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AN2834

How to get the best ADC accuracy

3.2.11

EMI-induced noise reduction

You can reduce EMI noise using proper shielding and layout techniques. The possible

sources of emission must be physically separated from the receptors. They can be

separated electrically by proper grounding and shielding.

Shielding technique

Placing ground tracks alongside sensitive analog signals provides shielding on the PCB.

The other side of the two-layer PCB should also have a ground plane. This prevents

interference and I/O crosstalk affecting the signal. See Figure 28.

Signals coming from distant locations (like sensors, etc.) should be connected to the PCB

using shielded cable. Care should be taken to minimize the length of the paths of these

types of signal on the PCB.

The shield should not be used to carry the ground reference from the sensor or analog

source to the microcontroller. A separate wire should be used as ground. The shield should

be grounded at only one place near the receiver such as the analog ground of the

microcontroller. Grounding the shield at both ends (source and receiver) might lead to the

creation of ground loops, with the result of current flowing through the shield. In this case,

the shield acts like an antenna and the purpose of the shielding is lost.

The shielding concept also applies to grounding the chassis of the application if it is metallic.

And it also helps remove EMI and EMC interference. In this case the mains earth ground is

used to shield the chassis. Similarly DC ground can be used for shielding in case of the

earth ground not being available.

Figure 28. Shielding technique

ADC

Ground

loop

Not recommended

Recommended

Sensor

Shielded cable

Sensor

Do not ground the shield at both ends

Ground the shield at the receiver end only

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