Anti-aliasing filter shield filter.jpg


The ControlBoard's circuit for measuring device capacitance/impedance is sensitive to high-frequency noise as can be seen in the following image of the feedback signal measured while actuating a test capacitor with a 1 kHz signal. Figures (a) and (b) show the time and frequency domain representations of the measured signal. The noise is a combination of higher-order harmonics and a high-frequency noise source of unknown origin. Figures (c) and (d) show the same signal after applying a low-pass filter.

Part of the problem is that the impedance measurement circuit amplifies high-frequency noise, as can be seen from the following image of the simplified circuit and transfer function. If we want to improve the accuracy of device capacitance/impedance measurements, we need to use a low-pass filter with a cutoff frequency (fc) higher than any frequencies of interest.

The anti-aliasing filter shield piggy-backs on top of the ControlBoard. It taps the signal from the A1 channel, routes it through a 4th-order, 50kHz (1% pass-band) Chebyshev low-pass filter, and sends the output to channel A2.


You can download the KiCAD files for the shield here.


To install the filter, simply plug it into the ControlBoard as shown in the images below. Note that you also need to enable the filter within Microdrop. From the menu, select Tools/DMF control board/Configuration/Edit settings, and make sure the Use_antialiasing_filter check box is ticked.

Last modified 4 years ago Last modified on 02/11/15 17:27:08