John Errington's Data Conversion Website
Signal conditioning
Before a signal can be measured it must be made to conform to the input requirements of the ADC. For example, an ADC may be capable of measuring voltages in the range -10 to +10V. However our incoming signal may be too small (perhaps 0 - 2mV), too big (100 - 300V) lie outside the range (100 - 101V) or be changing too fast to measure. The most important changes that usually need to be made to the signal are
- isolation
- bandwidth limiting (filtering)
- offset adjustment
- amplification
isolation: separates the signal from mains supplies etc. that could be damaging to the equipment. Examples include taking readings from mains-driven equipment, and also for patient monitoring where the patient must be protected from mains voltages
bandwidth limiting (filtering): the incoming signal may be changing too fast for the converter to measure. Electronic filtering is used to eliminate changes in the signal that are too fast to be properly measured (Nyquist criterion)
 Here the signal is changing slowly and we may be able to measure its value |
 Here the signal is changing quickly. If too fast to measure we will get unreliable readings from the ADC. |
offset adjustment: removes any fixed bias voltage from the signal allowing the remaining signal to be amplified. For example a signal of 2V on a base level of 100V would be too small to measure effectively. By removing the 100V offset as shown here we are left with a useful signal.
amplification: (gain) the size of the remaining signal is now adjusted to match the input range of the converter - e.g. a 2V signal is amplified to 20V to match the input range of the converter. The amount of amplification (gain) is output / input, so in this case a gain of 20 / 2 = 10 is used.
Sometimes the signal needs to be reduced. If a 100V signal is to be read by a converter with an input range of 20V then a gain of 20/100 = 0.2 is required.
Example question: a microcomputer is to be used to measure the temperature of a conductor on an electricity pylon. This is carrying electricity with a 50Hz ac sine wave at 440kV. The temperature is measured with a thermocouple which gives a maximum signal of 0.01V at 100 deg. C. The PC uses an integrating ADC with an input range of 0 - 10V. Explain the signal conditioning that is required.
Answer:
First, the extremely high voltage ac signal must be removed and the system protected with effective isolation. Now the signal from the thermocouple is filtered to remove any traces of the ac signal. This removal of noise will also be helped by choosing an integrating ADC which has inherent noise resistance. Offset is not required as the signal is already referenced to zero, but the signal will need to be amplified by a factor of 10V / 0.01V i.e. a gain of 1,000 is needed.