Signal conditioning Remote sensing using a high precision instrumentation amplifier
The instrumentation amplifier (IA) is the workhorse of sensing applications. In this article, Hooman Hashemi, Analog Devices will explore some ways to take advantage of these amplifiers’ balance and excellent dc/low frequency common- mode rejection (CMR) for use with resistive transducers (for example, strain gauge) when the sensor is physically separated from the amplifier. Hashemi will present methods to increase the noise immunity of such gain stages while making them less sensitive to supply variation and component drift. Measured performance values and results will also be presented to show the accuracy range in order to allow a quick evaluation for end-user applications
can produce a differential voltage that predictably changes in response to changes in a physical parameter - with the side benefit of providing temperature and time drift immunity. The differential voltage rides atop a large common- mode (CM) voltage. To amplify the small signal from a bridge, an instrumentation amplifier is used. The beauty of an IA is that, with little or no loading of the bridge elements, it can sense the differential voltage and reject the CM to a degree that is next to impossible to achieve by a traditional op amp, due to the high degree of external resistor matching required.
W
hen it comes to sensors, there is little competition for what a Wheatstone bridge (Figure 1) can do. The bridge
Figure 2: Remotely located sensor setup suffers from environmental noise pickup
The problem is that shielded twisted pairs are
not immune to all interference over long cable runs. In such a case, the well-balanced input(s) of the instrumentation cannot be relied upon to eliminate the CM pickup. The positive and negative amplifier inputs are not equally affected by the interference picked up by the long cable, and the inputs contain uncorrelated signals that CMR cannot eliminate. Therefore, it is not a surprise to find significant noise at the output of the circuit, as shown in Figure 3, due to this unbalanced response to what seems to be CM noise.
One solution for the successful extraction of
the small bridge differential voltage from the CM (dc and interference) is to use two pairs of shielded or unshielded twisted pair (UTP). Done this way, both IA inputs are balanced and subjected to the same CM noise pickup. This is shown in Figure 4. A device such as the LT6370, with low frequency CMR (120 dB) can reliably go to work on rejecting what afflicts both IA inputs. The result is a clean output waveform at a large distance, even in noisy environments. Having all the CMR horsepower of LT6370 at
Figure 1: Wheatstone bridge
The electronics involved in physical
measurements are often located far from the physical parameter under measurement. For instance, a strain gauge measurement, such as that buried under the tarmac at a truck weigh station or within the structure of a bridge, is unlikely to be located next to the electronics used to read the measurement. For example, when dealing with a two-wire quarter-bridge strain gauge such as Omega corporation’s SGT-1/350-TY43, placing the sensor remotely from the sensing amplifier, as shown in Figure 2, yields unsatisfactory results, even if shielded twisted pair sensor leads are used.
Instrumentation Monthly March 2019
Figure 3 (above): Troublesome 120 Hz pickup at the amplifier output (0.1 V/div, 2 ms/div)
hand, one could take this idea one step further and streamline the configuration by eliminating one pair of wiring, leaving a single UTP. This concept is illustrated in Figure 5, where U2’s inputs are kept balanced for good CMR. Note how the UTP leads look identical to U2 and have identical impedance to ground (R2, R4).
Figure 4 (below): Remote sensing using two unshielded twisted pairs of wire
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