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capacitively couple into the EMG leads. This problem can be eliminated by differentially

recording from a twisted pair of leads, or by a different placement of EMG leads.

3) The high-frequency components of the signal are filtered out. The source resistance and the

capacitance of the cable to the amplifier act as a simple RC low-pass filter. Electrical cables

can provide 30-100 pF capacitance per foot (100-300 pF/meter). If a cable has 1000 pF

capacitance and the source resistance is 10 kΩ then the cut-off frequency is about 16 kHz. A

50 kΩ source resistance with the same cable would filter the signal at 3 kHz. The appropriate

solution is to use a differential amplifier near the signal source.

Bridge Design for Pressure and Force Measurements

Pressure and force transducers are often constructed using strain gauges connected in a Wheatstone

bridge circuit. The basic circuit is shown in Figure 18.

Figure 18. The Wheatstone bridge circuit that is commonly used for strain-gauge measurements. VE is a

precision source that provides the excitation voltage. Typically one, two or four of the resistors in the bridge are

strain-dependent variable resistors. The strain is proportional to the difference between the voltages at A and B.

The Wheatstone bridge should be considered as two pairs of resistive dividers with the output voltage

being the difference between the voltages at A and B. The bridge circuit detects small changes in

resistance and its output is linear in the region of balance, i.e., when the voltage difference is small.

The greatest amplitude is produced when strain gauges are used for all four resistive elements,

although physical constraints may dictate only two or even one active element. When only two

gauges are used they should be connected in positions R1 and R2 and the gauges should be located

such that one increases and the other decreases in resistance during the applied pressure or force.

This can be achieved by placing the elements on opposites sides of the beam under strain. When

choosing strain gauges, remember that semiconductor types have outputs ten times higher than metal-

film types.

The sensitivity of a bridge circuit decreases with increasing temperature and some applications may

therefore require the addition of a temperature compensation circuit, which should be placed near the

bridge.

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Microsoft LPF-00004 Betreibershandbuch Seite 66