OpAmp As buffer, single power supply, Which OpAmp to use?

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Hi Everyone,

I want to use a buffer(voltage follower) for one input before passing it to my Avr ADC.

The Thing is i have a single power supply config( pin 4-GND pin 7-5volts.

pin 3 is fed in the input

pin 2 shorted to pin 6 (standard voltage follower circuit,

The thing is neither opamp 741 or lm318 or CA3140E outputs match the input volatage for the complete range of 0-5v input signal.

Any suggestions if there is any opamp there that i can have a drop in replacement that could maintain i/o characteristics

Or may be i should make changes to the circuit(i would hate to do that now that a PCB is ready)

Thanks Rodney

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The good thing is, nearly all OP-Amps use the same or at least similar pinouts, at least as DIP or SO8 size. So the layout can stay the same. So you need a single Rail-Rail op-amp. One example would be OPA347, but there are a lot to choose from.

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removed

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One of the key questions is the required signal voltage range. Since this is a unity gain buffer, the required range will be the same at the input and the output. But, what is really needed?

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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the signal range is 0v to 5v. no gain required. output should match input, which i dont get because of my single power supply

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OK, How close to 0V and how close to 5V?

There is NO amplifier that will get you all the way to exactly 5V with a 5V supply and NO amplifier that will get you all the way to exactly 0V without a negative supply.

Depending on the amplifier and the load resistance, you might get to within 50-80mV. 100mV is a bit more common. This with load resistances of, lets say, 10K or better.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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100mv off at the rails is fine. could u suggest me such opamps

Quote:

This with load resistances of, lets say, 10K or better.

do i have to use a resister between opamp output and Atmega128 adc input?

thanks[/list]

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No resistor needed. Better off with none. Some circuits have a lower input impedance and that can make it harder to get close to either rail.

What are your purchasing limitations? Are you in the US? UK? EU? Elsewhere?

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Most of the analog IC comanies should have a suitable type. The difficult thing is to get single versions in a DIP case. Dual or SO8 case would be easier. Possibile Types are:
OPA347 (low power),AD8041( too fast though),
AD8551 (zero drift), MCP6021.

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LTC1152

Tom Pappano
Tulsa, Oklahoma

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http://www.national.com/selguide

Op-amp selection tool. Its pretty handy to have on your desktop!