Best practices for accurate measurements

It is important to understand some basic principles in order to get the most accuracy out of your CurrentRanger. Here are some basic Ohm’s Law facts:

  • Any piece of wire is essentially a resistor, even if on the order of mΩ, the longer the wire the larger its resistance
  • Any terminal is also a resistor, even if very small
  • Any mechanical connection between two wires, or between a wire and a terminal, creates some resistance. The better/stronger the connection, the less resistance is induced into the overall end-to-end connection. The best lowest impedance (resistance in the AC world) connection you can nominally make is a welded/soldered connection
  • A weak/loose connection is a weak-link in your circuit which acts as an added resistance in your DUT circuit. Even a few tens of mΩ can create a significant voltage drop, which results in a measurement error. You may assume that there is zero resistance from your DUT’s power source to the CurrentRanger positive [+] terminal, but this can often be a fallacy especially in the µA and nA ranges.
  • If for whatever reason there is a significant voltage drop before or after the CurrentRanger terminals (other than purely your DUT load resistance), then some degree of error will be introduced in the measurement
  • It’s impossible to completely remove any residual resistances inherently present in physical wiring/connection/terminals, but with good practice and understanding these can be minimized
  • You should take into consideration that wiring and connections may contribute a very small resistance that could add a fraction of % of error to the measurement, but in general if all things are equal this error will be constant regardless of the meter you use, even with high end instruments
  • Any residual resistances described above (and any errors they create) will be more significant with very small DUT loads (µA range, nA range especially) and become insignificant as your DUT is a large load (upper mA range)

To make it most convenient to connect your DUT circuit on the input, the CurrentRanger has several terminal options described here. Probably the best and easiest to use of these is the thumb terminal which grabs the input wiring with a strong tension mechanism, thus minimizing any connection resistance. The “Gold” banana-jack terminals are probably the least consistent option when used to secure a wire as they can loosen up or give the impression of a good connection when in fact a wire can snap away easily. Banana jack leads and alligator clips are better candidates to use with the “Gold” and low profile banana-jack terminals.

It’s also a best practice to use thicker stranded wiring, from the power supply to the [+] terminal, and from the negative [‒] terminal to the DUT, and keep wires short whenever possible.

Capturing a fast transients with dynamic impedance

When you have a load with a dynamic impedance, it may be difficult to get repeatable measurements. An example is a MCU with a radio transceiver.

It’s important to ensure the transceiver’s load (antenna in the case of a radio transceiver) is a perfect 50Ω load, otherwise the scope captures will differ between measurements, since an actual antenna can slightly change impedance even as you move around it.

To make consistent and repeatable measurements, this can be achieved by soldering two 100Ω resistors in parallel (=50Ω resistor) instead of a real antenna, this way the transmitter sees a “perfect” load that absorbs all the energy. Here is a Moteino with a “perfect” 50Ω antenna load: