User Guide for FSR Sensor

FSR Integration Guide and Evaluation Parts Catalog Page 23

with Suggested Electrical Interfaces

FSR Current-to-Voltage

Converter

In this circuit, the FSR device is the input of

a current-to-voltage converter. The output

of this amplifier is described by the

equation:

VOUT = VREF • [-RG/RFSR].

With a positive reference voltage, the output

of the op-amp must be able to swing below

ground, from 0V to –VREF, therefore dual

sided supplies are necessary. A negative

reference voltage will yield a positive output

swing, from 0V to +VREF.

VOUT = (-RG • VREF) /RFSR.

VOUT is inversely proportional to RFSR.

Changing RG and/or VREF changes the

response slope. The following is an

example of the sequence used for choosing

the component values and output swing:

For a human-to-machine variable control

device, like a joystick, the maximum force

applied to the FSR is about 1kg. Testing of

a typical FSR shows that the corresponding

RFSR at 1kg is about 4.6kΩ. If VREF is –

5V, and an output swing of 0V to +5V is

desired, then RG should be approximately

equal to this minimum RFSR. RG is set at

4.7kΩ. A full swing of 0V to +5V is thus

achieved. A set of FORCE vs. VOUT

curves is shown in Figure 15 for a standard

FSR using this interface with a variety of

RG values.

The current through the FSR device should be limited to less than 1 mA/square cm of applied force. As with

the voltage divider circuit, adding a resistor in parallel with RFSR will give a definite rest voltage, which is

essentially a zero-force intercept value. This can be useful when resolution at low forces is desired.

Figure 15

FSR Current-to-Voltage Converter