John Errington's Experiments with an Arduino
Sensible results from your ADC readings
Bad layout or interference can cause unexpected results.
If you are experiencing inaccurate or unstable results from your analog measurements these sections may help you find out why this is happening.
Measure the right voltage!
Remember the voltage you measure is
between the
ADC input AND ADC GROUND.
If other parts of your circuit disturb that voltage you will get an incorrect reading.
The figure shows sections of your circuit and connection resistance cause by track or wire connections. This can be a major problem with long wires or when using a breadboard.
In this top figure the current in the power load causes a voltage across R3 R2 and R1; so the sensor ground potential Vs0 is not at the same voltage as the ADC ground Va0.
This better layout keeps the analog (low current) and power (high current) grounds separate.
The only (slight) remaining error in measuring the sensor output is due to the voltage across R3.
Managing interference
A wire is not the ideal perfect conductor we normally asume. It has resistance, inductance and capacitance. As a result if 2 wires are close together a signal on one can appear on the other. USUALLY we dont want this - its "INTERFERENCE" so called because it interferes with the signal we expect to see.
A strong signal on wire A can be "coupled" into wire B causing an unwanted signal
These figures show how this unwanted interference can affect our measurements.
Generally capacitive interference is only a problem if we have a high resistance input.
We can THINK
CAPACITIVE COUPLING induces VOLTAGES and
INDUCTIVE COUPLING induces CURRENTS
so it affects low impedance pathways.
Reducing interference
The simplest way to reduce the effects of unwanted interfernce is to provide separation between large signals and sensitive inputs.
So keep cables carrying large currents or voltages away from your input wires.
If this is not possible you can reduce capacitively coupled signals by using screened cable with the outer screen conductor earthed to your system.
Inductively coupled signals require a magnetic screen such as being contained inside a magnetically shielded area such as a closed ferromagnetic box.
