How to minimize the noise in your Otii measurements

March 30, 2020

To make sure your measurements are done in the best possible way, focus on minimizing noise in your everyday Otii measurements.

The smaller the signal you want to measure, the more critical it is to cancel all surrounding noise sources and their path into your setup. You’ll need the signal to noise ratio to be as high as possible.

Minimize induced disturbances from the mains

The 50 Hz magnetic field (60 Hz in some countries) from the power mains will be induced more or less in any conductor anywhere. That’s why you can see it on your measured signal.

*Figure 1. 50 Hz noise measured with Otii Software*

According to the transformer principle, one or several conductors in our circuitry induce the 50 Hz pickup that usually proves problematic. Screening – the way radio frequency circuits with coaxial cables us it – won’t work for the induced 50 Hz field.

The trick is to minimize induction through two actions:

ACTION 1: Minimize the loop area

Twist the battery + and – cables together
- With one (1) expansion port ADC: twist the ADC cable with the AGND cable
- With two (2) expansion port ADCs: twist both ADC cables together
NOTE: Always use the shortest possible wires.

*Figure 2. Twisted cables for both supply and ADC connected to the Otii Arc*

ACTION 2: Increase the distance between your equipment and the noise source

Increasing the distance between your equipment and the noise source can be very useful, because the magnetic field loses its intensity proportional to 1/r3.

WATCH OUT FOR:

Hidden main cables (or other cables that can pick up the mains) near your equipment.
An ESD protecting carpet on your desk (should be used in all electronic laboratories).

*Figure 3. The same measurement as in Figure 1. but measured with twisted, short cables, away from ESD protecting carpet*

Please pay attention to the fact that, if you minimize the 50 Hz noise, you will, of course, minimize higher frequency noise as well.

Ground loops can affect your measurements

If your device under test (DUT) has other electrical or galvanic connections* to the surrounding world, there will probably be a ground loop. Ground loops are well known for introducing many kinds of disturbances in electronic systems. Check this list when you connect your Otii in a measurement setup:

Only use the negative battery wire (black 4 mm jack) for power return.
Use a separate ground wire for signaling ground, attached either to DGND (Digital Ground) or AGND (Analog Ground) on the expansion port. This setup prevents disturbances and errors in measurements or digital signaling caused by the voltage drop in the negative battery wire.
Use the AGND as the reference point for analogue measurements.
Use the DGND as the reference point for digital signaling.
Only use one of the grounds if you’re using both analogue measurements and digital signaling.

* Examples of connections that generate a ground loop:

USB connections
TAG / debug connections to your computer
External power and audio/video connections

*Figure 4. Example of a ground loop. Remove the power to the computer to open the loop.*

In the above example, remove the power to the computer to disconnect the ground loop and minimize the risk of 50 Hz and more high-frequency disturbances.

Noise from an external DC source that affects your measurements

If the power from a USB-only connected Otii is not enough, you need to connect something to the DC port. However, a noisy power supply connected to the DC port of your Otii can introduce disturbances in your measurements. You should also keep in mind that the noise from the power supply can change depending on the load. In other words, you may not always see the noise in your measurements.

If you choose to power your Otii with an external DC adapter, we recommend that the adapter is:

From an established supplier
Low noise
Approved for IEC 60950-1 as a Limited Power Source

Many AC/DC adapters with a rating above 2A can be quite noisy, so be careful concerning the quality when you want proper measurements while you work in these ranges. Our recommendation would be XP Power VEL18US090-EU-JA (available e.g. at Digi-Key) – an adapter that has received plenty of good feedback.

Closing thoughts

Please keep in mind that you’re facing a very common but complex problem that requires several actions before it can be solved. To minimize the noise in your measurements, you’ll probably need to test different ways of removing the disturbances while you monitor the result.