# Basic op-amp doubt

1. Aug 31, 2011

### rama1001

I am just reading an op-amp datasheet. In that, they have written in features as "RAIL-TO-RAIL OUTPUT SWING" and in Applications as "MULTI-CHANNEL DATA ACQUISITION".

What does they mean? Can someone explain me in simple way.

2. Aug 31, 2011

### fleem

I'm pretty sure the "RAIL-TO-RAIL OUTPUT SWING" means the output voltage is capable of going very close to the positive and the negative supply voltages, as opposed to some devices that are not capable of outputting a voltage close to the power supply voltages (for example, they might only be able to get within a volt or two of the supply voltage). This limitation has to do with the voltage drop that exists across all semiconductor junctions, but you can minimize it with good internal circuit design of the chip.

The "MULTI-CHANNEL DATA ACQUISITION" might mean the op-amp can be used as a signal receiver, and maybe there are multiple op-amps in a single chip so that you can receive multiple signals? Just a guess.

3. Aug 31, 2011

### rama1001

Yes, you are right. There are two opamps inside the designed opamp.
Thank you so much.

4. Aug 31, 2011

### yungman

Rail to rail output means the op-amp output can drive all the way to the supply voltage rail. Multi-channel DA has nothing to do with whether it is single or dual or quad op-amp. You just happened to look at a dual op-amp that has rail to rail output which is very common.

Op-amp is an op-amp. You can find output rail to rail, input rail to rail or both. You have low noise, low input current etc. But it is still under the general definition of an op-amp. It just say it fit for multi-channels application, no more, no less.

Usually in data acq consist of ADC DAC. In the older days, ADC and DAC were powered by multiply supplies and usually need +/-15V, and the input of the ADC is not rail to rail. At the time, you don't need rail to rail op-amp. In fact, I always had to put limiting circuits to make sure the output of the driving circuit into the ADC did not go over the input range. Now a days, they mostly run on single +5V supply, rail to rail become more important.most of them now a days run on +5V single supply only. The ADC input range is from 0V to +5V, you want the driving circuits to be able to drive the full range of the supply voltage. Not only this eliminate multiple supplies( $$), it save the protection circuits needed (again$$\$)!!!

I designed a lot of data acq stuff dated back from working for LeCroy to embedded processors with build in ADC and DAC. This is my only understanding. They have different ICs that has analog switches with op-amp for MUX and buffering, but it is a different catagory. They are not op-amps by definition. An op-amp is an op-amp!!

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