Wheatstone Bridge

In Tasks 2 and 3 of previous chapter’s laboratory (diode and transistor temperature sensors), we measured V ab across the 1 Ω resistor to evaluate the current. We did not measure V a and V b separately and make a subtraction, as both are on a scale of sev

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Wheatstone Bridge

In Tasks 2 and 3 of previous chapter’s laboratory (diode and transistor temperature sensors), we measured Vab across the 1 O resistor to evaluate the current. We did not measure Va and Vb separately and make a subtraction, as both are on a scale of several volts (measured in 0–20 V scale), while their difference is only in millivolts (measured in 0–200 mV scale). If you have a DMM, you can connect one of its leads to the position a and the other to b to measure this tiny voltage drop. This is not really possible if you plan to develop a stand-alone sensing device. In fact, this type of voltage measurement is known as a differential measurement, a common metric particularly in biosensor applications. Many biosensors measure a tiny difference in voltage, current, or resistance in comparison with that of a blank or a negative control. If you try to make measurements for the target and the blank separately and evaluate their tiny difference by subtraction, you will need a sensor that has extremely high accuracy and sensitivity, perhaps with six or more significant digits. There is a simple solution for it: a Wheatstone bridge. In the past, it has been used primarily for strain gauge applications, but more recently for a bio-application in the form of a cantilever biosensor. In reality, a Wheatstone bridge can be used for any differential measurement for both physical and biosensors.

5.1

Wheatstone Bridge

A Wheatstone bridge is an electrical circuit used to measure a very small change in resistance, such as a 10 O decrease for a 10 kO resistive load. This small change is not readily detectable by a typical DMM in the 0–20 kO range. The Wheatstone bridge consists of four resistors arranged in a diamond configuration. An input DC voltage, or excitation voltage, is applied between the top and bottom of the diamond, and the output voltage is measured across the middle. When the output voltage is zero, the bridge is said to be balanced. One (or more) of the legs of the bridge may consist of a resistive transducer, such as a thermistor or a strain gauge (often as R4 J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, DOI 10.1007/978-1-4419-6022-1_5, # Springer Science+Business Media New York 2013

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5 Wheatstone Bridge

Vin

Fig. 5.1 A Wheatstone bridge

R4

R1 Ia

Vout

Ib

a

b R2

R3

in the figure below). The other legs of the bridge are simply completion resistors with resistance equal to that of a selected resistive transducer. As the resistance of one of the legs changes, the previously balanced bridge becomes unbalanced; this can occur when a temperature or strain from a resistive transducer changes, for example. The unbalance in the bridge causes a voltage to appear across the middle of the bridge. This induced voltage may be measured with a voltmeter, or the resistor in the opposite leg to the changed resistor may be adjusted to rebalance the bridge. In either case, the change in resistance that caused the induced voltage may be measured and converted to obta