Feature: Sensors
Figure 7: Circuit for simulating noise reduction with a symmetrical voltage source. A negative voltage has been added to the differential amplifier to keep the output within a visible range
because the impedance of these wires must be equal to maintain bridge balance across the entire RTD temperature range. Te equation that governs the circuit in Figure 3 is:
reduction, so they are not recommended in noisy environments.
Symmetrical supply connections A detailed study of noise compensated connections reveals that the problem arises because the noise signal being subtracted from the input signal has different characteristics. Tis is primarily due to the impedance difference at the two ends of the connecting cable. I suggest to use symmetrical supplies for the sensor, so that the impedance at both ends of the cable is equalized by the supply itself.
Tis circuit has twice the sensitivity of the previous one by using
two sensors, but this configuration does not correct the bridge’s delinearisation.
Noisy connections Te connection with compensated cable resistance presents problems due to its high noise level. Another connection that presents noise problems is the Kelvin connection, a simulation of which is detailed below:
Kelvin connection In the four-wire Kelvin connection, the wire resistance is compensated without any adjustment. Te V0 measurement is performed differentially, so the noise reduction is theoretically “maximal”. Figure 5 shows the circuit used for the simulation, with the results shown in Figure 6. Even though the noise level in the connecting wires has been
reduced compared to the previous configurations, the noise at the output is very pronounced. In practice this connection is unfeasible in terms of noise reduction, since the noise at V1 increases and the noise at V2 is constant, so the differential output has a high noise content. Other connections with differential current excitation for measurement also have this problem of insufficient noise
20 June 2026
www.electronicsworld.co.uk
where the first term is practically constant, and the second is a function of ΔRT. Figure 8 shows the simulation results. Complete noise cancellation is observed in the green output signal, but due to the characteristics of this connection, the non-linearity of the output signal with respect to the sensor response has been accentuated. It is worth noting that a negative voltage has been added to
Voltage excitation Te voltage excitation circuit is shown in Figure 7. It consists of two voltage sources, one positive and one negative but of same value, connected to the two wires of the sensor (a Pt100 RTD in this case) through a 100Ω resistor. Te resistance of the wires can be compensated for using the 50Ω trimmer method. In this case there are two trimmers, and they must be set at the same value for effective noise reduction. Te equation that governs the circuit is:
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