Signal-to-noise ratio is of touch screen controller performance, now as an industry standard accepted by everyone.
Signal-to-noise ratio of the problem is no industry standard measurement, calculation and reporting methods, especially in some typical systems, noise under the condition of high variability, such as mobile phones.
The two parts (
Signal and noise)
Measurement and calculation is largely dependent on the device under test (
DUT)
Representative, is the mobile phone.
It is important to note that although the signal-to-noise ratio as a performance measure has been widely accepted, industry experts understand that most of the market to promote high signal-to-noise ratio in the practical application does not guarantee.
In addition, in the noise environment, provides the high signal-to-noise ratio does not completely conform to its functional specification.
In
capacitive touch screen, the SNR signal is measured by finger capacitance add after the actual capacitance variation.
Finger capacitance depends on the sensor cover thickness, the finger size, DUT to the parasitic capacitance, and the sensor model.
Noise components depends on the internal noise and external noise controller, this paper will discuss these aspects.
Projected capacitive touch touch technology has been applied in many new smartphones, touch sensors will meet noise when used.
The noise from the display (
May be the LCD or AMOLED)
Coupled to touch sensors, the closer distance, the greater the noise.
Unlike analog display synchronization, this kind of LCD is usually peak noise.
The USB charger is usually also peak noise.
It is also the most likely to change, because in each equipment of AC/DC transformer structure and component is different.
Third party low-cost charger particularly easy to appear this kind of noise peaks.
Therefore, when touch controller didn't like cypressChargerArmor noise suppression technology, USB charger is OEM the most headache thing.
When all these external noise exist, we expect the touch controller not error report his finger or finger position.
They cannot be classified as ordinary, or gaussian noise or distributed.
This creates a problem for engineers and marketing personnel, to distinguish between no noise ADC signal-to-noise ratio.
In a wide range of measurement conditions, the signal-to-noise ratio has been able to have to say it's a miracle as metrics.
In addition, the signal-to-noise ratio cannot predict the most important and quantization noise related parameters of touch screen: the jitter (
Also known as the noiseless resolution)
And error touch report.
Fortunately, there is a signal to noise ratio measurement technique can predict the dithering of non-gaussian noise exists.
The signal-to-noise ratio of the noise affect how bad touch screen system affects the robustness of system, causing false touch and location.
Finger touch screen will interfere with the edge of the intersection of two transparent electrode electric field.
This capacitance is called mutual capacitance.
It's changed the capacitance sensor.
Intersection in transmitting and receiving electrode orthogonal intersections.
There are hundreds of such intersection in mobile phone touch screen.
Touch screen controller to measure all the change of the junction capacitance, and the measurement data into quantitative raw data.
By measuring each intersection, rather than the entire electrode, the controller will be able to create a 2 d touch sensor capacitance charts.
If the intersection near the fingers a big noise peaks, then in the position calculation algorithms will be added a wrong mark.
Then the algorithm convert raw data to coordinate;
According to noise peak value, the coordinates of the finger position report may be shaking, when static finger, may alternate between two coordinates.
When smartphones using touch screen interface, plugged into a USB charger, some unconscious enter or select these situations may occur.
We can conclude that, in the absence of standardized measurement method, the signal-to-noise ratio can be used as a performance measure, but is not perfect.
There are defined performance indicators, measuring steps, calculation method, touch screen controller suppliers (
001 - see the plath norms
49389).
OEM and mobile equipment can be used to quantify touch performance.
These specifications are necessary, can ensure repeatable test results, verify the touch screen performance, reduce the touch screen test hardware and firmware.
Typical performance test in addition to touch screen and controller interface to the hardware need metal finger simulator, fixture, oscilloscope, function generator, automatic machinery.
For example, a shaking of the standard measurement process is divided into seven steps, time coordinate finger position noise.
The measurements showed how much movement here, how many distance, we would expect is not moving finger.
This is a relatively simple parameter measurement, it is direct and immediate impact on the user interface.
In contrast, the influence of SNR on the touch screen performance is less direct.
Even in the noise environment, digital filter and position calculation algorithm can remove jitter, is to reduce the signal-to-noise ratio (
As a kind of performance measurement)
。
The signal-to-noise ratio as a performance index is not recommended, because it can't eventually give you a real sense of system performance.
This article is to want to tell you that don't meet with the dot, biased, signal-to-noise ratio doesn't tell us whether the system very well respond to touch.
This is why the touch controller leading manufacturers, such as the TrueTouch plath, a set of test and measurement method to evaluate the performance of the new touch screen design.
Reprinted from EDN electronic technology design
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