I am reviewing an schematic and it contains several reference to 1uF 0402 SMD capacitors, some of them are marked simply as "1uF" and others as "1u, NP0/C0G" , I think both are the same, is there any difference?
1uF, 0402 size in NP0/C0G is not available:
C0G (NP0) is the most popular formulation of the “temperature-compensating,” EIA Class I ceramic materials. Modern C0G (NP0) formulations contain neodymium, samarium and other rare earth oxides. C0G (NP0) ceramics offer one of the most stable capacitor dielectrics available. Capacitance change with temperature is 0 ±30ppm/°C which is less than ±0.3% C from -55°C to +125°C. Capacitance drift or hysteresis for C0G (NP0) ceramics is negligible at less than ±0.05% versus up to ±2% for films. Typical capacitance change with life is less than ±0.1% for C0G (NP0), one-fifth that shown by most other dielectrics.
Features & Benefits
C0G (NP0) formulations show no aging characteristics and have an operating temperature range from -55°C to +125°C
Voltage Range: 6.3V to 500V
Capacitance Range: 0.5pF to 0.1µF
Sizes: 01005 to 2225
Class 1 Caps
As you may have noticed in the chart, C0G is extremely stable (note that C0G and NP0 both have a zero, not an uppercase “O”). C0G is a Class 1 dielectric and an all-around capacitor superstar: the capacitance is not significantly affected by temperature, applied voltage, or aging.
It does, however, have one disadvantage that has become particularly relevant in this age of relentless miniaturization: it is not efficient with respect to volume. For example, if you go onto Digi-Key and search for a 0.1 µF C0G cap, the smallest in-stock part is a 1206. In contrast, you can find a 0.1 µF X7R cap in the 0306 package, and with a voltage rating (10 V) high enough for 3.3 V or even 5 V circuitry.
No not the same. A 1uF C0G, NP0 capacitor (if you can even find one!) will be very large and expensive (definitely not 0402). The default for a bypass capacitor or similar non-critical application would be something like X7R or X5R but that should be specified. The voltage rating should also be specified.
C0G, NP0 refers to an EIA Class I dielectric made with very low temperature coefficient and voltage coefficient, typically specified for capacitors in the pF but now easily available up to many nF, however uF levels of capacitance are (today) not easily available. At higher capacitances, dielectrics such as X7R and X5R, X5S and so on EIA Class II diectrics are used because they allow a physically small capacitor to be manufactured inexpensively.
The designer must work around their many non-ideal characteristics, which include (sometimes extreme) temperature and voltage coefficient (the capacitance may drop by 80% or more at rated voltage) but also microphonics (they tend to generate a voltage from vibration) and aging (the capacitance decreases with time).
There are also Class III dielectrics such as Z5U but they are significantly more horrible again in characteristics.
Graph below is from Kemet, a major manufacturer of this type (MLCC = Multilayer Ceramic Capacitors).
The graph emphasizes the temperature characteristics (and the relative values of the dielectric constants) but the voltage characteristics can be at least as important in design.
All ceramic capacitors have a different classification for their dieelectric. Other dieelectric classes are X5R, X7R, Y5V, etc. The dieelectric class only denotes their temperature range and capacitance stability within that range (DC bias stability not included).
But unlike the other classes, C0G/NP0 is known to have additional characteristics and the label carries additional connotations. C0G/NP0 means it is specifically calling for an capacitor whose capacitance is accurate (tight tolerance), stable with temperature and DC bias, and does not suffer from piezo-electric effects (microphonic noise). Usually for timing or filters.