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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

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Overview – Pinout & Basic Connections

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Overview – Basic Resolver Circuit

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Overview – Basic Current Shunt Circuit

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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

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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

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AMC1210 – Design Tips

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Design Tips – The Sinc Filter

• What is the sinc function?

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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’

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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

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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

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Design Tips – Calculating Bit Shift

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Design Tips – Calculating Bit Shift

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Design Tips – Calculating Bit Shift

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Design Tips – Calculating Bit Shift

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Design Tips – Calculating Bit Shift

• Should additional filtering be applied by the integrator, the filter parameters must be included in the previous calculations

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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

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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

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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

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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

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AMC1210 – Design Tricks

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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

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Design Tricks – Resolver Apps

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Design Tricks – Resolver Apps

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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!

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Design Tricks – Resolver Apps

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Design Tricks – Resolver Apps

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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