# Small capacitance change measurement using Capacitance to Phase conversion method

#### nagarajaj09

Joined Mar 20, 2017
2
I am currently researching on a method to detect small changes in capacitance using CAPACITANCE TO PHASE CONVERSION technique.However, I am stuck at a very basic concept. I am unable to determine the use of compensation capacitance at the input. It would be of great help if somebody could help me understand the basics of the compensation capacitance(Cc)and its purpose and the difference between with and without the compensating capacitor (along with waveforms in Ltspice if possible).

To make it clearer, I have attached 2 diagrams.
Diagram 1: WITHOUT THE COMPENSATING CAPACITANCE.

Diagram 2: WITH THE COMPENSATING CAPACITANCE (Cc).
In the simulation, I am unable find the difference in the waveforms between the circuits mentioned above (i.e Diagram1 and Diagram2).

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#### ErnieM

Joined Apr 24, 2011
8,376
Take a Long hard look at Figure 5 of the paper and understand what the 4 input capacitors are. Two are identicle fixed caps, two are sensors, one with a reference solution and the other with the unknown. One sensor will change in capacitance value and yield a phase shifted output proportional to the medium in the tube.

The output of the two first stage amps will thus vary in phase dependent on the change in capacitance. The rest is just a phase comparator and counter to measure this phase difference.

Such changes in capacitance may also be detected by any sort of RC oscillator. In a previous life we used a similar sensor as a liquid level detector by placing the sensor capacitance in a 555 timer running as table, then counted on a micro controller.

#### nagarajaj09

Joined Mar 20, 2017
2
Take a Long hard look at Figure 5 of the paper and understand what the 4 input capacitors are. Two are identicle fixed caps, two are sensors, one with a reference solution and the other with the unknown. One sensor will change in capacitance value and yield a phase shifted output proportional to the medium in the tube.

The output of the two first stage amps will thus vary in phase dependent on the change in capacitance. The rest is just a phase comparator and counter to measure this phase difference.

Such changes in capacitance may also be detected by any sort of RC oscillator. In a previous life we used a similar sensor as a liquid level detector by placing the sensor capacitance in a 555 timer running as table, then counted on a micro controller.
I understand the concept of the paper. Thank you. But i need to know why is he using a compensating capacitance and how can i view that on the waveform in LTSpice to prove it. I need to know why is he using the circuit in diagram 2 and not the circuit in diagram 1 and also i need to know how to add stray capacitances/noise of the wire in LTSpice . Please help me with understanding this issue.