"The key to accurate measurement is just controlling your environment"

Measurement Techniques

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Switching or Linear Power Supplies - which is better?
EMI Made Easy (kind-of)
Measurement Error - Floating Neutrals
LabVIEW Floating Neutral Measurement
Network Parameter Conversions
Diode Modeling
Short-Wave Infrared (SWIR) - Defect Detection
Time Domain Reflectometry
Dark Current Measurement

Measurement Tips

sample imageIt's Not Always What You Think

  So you go out and purchase a precision 12 bit DAQ module for an automated measurement of a circuit which has an output of +/- 10 volts.  You shield your wiring, ground your equipment, and even put line conditioners on your power strips.  But with all of this, your zero state measurement is 0.2v peak-to-peak.  Even grounding the input of the DAQ shows 40 bits of resolution is lost to noise.  So where is it coming from?

  The specifications for the DAQ show it to have more than enough bandwidth and resolution for the measurement.  However, no one at the manufacturer probably thought of checking the module with the actual supplied wall-pack.  The cheap switching regulator wall pack was producing the noise.  Looking at an FFT of the output showed 60Hz and harmonics bleeding through.  Measurement tip: Always establish a baseline measurement before attaching the circuit in test. 


Zero Volts Doesn't Always Mean Ground

Using one power supply or SMU (Source Monitor Unit) for a Source and another for Ground usually works but it depends on what you're measuring.  If you were measuring the current through a resistor or diode and the levels were in mA or uA ranges you'd probably be fine.  However, if your desired range was in pA or fA you would have a big problem.  Remember, a power supply at 0V is actually 0V +/- some uncertainty (offset).  A good supply would be in the 30-40uV range.  What makes it worse is the same uncertainty is built into the Source supply and it doesn't mean the offsets would move the same direction.  If you were sweeping the Source near this level your measurement would be garbage.  Better to use a regulated ground (GNDU) if available or actual Ground if noise levels permit.