what means rail to rail

[mesuty]

Dear friends ;

Could somebody explain what means RAIL TO RAIL in ADC-DAC systems.

And what is it for ?

[Ttelmah]

Any chip, has 'supply rails'. In the case of the PIC, normally 0v (Vss), and 5v (Vdd). A 'rail to rail' ADC, is able to read all voltages between these limits, without any 'gap' at the ends. Similarly a rail to rail DAC, can give an output ranging from Vss to Vdd in the case given. These abilities are relatively harder to do, than simple designes, since transistors all need some form of bias offset to work, so a input that includes '0v' genuinely in it's range, may need to use a PNP transistor, which then makes it hard to get working close to the Vdd rail. 'Rail to rail', is a very imprecise term really, with manufacturers often talking about op-amps as 'rail to rail', when the output still cannot actually 'reach' the supply rails, but only get within a few tens of mV of the rail. It is better/safer, to look at the actual input range specified on the data sheet, and verify that it does get as close as you need to the supply rails, and similarly that the output range limits are what is required....

Best Wishes

[SherpaDoug]

Significant sacrafices have to be made to get rail to rail operation. Especially rail to rail outputs tend to get very weak as the output actually approaches the rails. Be sure you really need that capability before choosing rail to rail parts.
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[kender]

Imagine a simple (non R2R) op-amp with it's V(+) = 5V and V(-) = 0V. It's output will be between 0.7V to 4.3V. It's output range is less then it's power supply range. The R2R op-amps have input and output range equal to the power supply range. Unlike non-R2R opamps, which are made of BJTs, R2R op-maps are made with FETs. Typically FET op-amps perform worse then BJT op-maps at higher frequencies.

Kender