Troubleshoot errors in low-voltage measurements
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
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.
A third channel is grounded with a 2-cm wire between data acquisition IN+ and IN- terminals, and between GND and IN+. This third channel is used to determine the internal system noise and thermal drift of the data-acquisition system itself. All experiments are conducted with an instruNet i423 card on its ±10 mV measurement range using instruNet World Oscilloscope/Strip chart software. This card provides software selectable 6 Hz and 4000 Hz two-pole analog low-pass filters, software selectable digital filters, and software selectable integration (averaging).
Many load-cell manufacturers recommend an excitation voltage of 10 V, which applies 285 mW into the load cell (10^2/350 = 0.285). That much power creates heat and temperature drift. Therefore, we prefer to run at a lower 3.3 V, which corresponds to a gentler 31 mW.
RFI couples into sensor signal
RFI involves radio waves that travel through air and couple into wires. This is explained by Maxwell’s equations, which state that a change in wire #1 current creates a magnetic field that flows through a loop of wire #2 and induces a current in that wire, which then converts to a voltage after traveling through resistance. The effect of RFI increases with increased source impedance (source is less strong to fight RFI); therefore, high source impedances and low level measurements are the most challenging. The experiments shown here explain how signal switching and sine waves can couple into your signals.
On/Off switching RFI: When an offending signal in the vicinity of a sensor wire makes a low-to-high transition, an upward spike couples into the wire, and when it makes a high-to-low transition, a downward spike couples (or vice-versa if RFI flux is in opposite direction). This is why we sometimes see spikes on a digitized waveform—they relate to an offending digital signal or device turning on or off.
Sinewave RFI: Alternatively, a sinewave can travel through air and couple another sinewave of the same frequency onto one’s signal. AM radio is approximately 1 MHz and FM radio is approximately 100 MHz, and both are notorious for entering the laboratory or factory.
How to Detect RFI
Set up your data-acquisition system to digitize from one channel as fast as possible with all analog and digital low-pass filters off and integration (averaging) off. Then view the resulting wave at different horizontal scales (e.g. 100 µSec to 50 mSec per full screen). Do this even if your ultimate experiment is to digitize multiple channels at a different sample rate with integration/filtering on. You might feel compelled to turn on filtering to make your signal look good. Yet for now, resist this temptation, and focus on learning more about your signal. The trick to understanding measurement error is to let go of your ultimate goal for a moment, and do some simple experiments. Figure 2 shows 350 µV spikes from a 200 Hz square wave where we digitize 8 ksamples at 166 ksamples/sec from our 350 O load cell.
Figure 2. High-frequency components from a square wave can couple into your signals, creating unwanted interference.
Find the source
While repeatedly digitizing oscilloscope traces, turn devices in the vicinity on and off (e.g. machine, pump, power supply), and view the effect on your digitized waveform. If you turn off a nearby power supply, and see spikes disappear, then that power supply is coupling into your sensor.
Is the offending signal traveling through air and coupling into your sensor cable, or does it travel through your ground wire? Try moving your sensor cable and look at the effect on the digitized wave. Does the position of the cable effect the plot? If so, RFI in the air is passing through a loop of wire (your cable) with a different physical geometry (different flux). Change in the radiated field due to moving cable is the telltale sign of through-the-air RFI. Added cable shielding might help, in addition to several other techniques discussed below.
Is your cable/sensor ground attached to external metal (e.g. device under test)? If so, physically disconnect and look at the effect on your signal. If your signal changes, then you know current is flowing along your ground wire due to an AC signal between your data-acquisition ground and device under test ground. This is called a "ground loop" and is often fixed by electrically isolating the sensor. The AC voltage difference between grounds is often caused by changing power, which involves changing current on the ground return path and its associated voltage drop on that ground wire. A typical difference between grounds is 15 mVac on top of 50 mVdc. To measure this with a sensitive data acquisition system, attach IN+ to ground #1, attach IN- to ground #2, digitize one channel as fast as possible and view 100 µSec to 25 mSec per full screen.
Will Differential Amplifier Common Mode Rejection Save Me? Data acquisition systems have differential inputs that measure the voltage difference between two inputs. All differential amplifiers have a specification for how much common signal on both inputs is rejected. A typical specification is 80 dB rejection at 60 Hz. This means that 1/10000th of 60 Hz on both pins is seen as a differential signal. For example, connect IN+ to IN- with a bare wire, apply 60 Hz, 1 Vrms between IN+ and GND, digitize, and you will see 60 Hz 100 µVrms between IN+ and IN-. The dirty little secret of data acquisition is this rejection gets worse by 20 dB per decade, which means you get 1/1000 rejection at 600 Hz, 1/100th at 6 KHz, 1/10th at 60 KHz and nothing beyond. Digital switching (e.g. spikes) often involve frequencies in excess of 60 KHz. Therefore, in many cases, amplifier common-mode rejection will not save you, especially with digital switching RFI.