The Functions of Signal Conditioners in Biomedical Measurement Systems

In a Biomedical measurement system, the electrodes pick up the bioelectrical potential whereas the transducer converts the physiological signal to be measured into a usable electrical output. The outputs from the Bioelectrodes/transducers are then converted into an electrical quantity that is suitable for operating a display or recording device with the help of a signal conditioner.

Some of the functions performed by a signal conditioner before relaying the signal to the recording device include:

Amplifying the signal

The biosignals available from the transducers are usually very small in magnitude. Therefore, amplifiers are needed to boost the level of the input signal to match the requirements of the recording systems or match the range of the analog-to-digital converter, thus increasing the resolution and sensitivity of the measurement.

Signal conditioners are usually located closer to the signal source, or the transducer, to improve the signal-to-noise ratio of the measurement by boosting the signal level before it is affected by the environmental noise.

Filtering

A filter is a circuit that amplifies some of the frequencies applied to the input and attenuates others. We have 4 common types of filters:

• High-pass filter –only amplifies frequencies above a certain value.
• Low-pass filter –amplifies frequencies below a certain value.
• Band-pass filter –amplifies frequencies within a certain band.
• Band-stop filter –amplifies all frequencies except those in a certain band.

Signal conditioners can also include filters to reject unwanted noise within a certain frequency range.

Isolation

Incorrect grounding of the system is one of the common causes of measurement problems and noise. Signal conditioners with isolation can prevent these kinds of problems. Such devices pass the signal from its source to the measurement device without a physical or galvanic connection by using transformer, optical or capacitive coupling techniques. The isolation also blocks high voltage surges and rejects high common mode voltages.

Linearization

Transducers like thermistors and thermocouples have a non-linear response to changes in the physical variable being measured. Signal conditioners have either hardware-based or software-based linearization routines for this purpose.

Excitation

Some transducers like strain gauges, thermistors require external voltage or current excitation. The signal conditioning part of the measurement system often provides the excitation signal. For example; strain gauges act like resistance devices in a Wheatstone bridge configuration, which require circuitry and an excitation source.