[sdiy] input buffer question

David G. Dixon dixon at interchange.ubc.ca
Tue Feb 9 21:38:42 CET 2010


I found a very nice little tutorial document on the web, in the form of
Analog Devices MT-036.  Here's the link:

http://www.analog.com/static/imported-files/tutorials/MT-036.pdf

It says that the best way to protect against input overvoltage and phase
reversal in unity-gain buffers (voltage followers) is with diode clamps on
the input to each rail (when not included inside the opamp, or even
sometimes when included if you want to protect the internal diodes) in
conjunction with an input current-limiting resistor, typically of at least
1k.  The best diodes to use in this case are Schottky diodes because of
their lower voltage drop, but I can't see why they would be significantly
better than regular diodes.  Sometimes, a 1k feedback resistor may also be
desirable, but is typically unnecessary for FET input opamps.

The input resistor should be fairly small, because it contributes to offset
voltage errors.  However, again, for FET input opamps this is typically not
a significant problem.

I've also checked the common-mode range of TL07x (a spec I didn't really
know how to interpret before) and it is -12 to +15V.  Hence, the lower diode
won't really protect against phase reversal, but how often does one
encounter a CV lower than -12V?  Also, powering these opamps from +/-12V
rails would preclude the possibility of phase reversal -- perhaps the one
good reason to use this voltage level (?).


> Scott Nordlund wrote:
> >
> > Clamping diodes may also offer the benefit of preventing the dreaded
> > "phase inversion" distortion that some op amps are susceptible to
> > in this configuration.  Not that it's likely to be a big issue if you're
> > powering it from +/- 15V.
> 
> Actually, quite the opposite.  Phase inversion/reversal is (copied from my
> website):
> 
> "Phase reversal happens when the input signals exceed the common-mode
> input range and negative feedback turns into positive feedback. The result
> is that the output of the op-amp latches on to one of the rails and stays
> there until power is removed. "
> 
> For the TL07x family the typical common mode input range is -12V to +15V.
> For example, V- at -15V and V+ to +15V (or vice-versa) will more than
> likely put it into the phase reversal state.
> 
> If you want to avoid phase reversal you could, for example, put anti-
> parallel diodes across the inputs.  In that case I would also suggest
> putting a feedback resistor in, even though in the unity-gain buffer case
> it doesn't have any effect on the ideal gain calculation.
> 
> Neil




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