IMC CANSAS - Filter

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SC16, SCI8, SCI16 voltage, current and temp. modes 381

This mechanism can increase noise or cause offset errors which accumulate due to non-linear

rectification in the input amplifier. This problem is called RF-interference and, depending on the circuit,

can be unavoidable with highly sensitive amplifiers having a large bandwidth. The amplifier's quality may

limit the problem, but complete RFI immunity can never be achieved. For this reason, it's generally

advisable to ground the common mode voltage supply of a switching power adapter.

In cases in which RFI problems occur with switching power adapters whose supply's static isolation

voltage potential is already determined and rules out electrical grounding, a large (ceramic) capacitance

connected between SUPPLY and CHASSIS could help! A suitable 1nF capacitor is already present

internally and should succeed in suppressing the problems described in most cases!

6.18.9 Filter

The signal passes through the following filter stages one at a time.

1. Hardware: Pre-filter for the ADC (analog-digital converter), which works according to the Sigma-

Delta procedure and requires a relatively high-frequency, fixed-frequency low-pass filter: Low-pass

60 kHz, 3rd order. This filter cannot be deactivated.

2. ADC: Low-pass effect of the ADC itself. Its cutoff frequency is around 8 kHz. Its characteristic is a

3rd order rectangular filter.

3. Noise suppression: Noise suppression filter, dependent on the sampling rate. This is a low-pass

filter with Hanning characteristics. See the table for the cutoff frequency. The cutoff frequency is

far above a channel's sampling frequency. The filter counteracts the aliasing and suppresses noise

and disturbances, but not in the sense of a perfect anti-aliasing filter. This filter cannot be

deactivated. With some modules and at top sampling rates, this filter stage is omitted.

4. Compensation: Compensation filter, dependent upon the sampling rate. This filter adjusts the

sampling instant. This compensates for the fact that measurement is based on the use of a

multiplexer. The filter returns data which simulate simultaneous sampling on all channels. This filter

can be deactivated with the option "Filter / No filter". The filter also is the main cause of delay on

the CAN-Bus. It also mainly determines the module's resulting cutoff frequency. The filter has

Hamming-characteristics. It is omitted for very slow sampling frequencies.

5. Averaging: Formation of a mean value. If the individual channels have different sampling rates,

then the multiplexer operates at only one single (high) sampling rate. The channels with slower

sampling rates are generated by forming arithmetical means of the sample values, where the

measured values from the previous filter stage are averaged over the whole sampling interval. With

the option "No filter", this averaging replaces the current value from the previous filter stage. With

very slow sampling rates, the value is also generated by taking averages.

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