Troubleshoot errors in low-voltage measurements

July 27, 2015 // By Glenn Weinreb
Troubleshoot errors in low-voltage measurements
Many data-acquisition systems connect directly to sensors. As with all measurement systems, you must contend with errors and do your best to minimize them. Errors caused by thermal drift, EMI/RFI, internal noise, wiring, grounding, and shielding all contribute to total measurement error. Fortunately, you can minimize errors if you know what's causing them.

For this article, we used an instruNet i423 digitizer, one of several systems designed to attach directly to many different sensors such as voltage, current, resistance, load cell, strain gage, thermocouple, and RTD. You can apply the techniques covered here to other data-acquisition systems as well.
We ran experiments with a load-cell sensor that measures 0 to 2 kg of force and internally contains four 350 O resistors that are bonded to a metal plate that flexes when pressed. Flexing of the plate changes the resistor values. You can think of this as a sensor with a 350 O source impedance that receives a 3.3Vdc excitation voltage and produces a ±10 mV signal with a 1.65 Vdc offset. The data acquisition differential amplifier sees ±10 mV and we will evaluate microvolt level errors. All pictures in this article are actual measurements from this setup. Figure 1 is a schematic of the sensor. Electrically, a load-cell sensor is the same as a strain gage and mV/V pressure sensor.

Figure 1. A strain gage is essentially a four-resistor bridge circuit where the voltage across it changes because of flexing.

We'll focus on these Error sources:

  • RFI Couples into Sensor Signal
  • 50/60 Hz Power Couples into Sensor Signal
  • Data Acquisition System Internal Noise
  • Thermal Drift and Sensor Instability

Test setup
Normally, sensors attach to a data acquisition system through a shielded cable. For the purpose of demonstrating RFI (radio waves coupling into signal wires), however, we break out the IN+ wire and induce an offending signal with a function generator. The function generator 5 Vrms output is connected to a bare wire that wraps around the sensor IN+ wire ten times. We've placed 270 O in series with the function generator output to facilitate 18 mA through the offending coil (5 Vrms / 270 = 18 mArms).

We've also attached a dummy sensor, one that's electrically similar to the load cell, to a second measurement channel. It consists of four independent thin-film resistors floating in air at the end of a cable, where the function generator is attached in the same way as the load cell. RFI couples more with increased source impedance. Therefore, the dummy sensor has the same 350 O source impedance as the load cell. The second channel is used to identify slight instability from within the load cell itself.

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