Performance comparison

AN4316

22/26

DocID024847 Rev 2

7

Performance comparison

In order to compare the performance of a whole touch sensing application, the usual

parameter is the Signal to Noise Ratio, well-known as SNR. But each company, each team

and even each engineer as its own method to compute it.

Here below we provide a way to compute it, but the designer must keep in mind that to get

correct results, he must compare apples to apples. So the test conditions and the method of

computation must be under control and reproductible else it is meaningless to compare two

results.

In the above equation:

•

“Signal” is the average of the delta measurements during a touch

•

“Noise” is the amplitude (delta max - delta min) without touch.

Figure 14. SNR computation

In order to make meaningful comparisons, the various SNR measurements must be

performed under similar test conditions.

The following parameters affect the results of the SNR:

•

hardware application i.e. layout, panel (dielectric, thickness, glue, ...), capacitance

value and quality, etc...

•

firmware application i.e. acquisition configuration (frequency, reference, ..), threshold

settings, etc...

•

test conditions i.e. object used to touch (standard test finger or genuine finger), way the

panel is touched (pressure, slope, ...), applied noise if any, etc...

SNR

Signal

Noise

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=