Protect Instrumentation Amp Inputs

[Ken Petrac]

I just blew a $35 instrumentation amp (differnetial) by accidientally
over-voltaging the inputs. Without degrading performance, how can I
modify the inputs to ensure this will not happen again?

It was an AMP01 and I thought it had a degree of internal protection.
Obviously needs something more.

Ken Petrac

 

[Ken Petrac]

The classic method is a series resistor followed by diodes to the
power rails. Picking the diodes depends on what's important. Can you
stand a bit of leakage current? I'm not familiar with the AMP01.

Something like this then?

www.analog.com/static/imported-files/tutorials/MT-069.pdf

It specifies low leakage diodes. What values would you recommend for
the zeners with a +/-6VDC supply?

Ken Petrac

 

[Spehro Pefhany]

I usually just tie gates to source and run them at Idss max, which is
in the 1.4 mA ballpark for these parts. Most opamp ESD diodes will be
happy with that. There is a tricky way to use one resistor to be
effectively in both sources, to reduce max current, but that will add
more series resistance and usually isn't needed.

This is the basic depletion-mosfet current limiter:

--------d s----+----s d------
g | g
+------+------+


These parts are rated for 500 volts or some such. The opamp power
rails have to sink any clamp current, of course.

You could do this with the bigger Supertex parts, for lower series
resistance but higher currents; then the added resistor might make
sense, to program the current limit.

Jfets would work too, but they usually have horrible parameter spreads
and lower voltage ratings. These Supertex parts are slick.

John

It would be nice if they could put 8 of them in an SO-8.

 

[Nico Coesel]

Note that most power supply regulators will let you lift the supply
line through the didoes using this scheme. You need to insure there is
some sort of protection device on the power supply rails to prevent
this from happening. Generally regulated supplies like to source but
not sink.

I agree. I rather use some resistors and capacitors for filtering and
a TVS or varistor to clamp the inputs.

 

[Spehro Pefhany]

I agree. I rather use some resistors and capacitors for filtering and
a TVS or varistor to clamp the inputs.

It's sometimes better to use separate devices for conducting the
current to the clamp and the clamp itself. A TVS might have 1,000 x
the capacitance of a switching diode. The clamp might be leakier too-
with separate devices you can bias the clamp beyond the operating
range.

 

[Grant]

It's sometimes better to use separate devices for conducting the
current to the clamp and the clamp itself. A TVS might have 1,000 x
the capacitance of a switching diode. The clamp might be leakier too-
with separate devices you can bias the clamp beyond the operating
range.

Yes, I posted a circuit here a while back that used low-leakage diode from signal
to a 4.3V voltage at low source impedance, by running several mA through the zener.

This let me reduce the signal series resistor to 1k (I think) and thus have
little effect on the signal integrity (eventually goes to a dual slope converter,
so more noise not an issue here). In my case I'm protecting the input of a
+/- 5V powered CMOS device (4052) from an opamp powered from +12V/-5V's output,
in order to get clean, accurate signal performance from -3V to 5V. No clamp
for -ve signal as opamp and cmos use same -ve supply.

Point is to make the clamp voltage low impedance so it absorbs the over-voltage,
not let CMOS diodes pump up the supply rails, which is a Bad Thing.

Grant.