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Knowing Your Instruments


It is very important to know the limitations of your instruments. I have been stupid enough to measure a DC voltage from my chip which had an output impedance close to a mega ohm with a multimeter whose input impedance was also close to a mega ohm! I kept seeing a lower than expected output voltage until I swapped the meter with a 10Gohm meter then I saw the correct value!

Multimeters

  1. Normal Digital Multimeters have an input resistance of 1Mohm or 10Mohm.

Usage Tips

  1. Doing a measurement with your DMM in the ACV position on your DC circuit will give a quick indication of any excess ripple on the supply when you don't have a scope at hand.
  2. You can almost always determine the leads of a bipolar transistor with an ohm meter. b-e and b-c junctions will measure like a diode with the b-c junction reading slightly lower than the b-e junction when forward biased.
  3. DMM can upset sensitive circuits from noise generated inside it. It is always good to test your instruments when in doubt by doing some sanity check measurements.
  4. When probing directly on a crystal of a uP, use 10kOhm or so resistor in series with the probe tip to prevent loading from stopping the oscillator.
  5. When measuring the voltage of a high impedance output always make sure your DMM input impedance is high enough so as not to load the output and change the reading.

Spectrum Analyzer

Usage Tips

  1. Making measurements near a spec-analyzer's noise floor will give 3dB errors.

Oscilloscopes

  1. Oscilloscope grounds are generally the common ground so you cannot connect 1 probe ground to something else and the other to something else. Use either diff probes or measure the 2 signals separately

Probes

  1. Oscilloscopes have an input capacitance of 13pF but that is quite different what the circuit would see. The circuit sees the probe capacitance. This shows the 10:1 attenuation on the probe.
    R2, R3 and C2 are tuned out to produce the best frequency response. See the article:Scope Probe Schematic
  2. The higher the attenuation on the scope probe the smaller is the probe input capacitance. For example a 1:1 probe will have an input capacitance of 50pF to 100pF while a 10:1 probe will have 9pF to 15pF of input capacitance.
  3. Also see this excellent article explaining the anatomy of probes and the type of probes and how they effect measurements and how to choose a probe here.

Usage Tips

  1. When probing directly on a crystal of a uP, use 10kOhm or so resistor in series with the probe tip to prevent loading from stopping the oscillator.
  2. It is easier to see what is happening on the ports using a scope when you trigger one channel against the CPU clock.


Copyright 2018 Milind Gupta