On Thu, 11 Oct 2007 22:06:18 -0400, Phil Hobbs
wrote:

Gents,

As I was helping someone with a TIA design this afternoon, I had
occasion to read the data sheet for the OPA211, which appears pretty
similar to the LT1028, except that it's a bit more stable and doesn't
come in a plastic DIP. It has a curve of THD+noise vs rms output
voltage that I found very odd (see attachment).

Everything goes swimmingly until the signal swing on the inputs reaches
about 1V rms, and then it hits the wall. This is at a kilohertz, mind,
so it isn't an ordinary slew rate issue. It's also _way_ within the CM
range, because the plot specifies +- 15V supplies.

Since the problem surfaces at the same input voltage swing makes it look
like something in the input stage, but what in the world could be as
slow as that?

So what's up with this amp?

Cheers,

Phil Hobbs

That graph makes no sense whatever.

If the numbers were different, I'd say that some input back-to-back
diodes were conducting... but not at 1 KHz. Also, fig 3 sort of
contradicts fig 4. 140 dB is an interesting number, too.

Is it an artifact of the test circuit, fig 41?

If you figure it out, let us know.

John