The DTC is fixed by the components in the internal amplifier of the ICP sensor. The specifications of the ICP force sensor shown in this catalogue list the DTC for each force sensor.
When testing with an ICP sensor, two time constants must be considered, one for the time constant of the fixed value sensor and the other for the time constant of the coupling circuit used in the signal conditioner.
When the ICP sensor is subjected to a step function input, the amount of charge Δq generated is proportional to the mechanical input. According to the law of static electricity, the output voltage is ΔV= Δq /C, where C is the total capacitance of the sensing element, amplifier and ranging capacitor. The MOSFET amplifier then amplifies the voltage to determine the final sensor sensitivity. After the initial step input, the charge signal decays according to the formula q= Qe-t /RC, where:
q= instantaneous charge (pC)
Q= initial charge (pC) )
R= bias resistance value (ohms)
C= total capacitance (pF)t= time after t0
e= natural logarithm Cardinality (2.71 8)
This equation is also graphically represented in Figure 6 below:
Figure 6: Standard DTC Curve
The product of R and C represents the sensor’s DTC (in seconds). Standard sensor sensor time constants range from a few seconds to> 2000 seconds. A special time constant can be provided by changing the resistance value R in the built-in microelectronic amplifier of the sensor.
Most readout instruments have high input impedance, > 1 megohm. For these systems, the sensor DTC as previously described becomes the dominant value and can be used to determine the signal discharge rate. However, for signals coupled to low impedance readout devices, typically <1 megohm, it is necessary to determine the system time constant. This will be further explained in the next section.