23-08-2012, 02:49 PM
Analog-to-Digital Converter Design Guide
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SELECTING THE RIGHT ADC
Selecting the most suitable A/D converter (ADC) for your application is based on more than just the precision or bits. Different architectures are available, each exhibiting advantages and disadvantages in various data-acquisition systems. The required accuracy or precision of the system puts you in a category based on the number of bits required. It is important to always design your system to allow for more bits than initially required: if an application calls for 10 bits of accuracy, choose a 12-bit converter. The achievable accuracy of a converter will always be less than the total number of bits available.
Depending on the system requirements, your accuracy might be better expressed in micro-volts, decibels or LSBs (least significant bits). A FFT showing the frequency spectrum of a device can be useful in determining the noise performance of a given device. All Microchip stand-alone ADCs show typical performance data for AC specifications, such as THD, SINAD and SNR. The following table shows performance, in dB and V/V, for 8- through 24-bit converters.
Two Application Examples:
Seismic Recording16 different devices are connected to a central processing unit to monitor vibrations. Each device measures the signal at an extremely fast rate, less than 100 μS. High accuracy is not required due to the large signal size, but speed is of the utmost importance. A high-speed SAR converter would be the best selection for this application.
Voice-Band RecordingThe human ear can detect signals from roughly 20 Hz up to 20 kHz. If the application is a telephone intercom system where high-fidelity audio is not a concern, 60-70 dB of dynamic range is sufficient. Based on these bandwidth and dynamic range requirements, either a medium speed (50-200 ksps) SAR or delta-sigma converter would work in this application.
DEVELOPMENT TOOLS
The MXLAB software tool provides data acquisition, analysis and display in a Windows®system environment. Additionally, analysis can be made of the digital potentiometer shutdown, reset and daisy-chain operations. The MXLAB software can determine digital potentiometer settings based on gain inputs (dB or V/V), filter cutoff frequencies and offset voltage levels. The MXLAB software can be downloaded free from the Microchip web site at www.microchip.com.
The MXLAB Windows software contains a variety of tools to interface to the MXDEV®Analog Evaluation System. These tools provide different methods for troubleshooting the analog circuit in either the time or frequency domain:
[attachment=32518]
SELECTING THE RIGHT ADC
Selecting the most suitable A/D converter (ADC) for your application is based on more than just the precision or bits. Different architectures are available, each exhibiting advantages and disadvantages in various data-acquisition systems. The required accuracy or precision of the system puts you in a category based on the number of bits required. It is important to always design your system to allow for more bits than initially required: if an application calls for 10 bits of accuracy, choose a 12-bit converter. The achievable accuracy of a converter will always be less than the total number of bits available.
Depending on the system requirements, your accuracy might be better expressed in micro-volts, decibels or LSBs (least significant bits). A FFT showing the frequency spectrum of a device can be useful in determining the noise performance of a given device. All Microchip stand-alone ADCs show typical performance data for AC specifications, such as THD, SINAD and SNR. The following table shows performance, in dB and V/V, for 8- through 24-bit converters.
Two Application Examples:
Seismic Recording16 different devices are connected to a central processing unit to monitor vibrations. Each device measures the signal at an extremely fast rate, less than 100 μS. High accuracy is not required due to the large signal size, but speed is of the utmost importance. A high-speed SAR converter would be the best selection for this application.
Voice-Band RecordingThe human ear can detect signals from roughly 20 Hz up to 20 kHz. If the application is a telephone intercom system where high-fidelity audio is not a concern, 60-70 dB of dynamic range is sufficient. Based on these bandwidth and dynamic range requirements, either a medium speed (50-200 ksps) SAR or delta-sigma converter would work in this application.
DEVELOPMENT TOOLS
The MXLAB software tool provides data acquisition, analysis and display in a Windows®system environment. Additionally, analysis can be made of the digital potentiometer shutdown, reset and daisy-chain operations. The MXLAB software can determine digital potentiometer settings based on gain inputs (dB or V/V), filter cutoff frequencies and offset voltage levels. The MXLAB software can be downloaded free from the Microchip web site at www.microchip.com.
The MXLAB Windows software contains a variety of tools to interface to the MXDEV®Analog Evaluation System. These tools provide different methods for troubleshooting the analog circuit in either the time or frequency domain: