Analog

Cornell University ECE4760
Analog techniques

AntiAliasing the ADC input
All sample data systems need bandwidth limitiation on their input to avoid aliasing. A simple RC filter will sometimes work, but a higher order active filter is often necessary. It is possible to build a 3rd order Butterworth or Chebychev filter with one opamp, or even one transistor! There are many possible designs but a 3-pole active filter design utility by Stellar Coding is easy to use to build a opamp version. Note that in the circuit diagram below (from the web page) the opamp is configured as a unity gain follower. That means that we could use a single BJT as an emitter follower, if the input and output offsets worked out right. A commercial microphone module, such as Electret Microphone Amplifier have an ouput offset of Vdd/2. If powered by 5 volts then the output offset in 2.5 volts, with an active range of +/-1 volt from 2.5 volts. Using this to feed the BJT circuit (ltspice .asc) below subtracts one silicon diode drop, 0.7 volts, from the 2.5 to make 1.8 volt offset, just about perfect for the PICO ADC. Either filter rolloff is 60 db/decade. The BJT filter is low power, but will not be as ideal as the opamp version. If you power the microphone from 3.3 volts, then just use the opamp circuit.

Another approach to antialiasing is to use a clock-tunable analog filter. Such devices are made by Analog Devices and Maxim Semiconductor. They are sample-data device, but the sample rate is so high that they do not alias in the range of their own cutoff frequency. Restricting the Analog device search to 8-pin PDIP only gives 4 choices: an 8-pole elliptic filter with a very sharp cutoff, a linear-phase 5-pole bessel, and two 5-pole Butterworth filters.

Building a compact motor controller.
Solderless proto-boards do no work well for high current motor controllers. They melt.
A compact design for the ece4760 motors lab is fabricated on a small piece of solder board. The
image is kind of a hacked together mash-up of a schematic and photograph, but hopefully shows the connections.
The rectangular black object at the top is a SPST kill switch. The DIP socket is wired for a 4N35 optoisolator.
Below that is N channel MOS FET.