On Mon, 15 Mar 2010 10:30:47 +0800, "Xiaofan Chen" said:

> > The LM358 has a PNP input stage and is fully valid to the negative rail.
> > Input voltage is only limited near the positive rail for this opamp.
> 
> Yeah it is okay to run with negative rail. But the circuit the OP
> mentioned does not have a negative power supply.
> http://www.edn.com/article/CA6451249.html
> It connects V- to Ground. But anyway, this may not be a real
> issue since the accuracy requirement is not that high.

That is not what I was saying. 

The negative rail of the opamp is ground in this circuit, yes. 

The LM358 is a "single supply" opamp. Both input and output are valid to
the negative rail. That is the definition of a single supply opamp.

In order to achieve outputs valid to the negative rail, they used a
class B output stage. If they had added the bias for class AB it
couldn't have swung all the way negative. I believe there is even a
resistor from output to the negative rail to make it get to the negative
rail.

In order to achieve inputs valid to the negative rail they used PNP
transistors in the input stage.

It's old-school but very inexpensive.

Even if the negative rail is ground, this circuit is valid. It is not
affecting the accuracy. There is no need for a "rail to rail" opamp in
this circuit.

I've used this opamp and the LM324(the quad version of the LM358) in
this type of circuit for decades :)

Cheerful regards,

Bob

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