infineon AP32297 A/D Converter Supply and PCB User Guide

A/D Converter Supply and PCB Design Guideline
AP32297
AURIX™
TriCore™ 32-bit
Application Note

About this Document

Scope and purpose
This document describes details about PCB design and filter components at A/D converter supply pins. Some architectural differences between the Infineon AUDO™ and AURIX™ family of products require special care when connecting the A/D converter module to the PCB.
The AUDO™ Family is a single core architecture with CPU frequencies up to 180MHz and has a ground concept with separated ground pins for digital and analog modules. The AURIX™ 32 bit microcontrollers have a multi core architecture with up to 300MHz CPU frequency with an integrated cross bar for providing this high performance.

To get more signal pins for AURIX™ 32 bit microcontrollers and because of noise implications, there is only one common internal ground for digital and analog modules. This common ground is realized with an EPad for QFP packages and with a center Vss ball block for BGA packages. Because of the high performance AURIX™ microcontrollers draw a high core supply current. The resulting high ground current in combination with the common GND concept can increase GND noise compared to the AUDO™ Family. Therefore AURIX™ 32 bit microcontrollers require special care in respect to the analog supply and ground concept to get a good noise performance from the A/D converter. When using DC/DC converters for analog supply voltage this also has to be considered for filter components at analog supply and input voltages.
The resolution of a high performance A/D converter is smaller than 1 mV. Care must be taken in the system setup otherwise system noise is measured instead of small signal levels.
The document is split into the following sections:

• Analog Supply Voltage Filter Calculation (VDDM)
• Reference Supply Voltage Filter Calculation (VAREF)
• Analog Input Filter Calculation (ANx)
• PCB and Design Guidelines

Analog Supply Voltage Filter Calculation (VDDM)

The analog supply domain is separated from the main EVR supply domain and can be supplied by separate external regulators or trackers. A mixed supply scheme is possible with a 5 V A/D converter domain (VDDM = VAREFx = 5 V) and the remaining system running on 3.3 V supply (VEXT = VDDP3 = 3.3 V). When sourcing the analog supply voltage VDDM from a DC/DC converter, special care has to be taken to the analog supply filter. In this case the analog supply filter cutoff frequency has to be below the DC/DC converter switching frequency to suppress switching noise. A maximum noise amplitude of about 20 mV is allowed to met the specified A/D converter error values.

1.1 How to calculate an analog Supply Voltage Filter at VDDM?
A low pass RC filter is recommended for the analog supply voltage VDDM as shown in Figure 1. When supplying VDDM from a DC/DC converter a RC filter is essential to prevent high noise at the A/D converter results.

Steps for filter calculation

• Determine maximum analog supply rms current IDDM for every single VDDM pin.
  Depending on device type more VDDM pins are provided.

• Check reference voltage maximum specification in the Data Sheet.
  A typical value is: VAREF_max = VDDM + 50 mV.
  This allows a maximum voltage drop at the filter resistor RDDM of VRDDM_max = 50 mV.

•  Switching frequency fDC/DC of a DC/DC converter has to be suppressed.
  The cutoff frequency of the RC filter has to be below the DC/DC converter switching frequency.

• Calculate RDDM_MAX value and select appropriate resistor value from E24 table.

• Calculate CDDM_MIN value and select appropriate capacitor value from E24 table.
  Note: Select filter component tolerances to the application demands.
  Typical Electronic Industries Alliance (EIA) tolerances are: E12: 10% tolerance, E24: 5% tolerance, E48: 2% tolerance, E96: 1% tolerance

Figure 1 Block Diagram of the Analog Supply RC Filter at VDDM
1.2 Calculation Example
Reference Supply Voltage Filter Calculation (VAREF)

Reference Supply Voltage Filter Calculation (VAREF)

The reference voltage input VAREF is switched to parts of the internal switched capacitor field CAREFSW while the charge redistribution phase of a conversion is performed. A stable and noise-free reference voltage is required to get accurate conversion results. The A/D converter result is more sensitive to noise on VAREF than to noise on VDDM because of the direct VAREF influence to the conversion result via the internal capacitor field. Details of the required filter are shown in Figure 3.

