MNX10010 / Rev B
4/11/2008
6
COUPLING TIME CONSTANT AND LOW FREQUENCY
RESPONSE
The low frequency capability of a piezoelectric measurement system may be
limited by several factors about which the average user of such systems may not
be aware.
The specifications each sensor will delineate the discharge time constant which
controls the basic low frequency response of that sensor. However, the power
unit and the readout may also play a part in this specification and may, in many
cases, be the limiting factor rather than the sensor itself.
The low frequency response of any AC coupled system is limited by the coupling
time constant (TC) of the composite measurement system. The coupling
capacitor and the 1 megohm ‘pulldown’ resistor in each channel of the PS03, in
parallel with the DC input resistance of the readout instrument constitute a first
order high pass filter.
The TC of this filter is the product of the 10µF capacitor and the parallel
combination of the 1 megohm pulldown resistor and the input resistance of the
readout instrument.
The relationship between the coupling TC and the lower -3db frequency is:
If the readout instrument is of the order of 10 megohms, it may be ignored. The
TC is then 10µF x 1 megohms = 10 seconds. The lower -3db frequency is then:
If the readout instrument input resistance is 1 megohm, the parallel combination
of the pulldown resistor and the input resistance is 500k ohms. The TC is then
10µF x 500,000 or 1 second. The lower -3db frequency is then:
Eq
1.
0.16
f
-3db
= --------------
TC
Where: TC is the product of R and C, in Seconds
Eq
2.
0.16
f
-3db
= ------------ = 0.016 Hz
10
Eq
3.
0.16
f
-3db
= ----------- = .16 Hz.
10
