Question

How does cable length affect PR transducer performance?

Answer

A common request is that we provide piezoresistive (PR) accelerometers and pressure transducers with long cables? Why are the stock transducers supplied with short cables? This article will provide some answers and guidance for making a decision on cable lengths. For convenient installation, most miniature PR devices use a light four conductor cable. Larger industrial units often have a heavier duty cable. Here we will limit our discussion to miniature measurement devices such as the Endevco 8500 series of pressure sensors and accelerometers. Cable length affects the frequency response of the measurement and creates a voltage drop. This article will discuss the voltage drop problem. If a cable run of 250 feet is required, should a transducer be ordered with a 250 foot cable? Well, let's take a look at this scenario. First, let's assume that the transducer was ordered with 250 feet of cable. The sensor's integral cable uses #34 wires. The typical resistance of #34 wire is 2.6 Ohms per 10 feet. If the total cable length is 250 feet, we need to use 500 feet since two wires of 250 feet are required to complete the circuit.

Assume the following operating parameters:

Transducer input resistance: 1000 Ω
Excitation voltage: 10 VDC
Total wire resistance 130.5 Ω

By Ohm's law

I = E / R
I = 10 VDC / 1130 Ω
I = 9 mA

The voltage drop = .009 Amps X 130.5 = 1.17 Volts

If we were to splice, or connect #22 wire to the short transducer lead we would reduce our wire resistance to 0.16 Ω per 100 feet making the total resistance equal to only 8 Ω, resulting in a voltage drop of 0.072 Volts.

As can be seen, in the above examples, without using the larger gage wire there is a greater than 10% reduction in the excitation voltage. Since piezoresistive devices are ratiometric, the 10% reduction in voltage will result in a proportional reduction in sensitivity.

When using an extension cable with #22 wire, a 6-conductor shielded cable is recommended. While 4 wire cable works well, the addition of the two extra wires allows for remote sensing of the excitation voltage. Remote sensing lets the power supply adjust the voltage closest to the sensor rather than at the power supply.