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Issue #214 May 2008
Where Analog And Digital Collide
An Easy-To-Use LCR Meter
Third Place Microchip 2007 Design Contest
by Miguel Rusch
Start | LCR Meter | Back to Basics | The Big Picture | Creating a Wave | Analog Stages | Signal Conditioning | User Interface | Firmware | Take a Measurement | System Performance | Further Development | What's Next? | Sources & PDF
SIGNAL CONDITIONING
The flow of signal conditioning stages is illustrated for each test frequency in Figure 5. Before being sampled by the dsPIC ADC, each signal is passed through an LP antialiasing filter to ensure that the Nyquist-Shannon sampling theorem criterion is met. In this case, the sample rate (Fs) is 100 kHz and the ADC is 10 bits; therefore, the filter needs to reduce the signal strength by approximately 61 dB between 10 kHz (highest frequency of interest) and 50 kHz (Fs/2). This task was achieved by a sixth-order Butterworth filter. Microchip’s free FilterLab 2.0 was used to design this filter and generate the passive component values.
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| Figure 5 — Test waveforms are first passed through antialiasing filters (sixth-order Butterworth) before being digitized by the 10-bit ADC at 100 ksps. Down-sampling and FIR low-pass filtering is performed on data from the 100-Hz and 1-kHz tests. The final stage before data analysis is IIR band-pass filtering. |
Once the signal has been suitably band-limited, it is digitized by the 10-bit ADC. The next action is dependent on the test frequency. In the case of a 10-kHz test, the signal is passed directly to an IIR band-pass filter that limits the bandwidth to only the frequency of interest. In the case of the 1-kHz and 100-Hz tests, the signal must first be low-pass, FIR-filtered, and decimated. The rate of decimation is dependent on the test frequency, either at a rate of eight or 16. Decimation is required in this system at lower test frequencies to minimize the size of data needed to represent a single test cycle. It allows compliance with Nyquist criterion while only relying on a single antialiasing filter.
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