Requirements to the reference supply voltage for filter calculation

• Provide maximum required rms current at VAREF
• Provide charge demand for internal switching capacitors
• Suppress DC/DC converter switching noise fDC/DC
• Suppress general system noise and/or white noise

Reference Supply Voltage Filter Calculation (VAREF)
2.1 How to calculate an analog Reference Supply Voltage Filter at VAREF?
Steps for filter calculation

• Calculate RAREF_max
  – Determine maximum reference rms current IAREF for every single VAREF pin.
  Depending on device type more VAREF pins are available
  – Calculate reference current per VADC for a fixed conversion rate via specified charge per conversion QCONV
  – Determine maximum reference supply current IAREF for every single VAREF pin via number of connected VADC and DSADC modules
  – Define maximum allowed voltage error at RAREF with the assumption: VRAREF = LSB12 / 2
  – Calculate maximum value for resistor RAREF and select appropriate resistor value from E24 table

• Calculate CAREF_min
  – Calculate precharge factor (precharge feature enabled)
  – Define maximum allowed charging error CAREFSW

•  Calculate filter cutoff frequency
  – Check whether calculated values of RAREF and CAREF do suppress DC/DC converter switching noise frequency fDC/CD

• Adapt CAREF to suppress system noise
  – Using the maximum accuracy of a 12 bit A/D converter requires not only filtering DC/DC converter switching noise but also white noise and deterministic noise from the system at pin VAREF

Figure 3 Block Diagram of the Analog Reference Supply RC Filter at VAREF

2.2 Calculation Example

Reference Supply Voltage Filter Calculation (VAREF)

A minimum buffer capacitance CAREF = 270 nF is necessary to provide enough charge to the internal conversion capacitor net.

2.2.3 Calculate RAREF /CAREF Filter Cutoff Frequency
This cutoff frequency suppresses the DC/DC converter switching noise frequency of fDC/DC = 480kHz.
2.2.4 Adapt CAREF to suppress System Noise

Using the maximum accuracy of a 12 bit A/D converter requires filtering the DC/DC converters switching noise and providing enough current for ADC conversion at pin VAREF. The RC filter circuit at the reference input voltage also has to suppress white noise and deterministic noise from the system which is higher than the A/D converter resolution. Therefore the calculated value of CAREF has to be increased to a higher value to get a lower cutoff frequency.
A filter selection with a cutoff frequency of about 10kHz is a recommended setting. It has typically sufficient attenuation to suppress system noise in a system.

Using a VAREF filter circuit with RAREF = 6.8Ω and CAREF = 2.2µF has sufficient attenuation to suppress system noise in a typical application. The filter curve is shown in Figure 4. In case of increased noise on the A/D converter results a further cutoff frequency decreasing can help to suppress this noise. This is typically done by increasing the value of CAREF. The ESR (Equivalent Series Resistance) value of the uses ceramic capacitors should be as low as possible.
Note: Increasing RAREF also decreases the cutoff frequency, but this can cause a certain additional gain error to the result because of the increased voltage drop at RAREF.

Analog Input Filter Calculation (ANx)

During the sample phase, the conversion control unit connects the capacitors of the conversion C-net to an analog input channel via a multiplexer. The internal switched capacitor field CAINSW is then charged or discharged to the voltage level of the connected analog input channel.
In typical applications the external capacitance value has to be high enough that the total charge, which is necessary to load the internal capacitor field CAINSW of the A/D converter, is provided by the external capacitor CEXT. The resistance of the analog source has to consider the cycle time, the duration from the start of a conversion to the next conversion start of the same analog channel, and protection of the input in case of an overload situation. The details for how to calculate the circuit at the analog input are described in the application note AP56003 “A Guide to the Analog Part of the A/D converter”.
Because of the common ground concept of the AURIX™ Family some of the periodic system noise can be distributed also to the “analog” ground area of the PCB. If there is no appropriate filter circuit at the analog input, this noise can cause an increased Offset Error!
That is why the calculated external filter circuit based on the charging demands also has to be checked whether it fulfils the Nyquist-Shannon sampling theorem. The filter at the analog input needs a time constant of at least two times the sample time of the considered channel.
Due to the stopband attenuation of a passive low-pass filter, it is recommended to use a time constant τfilter of the external bandwidth limitation filter of at least four times higher than the configured sampling time of the A/D converter. The details are shown in the block diagram below.

