AN4038 datasheet(9/39 Pages) STMICROELECTRONICS

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AN4038

Test setup overview

Doc ID 022686 Rev 1

9/39

2.2

Test environment description

The detection distance measurements were performed automatically using a three-axes

table. The table simulated the approach of a finger or hand and guaranteed high precision

and repeatability of the measurement. The finger was simulated by a conductive pen

(diameter 10 mm) and the hand with a copper plate (80 mm x 80 mm). The approach speed

used for the detection distance was 60 mm/second.

The default parameters for measurements of the STM8TL5xxx were:

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CS (sampling capacitor) = 0 (register value)

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Target reference value = 1000

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Threshold = 30

The front panel, which was used with all sensors, was made of acrylic material with a

1.5 mm thickness. It was glued to the sensor board with a 100 µm of pressure sensitive

adhesive.

The noise graphs in this application note are expressed by the standard deviation. The

standard deviation (see Equation 1) is used to measure the dispersion or variability that

exists from the mean (average) value.

Equation 1: Standard deviation

Where X is the mean of the sampled values. The standard deviation was calculated using

5000 samples.

The firmware test developed for the detection distance measurements is based on the

STM8TL5x_STMTouch_Lib_V0.1.0.

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