blowing up OPA277's

I got burned a number of years ago with a chip design that simulated well, but latched up in real life :-(

Since then I've taken to sweeping power supplies slowly on and off and watching for current-flow aberrations.

Of course this depends on the quality of the Spice model... mine were device-level, yours may be... crap.

Sounds like your situation is a lot of capacitance on VREF dumping thru the ESD diodes when you turn off the supplies. Maybe some discrete diodes from there to rails, or some kind of snuff circuit? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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             I'm looking for work... see my website.

ybe a year ago.) The circuit works fine in spice, but turning it on and of f a few times.. (or something) is causing the opamp protection diodes to fa il. (All of a sudden the inputs are spitting out DC voltages.)

Can't be that:

7.3.3 Input Protection The inputs of the OPAx277 series are protected with 1-k? series input resistors and diode clamps. The inputs

diodes conduct current when the inputs are over-driven. This may disturb the slewing behavior of unity-gain follow er applications, but will not damage the operational amplifier.

What the heck does inputs spitting out DC voltages mean?

maybe a year ago.) The circuit works fine in spice, but turning it on and off a few times.. (or something) is causing the opamp protection diodes to fail. (All of a sudden the inputs are spitting out DC voltages.)

t resistors and diode clamps. The inputs

on diodes conduct current when the inputs

ower applications, but will not damage the

The circuit seems to work once. Then I add the next bit to it. Power on and something's wrong. The reference voltage is set by a resistor string and the voltages along the string are now not right... once they went negat ive at one opamp input... another time it was a positive over voltage.

I unhooked the resistor string, and measured voltage ~-9V .. it's weird.

I ripped it up and am now using opa134's.. everything is fine so far.

George H. (gotta run.)

With the excellent input protection they talk about, it sounds like you have a transient supply inversion on power-up or power-down. A Schottky rectifier from each supply to ground would fix that.

Cheers

Phil Hobbs

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http://electrooptical.net

Ummm. maybe a series resistor in that Vref path would be a good idea.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

maybe a year ago.) The circuit works fine in spice, but turning it on and off a few times.. (or something) is causing the opamp protection diodes to fail. (All of a sudden the inputs are spitting out DC voltages.)

OK, (sometimes lotsa talk, indicates a potential problem.) I don't really know, but I've found this opamp blown in other circuits.... most of the time with a big integrating cap as feedback.

Anyway.. I don't think Vos is all that important, so I'll use a different opamp.

I would like to ask about everyone's fav. low offset opamp.

George H.

aybe a year ago.) The circuit works fine in spice, but turning it on and o ff a few times.. (or something) is causing the opamp protection diodes to f ail. (All of a sudden the inputs are spitting out DC voltages.)

Oh my, the reference is big resistor string... about 110k ohm total.

5V reference, the first resistor is ~72k ohm. (2V-0.5V in 5 steps) (I was thinking that might be a bit high and I should cut it down some.)

George H.

Never mind. I thought it might be coming from a reference chip.

Major current out of the inputs is a pretty clear indication something bad has happened in that area. Usually power supply spike-related failures cause the output to rail.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

Hmm OK. Maybe some R in the the power lines? I've ripped out all the '277's but I could put one back... As I said I've had these failing at the

e maybe a year ago.) The circuit works fine in spice, but turning it on an d off a few times.. (or something) is causing the opamp protection diodes t o fail. (All of a sudden the inputs are spitting out DC voltages.)

re

put resistors and diode clamps. The inputs

tion diodes conduct current when the inputs

llower applications, but will not damage the

or string

ative at

This can't be happening at DC, have you scoped it out in the time domain?

Agreed with Phil, this sounds like a power supply problem. Hang a scope on your supply rails and watch for bad stuff happening at turnon or turnoff?

Another common cause of "works once" is large undamped LC filters that ring way above the bus voltage.

-- john, KE5FX

I have heard that some cheap variable lab supplies exhibit a high voltage transient at turn-on. I checked mine at 12V and did not see anything. If in doubt, a fairly large low ESR capacitor, as well as perhaps a small inductor, and maybe a TVS diode, would eliminate this possibility.

Paul

ere maybe a year ago.) The circuit works fine in spice, but turning it on and off a few times.. (or something) is causing the opamp protection diodes to fail. (All of a sudden the inputs are spitting out DC voltages.)

here

e

input resistors and diode clamps. The inputs

ection diodes conduct current when the inputs

follower applications, but will not damage the

stor string

egative at

.

Not closely... only after it failed. I'll look at the power supply today, it's an old HP triple output 6235A... I've been using it forever without any issues.

thanks to everyone else with their ideas.

George H.

e:

here maybe a year ago.) The circuit works fine in spice, but turning it o n and off a few times.. (or something) is causing the opamp protection diod es to fail. (All of a sudden the inputs are spitting out DC voltages.)

there

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s input resistors and diode clamps. The inputs

otection diodes conduct current when the inputs

n follower applications, but will not damage the

sistor string

negative at

rd.

n?

.

I had a very similar problem a few years ago...it turned out to be the HP 6

218C I was powering the circuit with. Transient when powering up.

ote:

ad here maybe a year ago.) The circuit works fine in spice, but turning it on and off a few times.. (or something) is causing the opamp protection di odes to fail. (All of a sudden the inputs are spitting out DC voltages.)

is there

tance

ies input resistors and diode clamps. The inputs

protection diodes conduct current when the inputs

ain follower applications, but will not damage the

resistor string

nt negative at

eird.

r.

ain?

...

6218C I was powering the circuit with. Transient when powering up.

Well the power supply is perfectly well behaved in turn on and off. Circuit works fine with a different opamp. I'm moving on, but would like to hear from anyone using the oap227 (and cou sins) who has had "issues".

George H.

I've used similar opamps and have issues... the 2 aren't really connected though :)

NT