The DTC is fixed by the components in the ICP sensors internal amplifier. Specifications for the ICP force sensors shown in this catalog list the DTC for each force sensor.
When testing with ICP sensors, there are two time constants that must be considered for low frequency determination, one being that of the sensor which is a fixed value, and the other that of the coupling electrical circuit used in the signal conditioner.
When an ICP sensor is subjected to a step function input, a quantity of charge, Δq, is produced proportional to the mechanical input. According to the law of electrostatics, output voltage is ΔV = Δq/C where C is the total capacitance of the sensing element, amplifier, and ranging capacitor. This voltage is then amplified by the MOSFET amplifier to determine final sensor sensitivity. After the initial step input, the charge signal decays according to the equation q = Qe-t/RC where:
q = instantaneous charge (pC)
Q = initial quantity of charge (pC)
R = Bias resistor value (ohms)
C = Total capacitance (pF) t = time after t0
e = base of natural log (2.71 8)
This equation is also graphically represented in Fig. 6 below:
Figure 6: Standard DTC Curve
The product of R and C represents the DTC (in seconds) of the sensor. Sensor time constants vary from just a few seconds to >2000 seconds for standard sensors. Special time constants can be supplied by altering the resistor value, R, in the sensors built-in microelectronic amplifier.
Most readout instruments have a high input impedance, >1 Megohm. For these systems, the sensor DTC as previously discussed becomes the dominant value and can be used in determining signal discharge rate. However, for signals coupled to low impedance readout devices, generally <1 Megohm, it is necessary to determine the system time constant. This will be explained further in the following section.