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What is the difference between the 'cap_sw'
and 'cap_var'
primitives?
Both the 'cap_sw'
and 'cap_var'
primitives describe a capacitor of which capacitance can vary. The key difference between the two lies in what controls the capacitance. For example, the capacitance of the 'cap_sw'
primitive is controlled by the input signal 'C'
. On the other hand, the capacitance of the 'cap_var'
primitive varies with the voltage across its two terminals, 'pos'
and 'neg'
.
The following two examples illustrate the difference between the two primitives. Both the examples measure the voltage Vout of a capacitor as it is charged by a fixed current of I0=3mA. The voltage is expected to increase linearly with a slope of I0/C, where C is the capacitance.
The first example uses the 'cap_sw'
primitive, of which capacitance is controlled by the input signal 'C'
. The 'C'
signal starts at 30pF and switches to 60pF at 20ns. As expected, the slope of the Vout waveform changes from 3mA/30pF=0.1V/ns to 3mA/60pF=0.05V/ns at 20ns.
The second example uses the 'cap_var'
primitive, of which capacitance varies with the voltage across its two terminals. The 'cap_var'
primitive has its 'C'
parameter defined as '\{30.0p, 3.0, 60.0p}
, which means the capacitor has a capacitance of 30pF when the voltage is below 3.0V and 60pF above 3.0V. The simulated waveform shows that the Vout waveform initially rises with a slope of 3mA/30pF=0.1V/ns. As the capacitor voltage reaches 3.0V, the slope changes to 3mA/60pF=0.05V/ns.
For more information, please refer to the documentations on the 'cap_sw'
(link) and 'cap_var'
(link) primitives.
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