The conversion of an analog input signal to digital data is performed by an Analog to Digital Converter (ADC). Typically, an ADC is not able to convert any input voltage level, and is in fact limited to a maximum voltage (usually 1.2V for example). However, depending on the specifications, transducers don’t necessarily have a matching output voltage. The input signal therefore needs to be amplified (or reduced) to match the measurement signal “range”. This measurement range is what a user can configure or, alternatively, is what automatically is detected by data acquisition systems.
Soft and Hard clipping of a signal
At some point during data acquisition an event could occur where the input signal falls outside the measurement range. This is visualised in Figure 1. Müller-BBM’s MKII Data Acquisition Hardware distinguishes in this situation between the following signal levels:
- – The signal is equal or less than 100% of the measurement range: The signal is quantised normally with the greatest possible accuracy.
- – The signal is in between 100% and 150% of the measurement range: The signal is ’soft-clipped’, meaning the signal is still quantised but its linearity is no longer guarantied, introducing (small) higher harmonic distortions
- – The signal is higher than 150% of the measurement range: The signal is ‘hard-clipped’, meaning the signal is no longer digitised yielding completely wrong values. The system is completely in overload yielding unusable measurement data.
The bottom-line here is that a user should carefully choose the measurement range of the measurement channels. Should the measurement range be too small, PAK yields an overload message and the corresponding data is marked as such. Small overloads do however not yield complete loss of data but can still be used for everyday processing. Always prevent hard clipping of the signal however, as this does require a renewed measurement campaign
• One should typically use the auto-range functionality while starting your first measurement. Drive the test object to it’s maximum output and activate the auto-range. As such optimal settings are obtained reducing the chances of even soft-clipped signals.
• ICP type sensors often have a maximum output range of +-5V which is incompatible with the (most applicable) standard +-10V input range of the MKii front-end. Use the PAK Equipment Manager in this case, specifying the sensor’s lower overload setting. Should PAK detect a voltage higher than 5V with a 10V measurement range, the data is still marked as Overload.