Analog Input Filter Calculation (ANx)
3.1 How to calculate a Noise Filter at the ANx?
The time constant τANx of the external analog input circuitry calculated on the base of AP56003 has to fulfil these conditions:

Note: In typical applications the external analog input filter fulfils the Nyquist-Shannon sampling theorem.
Only systems using external op-amps sometimes leave the external capacitor CEXT. In these cases an external capacitance CEXT is required to suppress noise at the analog input. If the external input capacitance is in the range of CEXT < (2r * CAINSW) then the resistance of the analog source and the sampling time have to be adjusted such that the internal capacitor field CAREFSW is charged to a sufficient accuracy. Details are described in AP56003.

3.2 Calculation Example
The assumed values for this example:

Filter Circuit Calculation Example Results Summary

The recommended settings from the example are:

The filter block diagram and the filter curves are shown in the figures below:

PCB and Design Guidelines

Here we provide a short introduction to mixed signal board design and offer a list of guidelines for optimum printed circuit board layout for AURIX™ microcontrollers with special consideration for the A/D converter. A general system wiring of the supply pins is shown in Figure 9.

5.1 Ground Plane
Inside the AURIX™ microcontrollers the analog ground and the digital ground are already shorted on-chip.
Therefore the PCB ground plane is no more separated in an analog and a digital part. AURIX™ microcontrollers require only one common ground plane for analog and digital signals on the PCB.
Note: This common ground plane is the most important reference point in the system!
The ground node is an impedance where offsets are caused by static and dynamic currents. This voltage offsets have to be smaller than the desired target resolution of the A/D converter.
Guidelines for the ground plane

• Keep the ground plane impedance as low as possible.

PCB and Design Guidelines

• All chip grounds, analog and digital, have to be connected to the PCB common ground plane.
• The sum of all injected currents to the ground plane is known and must be checked whether it fits to the ground plane impedance.
• Remove noisy system parts from the PCB or reduce ground plane impedance.

5.2 Component Placing
Guidelines:

• Partition the board with all analog components together in one area and all digital components in the other. An example is shown in Figure 10.
• Analog and digital signals have to be separated with sufficient spacing between to prevent noise coupling.
• Shielding of sensitive analog signals can suppress noise coupling.
• Mixed signal components, including the microcontroller, should only bridge the analog and digital areas. Rotating the microcontroller can often make this task easier.
• The microcontroller has to be soldered on the PCB. Do not use a socket!
• Minimize PCB complexity to keep the PCB to a small size.
• Keep all traces as short as possible.

5.3 Power Supply and Supply Voltages
Guidelines:

• Place de-coupling capacitors as close as possible to the microcontroller pins, or positioned for the shortest connection to pins with wide traces to reduce impedance.
• If both large and small ceramic capacitors are recommended, position the small ceramic capacitor closest to the microcontroller pins.
• Use capacitors with small ESR values.
• Use separate power supplies for analog and digital modules.
• Prevent power supply noise injection to the GND plane (i.e. switched mode power supplies).
• Use high quality connectors to connect the PCB to an external system to ensure high quality input signals.
• Connections to external system should be as short as possible.

5.4 Signal Traces
Guidelines:

• Analog signal traces are placed in the analog area.
• Digital signal traces are placed in the digital area.
• Regions between analog signal traces and also regions between digital signal traces should be filled with copper, which should be electrically attached to the common ground plane. These regions should not be left floating as this increases interference.

