In l293d datasheet i found that term "High-Noise-Immunity" in the features. so what does it mean?
i didn't find anything explain what it means in the internet
Electronic – what does High-Noise-Immunity mean
amplifierdatasheetdc motorlogic-gatesmotor controller
Related Solutions
"Volt per square root hertz".
Noise has a power spectrum, and as you might expect the wider the spectrum the more noise you'll see. That's why the bandwidth is part of the equation. The easiest is to illustrate with the equation for thermal noise in a resistor:
\$ \dfrac{v^2}{R} = 4kT\Delta f \$
where \$k\$ is Boltzmann's constant in joules per kelvin, and T is temperature in kelvin. \$\Delta f\$ is the bandwidth in Hz, just the difference between maximum and minimum frequency.
The left hand side is the expression for power: voltage squared over resistance. If you want to know the voltage you rearrange:
\$ v = \sqrt{4kT R\Delta f} \$
That's why you have the square root of the bandwidth. If you would express the noise in terms of power or energy you wouldn't have the square root.
All noise is frequency related, but energy spectra may differ. White noise has an equal power across all frequencies. For pink noise, on the other hand, noise energy decreases with frequency. Flicker noise is therefore also called \$1/f\$ noise. In that case bandwidth in itself is meaningless.
The left graph shows the flat spectrum of white noise, the right graph shows pink noise decaying 3dB/octave:
You can make noise visible on an oscilloscope, but you can't measure it that way. That's because what you can see is the peak value, what you need is the RMS value. The best thing you're getting out of it is that you can compare two noise levels, and estimate one is higher than the other. To quantify noise you have to measure its power/energy.
BSC means "Basic Spacing Between Centers". This is often used if the reference lines don't refer to a physical point or edge, like in the case of the pins: the reference is in the middle of the pin, instead of one of the edges.
BSC SQ = BSC square, i.e. in both X and Y dimensions.
The abbreviation BSC is misused in the case of the package outline, and overall dimensions, as they are specified by physical edges which can be measured.
Usually a mechanical drawing of a package will show minimum and maximum values. BSC SQ must be regarded as nominal, so may deviate, but it doesn't say by how much.
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Best Answer
It's right in front of you:
What they mean by "high noise immunity" is that it has a high (relative to TTL logic levels) threshold for switching between a low input level and a high input level. So small noise variations when the input voltage is low won't cause spurious switching.
In general, the bullet points on the front page of the datasheet are marketing and not technical. Sometimes they are useful for determining the intended application of a device, but you should generally ignore them and read the actual specification tables if you want to know how the device performs.