Interface Circuits

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The key to optimization of a film sensor is in the proper design of the interface circuit. Your particular application will dictate your interface design. In some cases, no interface will be required. When an interface is required, here are some steps to take in developing your circuit:

1. Consider the frequency range and signal amplitude requirements.
2. Consider the load resistance required to facilitate low frequency operation, and to minimize signal loss.
3. If only low signal levels are required, no interface or a unity gain buffer might work.
4. If a high value load resistance is required, a low leakage high, impedance buffer-amplifier is recommended.

PVDF film acts both as a voltage source and as a capacitor. The area between the silver ink electrodes is active area. The parts of the film that are not between electrodes can be ignored. They will not produce a charge or act as a capacitor. The thickness of the material and the area covered by the electrodes determines the capacitance (proportional to the area and inversely proportional to the thickness).

Here is a model of the equivalent circuit for a film sensor.

The dashed line represents the active film area. The voltage source (Vs) is directly proportional to the applied stimulus. The film's capacitance ( C ) is always present and forms a frequency dependent impedance. Therefore, the voltage across the output (Vo) will also vary with frequency.

The film capacitance can be considered a source impedance. When connected with an interface device (amplifier, preamplifier or other device), there is a resistance added to the circuit. The series connection of the film capacitance and the load resistance forms a voltage divider. The impedance of a capacitor varies with frequency. Therefore, the voltage across the interface will also vary with the frequency of operation. As the frequency drops, the impedance of the film increases. This lowers the voltage to the interface device.

One important aspect of the interface circuit is its input resistance or load resistance. This affects the ability of the film to sense low frequencies. It also affects the signal amplitude. Increasing the load resistance will improve the ability of the sensor to detect low frequencies, but it will also degrade the output amplitude. This is called the loading effect. You will want to target your circuit to cover the low frequencies needed for your application and yet minimizing the loading effect.

Frequency Response

To determine the frequency response of your circuit, you need to consider the time constant. This is the time required for a signal to decay to 70% (-3dB) of the original amplitude. The time constant is equal to the input resistance (load) times the sum of the film capacitance and the input capacitance. Time constant = Ri(Cf + Ci).

The circuit will exhibit the characteristics of a RC high-pass filter. The cutoff frequency is inversely proportional to the time constant. Raising the input resistance lengthens the time constant and lowers the cut-off frequency.

If a film that has a capacitance of 500pf is interfaced with a circuit with a 1M ohm load resistor and an FET (no capacitor), the cut-off frequency is 318.47Hz. If the input resistance is increased to 10M ohm, the cut-off frequency is reduced to 31.85Hz. However, the signal has also decreased because the load resistance is higher.

Another way to lengthen the time constant would be to increase the film capacitance. One way to do this is by increasing the active area of the film. Quadrupling the capacitance in the first example above would make the cut-off frequency 79.61Hz.

Typical interface schematics

Recommended components are low-leakage such as JFET-4117, op-amp LMC660, LF353, OP80.

Unity gain buffers:

Non-inverting Voltage Amplifier:

Charge amplifier:

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