5.5 Clock Generation
Guidelines:

• Locate the quartz crystal, ceramic resonator or external oscillator as close as possible to the microcontroller.
• Use a ground island for oscillator load capacitor ground which is directly connected to oscillator ground.
• Keep digital signal traces, especially the clock signal, as far away as possible from the analog input and voltage reference pins.
• Avoid multiple oscillators or asynchronous clocks. Best results are obtained when all circuits are synchronous to the A/D converter sampling clock.

Trademarks of Infineon Technologies AG
AURIX™, C166™, CanPAK™, CIPOS™, CIPURSE™, CoolGaN™, CoolMOS™, CoolSET™, CoolSiC™, CORECONTROL™, CROSSAVE™, DAVE™, DI-POL™, DrBLADE™, EasyPIM™, EconoBRIDGE™, EconoDUAL™, EconoPACK™, EconoPIM™, EiceDRIVER™, eupec™, FCOS™, HITFET™, HybridPACK™, ISOFACE™, IsoPACK™, iWafer™, MIPAQ™, ModSTACK™, my-d™, NovalithIC™, OmniTune™, OPTIGA™, OptiMOS™, ORIGA™, POWERCODE™, PRIMARION™, PrimePACK™, PrimeSTACK™, PROFET™, PRO-SIL™, RASIC™, REAL3™, ReverSave™, SatRIC™, SIEGET™, SIPMOS™, SmartLEWIS™, SOLID FLASH™, SPOC™, TEMPFET™, thinQ!™, TRENCHSTOP™, TriCore™.
Other Trademarks
Advance Design System™ (ADS) of Agilent Technologies, AMBA™, ARM™, MULTI-ICE™, KEIL™, PRIMECELL™, REALVIEW™, THUMB™, µVision™ of ARM Limited, UK. ANSI™ of American National Standards Institute. AUTOSAR™ of AUTOSAR development partnership. Bluetooth™ of Bluetooth SIG Inc. CAT-iq™ of DECT Forum. COLOSSUS™, FirstGPS™ of Trimble Navigation Ltd. EMV™ of EMVCo, LLC (Visa Holdings Inc.). EPCOS™ of Epcos AG. FLEXGO™ of Microsoft
Corporation. HYPERTERMINAL™ of Hilgraeve Incorporated. MCS™ of Intel Corp. IEC™ of Commission Electrotechnique Internationale. IrDA™ of Infrared Data
Association Corporation. ISO™ of INTERNATIONAL ORGANIZATION FOR STANDARDIZATION. MATLAB™ of MathWorks, Inc. MAXIM™ of Maxim Integrated Products, Inc. MICROTEC™, NUCLEUS™ of Mentor Graphics Corporation. MIPI™ of MIPI Alliance, Inc. MIPS™ of MIPS Technologies, Inc., USA. muRata™ of MURATA MANUFACTURING CO., MICROWAVE OFFICE™ (MWO) of Applied Wave Research Inc., OmniVision™ of OmniVision Technologies, Inc. Openwave™ of Openwave Systems Inc. RED HAT™ of Red Hat, Inc. RFMD™ of RF Micro Devices, Inc. SIRIUS™ of Sirius Satellite Radio Inc. SOLARIS™ of Sun Microsystems, Inc. SPANSION™ of Spansion LLC Ltd. Symbian™ of Symbian Software Limited. TAIYO YUDEN™ of Taiyo Yuden Co. TEAKLITE™ of CEVA, Inc. TEKTRONIX™ of Tektronix Inc. TOKO™ of TOKO KABUSHIKI KAISHA TA. UNIX™ of X/Open Company Limited. VERILOG™, PALLADIUM™ of Cadence Design Systems, Inc. VLYNQ™ of Texas Instruments Incorporated. VXWORKS™, WIND RIVER™ of WIND RIVER SYSTEMS, INC. ZETEX™ of Diodes Zetex Limited.
Trademarks Update 2014-07-17

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Edition 2015-04
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Document reference
AP32297

References

• Semiconductor & System Solutions - Infineon Technologies

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