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Subject: Re: Rail to Rail OP-amp?

Date: 12/09/02 at 7:10 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Rail to Rail OP-amp? posted by Wang on 12/09/02 at 1:18 AM:

Hello Wang,

The difference is in the voltage swings that the op-amp can produce at its output and/or work with at its inputs. A rail-to-rail op-amp will be able to swing the output voltage to within some mV of the supply rails, and can also work with a common mode input voltage anywhere between the supply rails.

This feature is more likely to be useful when you are using the device with low supply voltages, where the output swing or input common mode range limitations of ordinary op-amps can be too restrictive.

A 'rail-to-rail' op-amp might actually only be rail to rail at its output, or only at the input. Some are rail-to-rail at the inputs and the output. The only way to be sure of getting what you want is to check the data sheet to see what the manufacturer means by 'rail-to-rail'.

The best information for choosing op-amps is probably the manufacturers product guides, where they list their various offerings in a table that allows you to compare specifications such as input offset voltage, input bias current, and gain-bandwidth product. But you have to know what things are most important for your application.

Hope this helps,

David Ashby

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