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ANALOG SYSTEM DESIGN

by George Martin

Start ý Considerations ý Transducer Interfaces ý Thermocouples ý ADCs ý Almost Thereý ý Sources and PDF

ALMOST THEREý

Ready to put it all together? Let me throw in one more topic, noise and filtering. If you have a 12-bit ADC and youýre going to measure an input signal, that signal will come with some noise. Letýs first look at the frequency of the noise. If youýre looking at oil pressure, there is no need to have any frequency greater than the engine speed, or perhaps a multiple of engine speed. So, 6000 rpm is 100 Hz, and if you picked a multiple of 12, then 1200 Hz is the maximum frequency youýre interested in.

You should have a filter that attenuates frequencies greater than 1200 to a level of one part in 4096. This is so the noise does not show up in the A/D readings. There are a lot of filters and filtering techniques, but after the higher frequencies are in the A/D readings, thereýs no way to get them out. You can, however, sample at two or four times the Nyquist rate and then use a digital filter on the A/D readings. That would move your filter from 1200 Hz to 4 ý 1200 = 4800 Hz, and you only have to filter down to one part in 1024. You can get the other two bits out in the digital filter. This makes for an easier analog filter design. Also, the factor of 12 put in the pressure reading probably comes from the number of teeth on the oil pump drive. You can pick up tooth chipping!

After youýve selected a sampling rate and input filter for each of the channels, itýs time to start the design. At this point, you can probably pick some supply voltages and input power supply design approaches.

If you look up thermocouples on the ýNet, youýll find a lot more information. Good luck in your analog endeavors.

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