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Rail to rail op amps transition region

Question asked by Kurt on May 26, 2013
Latest reply on May 29, 2013 by OliverSedlacek

I am designing an EEG and I am under very tight space constraints. I have the intermittent help of an expert but he's been unresponsive lately and I hesitate to bug him too much. Basically I have an instrumentation amplifier acquiring signal from the scalp and two stages of amplification after that. There's a voltage reference, right leg drive and everything. There are two channels, so we're actually talking about 2 instr amps and 4 op amps (2 per channel) for further amplification.

 

The expert has told me NOT to use rail to rail op amps for the second stage of amplification due to the weird discontinuities when the rail to rail threshold is crossed (I believe that corresponds to an alternative amplification circuitry kicking in). So for instance, he said don't use AD8609 but use MCP609. Supposedly the latter has "more similar" input stages to the TLC272 which is what was on the reference design I am following.

 

So the problem is, the MCP609 is only available in TSSOP nothing smaller... and I'd really like to use QFN or DFN... and have 4 op amps in one chip. I've looked everywhere... analog, intersil, microchip, TI.... no one seems to manufacture quad op amps in small packages. I wonder why that is and I have still to find a suitable IC to use in my design.

 

It should have low offset voltage, low noise, and NOT be rail to rail. My supply is 3.3V.

 

What is the problem here? Why no quad op amps in small packages??

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