3 Overview – What the heck does it do? A four channel digital filter for delta-sigma modulatorsIsolated current shunt & resolver applications with AMC120XFlexible filter configuration for use with ADS120XTypical Delta-Sigma ADC Block Diagram+_Analog Input∫1-Bit DACComparatorDecimationFilterDigitalInterfaceSerial/Parallel BusClockingAMC120X / ADS120XAMC1210
5 Overview – A Brief Look at Modulators Device NameResolution (More later...)Input RangeChannelsSample RateReference?Isolation?ADS120124Vref11kSPSInt / ExtNoADS120216+/- 320mV40kSPSInternalADS1203ADS1204+/- 250mV4ADS1205+/- 2.5V2ADS1208100mVADS1209AMC1203IntYesAMC120478kSPSAMC1201?AMC1204B78kSPS?AMC1304Family* 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 & softwareAMC120X/ADS120X EVM – Very small DB9 connector evaluation modules featuring just the modulator and footprints for decoupling/filtering passivesMATLAB & DOS Pattern Generators for the Signal GeneratorAMC1210 In Motor Control Applications Application Report
10 Overview – Register Overview General Registers:Control: Pin polarity, interrupt enable, depth of patternPattern Generator: Shift register for pattern generatorClock Divider: Filter enable, phase calibration, signal generator control, modulator clock frequencyFilter Registers:Control: Modulator clocking options, sample-and-holdSinc Filter: Filter architecture, oversampling ratioIntegrator: Bit-shift, data-format, demodulation, oversampling ratioThresholds: High and Low thresholds used by the comparatorComparator: Flag enables, comparator structure, oversampling ratio
11 Overview – Common Applications Resolver / Motor Control:Isolation isn’t completely necessary, ADS120X devices fit wellFilter to filter and filter to excitation synchronization is criticalWhat’s a resolver?Considered the ‘true analog’ counter-part to ‘digital’ encodersSystem of 3 windings; a primary or ‘excitor’ winding and two secondary windings placed 90 degrees out of phaseCurrent Shunts:Isolation is important, AMC120X devices fit wellDigital comparator accommodates for alarm conditions common in current shunt monitorsGeneral Data-Converter:Flexible digital filter capable of fitting to a variety of applications
13 Design Tips – The Sinc Filter What is the sinc function?
14 Design Tips – The Sinc Filter ‘Sinc Filter’ can be used in two contextThe 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 oversamplingFirst, determine the possible values output by the filter unit by examining the oversampling ratio and sinc filter structure:Sinc1: - x to xSinc2: - x2 to x2Sinc3: - x3 to x3Sincfast: - 2x2 to 2x2Next, determine the number of bits required to represent those values, taking care to include the sign bitSinc1: log2(x) + 1Sinc1: log2(x2) + 1Sinc1: log2(x3) + 1Sincfast: log2(2x2) + 1Finally, apply integer truncation and the appropriate rounding then subtract 16 to calculate the shifts required
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-converterVref/(2(bits-1) -1)Where bits is precisely the number of bits of data recovered from the deviceIf this is greater than 16, the value should be truncated to 16 bitsIf 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 datasheetThe frequency data will be produced from the sinc filter can be expressed as:FData_Sinc = FModulator / SOSRSimilarly, the frequency data will be produced from the integrator filter (if active) can be expressed as:FData_Integrator = FData_Sinc / IOSRThe 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 metA typical resolver application synchronizes the frequency of the carrier signal with the frequency of the motor control loop, usually between 8- 20kHzThe 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 coilsOur resolver on hand required a relatively high frequency carrier: 16kHzSharing 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 carrierfCLK = 32MHzNCDIV = 2NPAT = 1000SOSR = 125IOSR = 8N = 2
27 Design Tricks – Resolver Apps Synchronicity is absolutely key for a successful resolver applicationA synchronous sinc filter enable is possibleMFE bit in the Clock Divider RegisterIndividual filter enable bits in each Sinc Filter registerResolver applications, however, also require utilizing the integrator filterThe integrator filter becomes active and starts integrating as soon as it is enabled and it sees clocks from the modulatorThere is no synchronous reset for the integrator filtersSolution:Stop the system clocks for the AMC1210 and ADS1205 until we are ready to convertIssue a reset between acquisition blocks before writing registers
30 Design Tricks – Resolver Apps Just behind synchronization in importance is minimizing zero crossing error during phase calibrationPhase calibration has a small chance to fail if the signal that phase calibration is performed against is too weak in magnitudeSolution: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 processThe good news is...once the device is up and running the position data is reliable and exhibits no phase inversion issues!