Can we adjust the filter settings on a BioSemi system ?
 

Yes, but only in software. There is one exception: the high pass filter on the new ActiveOne system can be bypassed to achieve a DC response. The decision to choose for fixed filter settings is the result of the increased processing speed of the PCs in the last years. With a typical PC, it is no problem to filter the data provided by our systems (up to 500 kSamples/s) on line. Given this modern processing power, we decided to keep the front-end simple, compact and reliable by eliminating all switches and additional components that would be needed for adjustable filters.

The frequency response of our systems is plotted in the figure below. The low-frequency signals have a lower gain to prevent saturation of the amplifier and ADC on large electrode offset potentials. However, note that the DC signals are not rejected. The low-pass filter is necessary to prevent aliasing effects of high frequency electrode and amplifier noise. We use a 5th order Bessel filter for optimal pulse response (no overshoot). The -3 dB frequency of the filter is placed at 1/4 of the (fixed) sample frequency. The filter provides approx. 20 dB suppression at the Nyquist frequency (half the sample rate). This setting has proved to be a good tradeoff between a maximal analog bandwidth, and minimal additional noise by aliasing.

 
The response of the ActiveOne in Semi-AC mode can be equalized in software to "flat from DC" response. This allows DC measurements with an electrode offset range of +/- 330 mVolt. (Larger then 19 bit performance) It is also possible to use the ActiveOne in simple DC mode. This has the advantage that no software equalizing is necessary and that there is immediate signal recovery after amplifier overload conditions. However, the electrode offset range is limited to +/- 33 mVolt in this mode. (Sufficient with active electrodes)
 
ActiveOne response (Semi-AC mode)
 
ActiveOne default response. 1uV bitstep in the passband and reduced gain at low frequencies to handle large electrode offsets.
 
DC input range is +330mV to -330mV
AC input range : 66mVpp
Software equalizing allows DC response.
   
ActiveOne response (DC mode)
 
Since the offset Voltages of the active electrodes are typically very low (<10mV), and because of the sintered Ag-AgCl electrode material, the ActiveOne system can be operated in DC mode.
 
Input range is +33mV to -33mV
No delay after amplifier overload.