Re: INA103 differential stage

Phil Allison wrote:

> "Pooh Bear"
> > Phil Allison wrote:
> >>>
> >> >> ** No - all you need is two op-amps.
> >> >>
> >> >> Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm
> >> >> input and feedback resistors.
> >> >>
> >> >> Balanced low noise in - balanced line level out.
> >> >
> >> > Phil ! Are you feeling unwell ? He said a mic pre ! That configuration
> >> > will be
> >> > noisy as hell.
> >>
> >> ** Try reading the context - dickhead.
> >>
> >> The INA103 allows access on pins 5 & 12 to the differential stage's
> >> outputs.
> >>
> >> http://focus.ti.com/lit/ds/symlink/ina103.pdf
> >>
> >> The NE5532 would merely buffer those outputs.
> >
> > Ok, I follow you now. From what he said ( about simply amplifying each leg
> > of
> > the mic input separately ) I got a different picture of what you meant.
> >
> > Why not buffer those points using a voltage follower configuration though
> > ?
> > Slightly less noisy without those 4k7s.
>
> ** Are you feeling well - Pooh ??
>
> The reason for preferring unity gain inverting is that it produces less
> HD - since there is no common mode voltage at the inputs.

That's an interesting assertion. I recall that being said in the 70s. The
reason being supposed 'common mode failure' of the input pair.

For a while I followed the 'inverting is best' suggestion but eventually
abandoned it since it generally leads to increased noise. I see no sign of this
issue with modern op-amps.


> The self noise from a unity gain inverter, using an NE 5532 and 4.7 kohms is
> about 3 uV rms ( 1 uV from the resistor, 1 uV from the op-amp & noise
> gain of 2 ).
>
> Relative to a 1 volt output level, this is a s/n ratio of 110 dB -
> exceeding the spec for the INA103.
>
> The noise from the preceding op-amp is gonna be more in any case - even at
> unity gain the INA103 is speced at 10 uV output noise ( 70 nV rt Hz) .

I'm just getting very fussy over *all* noise contributions these days. As you
correctly point out, even 4k7 makes a significant difference to the overall
noise of a 5532.

Graham


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