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
+
_
∫
Comparator
Decimation
Filter
Digital
Interface
Serial/Parallel Bus
Clocking
1-Bit DAC
AMC120X / ADS120X
AMC1210
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Overview – Delta Sigma Modulation
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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
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 820kHz
• 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
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