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blowing up OPA277's

Started by George Herold November 4, 2016
Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail.  (All of a sudden the inputs are spitting out DC voltages.)  

A circuit fragment looks like this,


Vin----RRRRR-----+----more of the same... 
                 |
                R1
                 |
                 +--|>|---+
                 | |\     |  
                 +-|-\    |
                   |   >--+
V ref--------------|+/
                   |/     

I've found the OPA277's to be a little "delicate" in the past, is there
something bad about the diode in the feedback path?  

I'm going to try a different opamp type.

Oh, I also use the similar OPA227... faster more Vos. 
In the spec sheet for the opa227 they recommend a minimum resistance 
of 500 ohms in the feedback path.    

George H.          
On Fri, 4 Nov 2016 11:28:14 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > >A circuit fragment looks like this, > > >Vin----RRRRR-----+----more of the same... > | > R1 > | > +--|>|---+ > | |\ | > +-|-\ | > | >--+ >V ref--------------|+/ > |/ > >I've found the OPA277's to be a little "delicate" in the past, is there >something bad about the diode in the feedback path? > >I'm going to try a different opamp type. > >Oh, I also use the similar OPA227... faster more Vos. >In the spec sheet for the opa227 they recommend a minimum resistance >of 500 ohms in the feedback path. > >George H.
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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
On Friday, November 4, 2016 at 2:28:18 PM UTC-4, George Herold wrote:
> Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > > A circuit fragment looks like this, > > > Vin----RRRRR-----+----more of the same... > | > R1 > | > +--|>|---+ > | |\ | > +-|-\ | > | >--+ > V ref--------------|+/ > |/ > > I've found the OPA277's to be a little "delicate" in the past, is there > something bad about the diode in the feedback path? > > I'm going to try a different opamp type. > > Oh, I also use the similar OPA227... faster more Vos. > In the spec sheet for the opa227 they recommend a minimum resistance > of 500 ohms in the feedback path. > > George H.
Can't be that: 7.3.3 Input Protection The inputs of the OPAx277 series are protected with 1-k&Omega; series input resistors and diode clamps. The inputs can withstand &plusmn;30-V differential inputs without damage. The protection diodes conduct current when the inputs are over-driven. This may disturb the slewing behavior of unity-gain follower applications, but will not damage the operational amplifier. What the heck does inputs spitting out DC voltages mean?
On Friday, November 4, 2016 at 2:55:09 PM UTC-4, bloggs.fred...@gmail.com wrote:
> On Friday, November 4, 2016 at 2:28:18 PM UTC-4, George Herold wrote: > > Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > > > > A circuit fragment looks like this, > > > > > > Vin----RRRRR-----+----more of the same... > > | > > R1 > > | > > +--|>|---+ > > | |\ | > > +-|-\ | > > | >--+ > > V ref--------------|+/ > > |/ > > > > I've found the OPA277's to be a little "delicate" in the past, is there > > something bad about the diode in the feedback path? > > > > I'm going to try a different opamp type. > > > > Oh, I also use the similar OPA227... faster more Vos. > > In the spec sheet for the opa227 they recommend a minimum resistance > > of 500 ohms in the feedback path. > > > > George H. > > Can't be that: > > 7.3.3 Input Protection > The inputs of the OPAx277 series are protected with 1-k&Omega; series input resistors and diode clamps. The inputs > can withstand &plusmn;30-V differential inputs without damage. The protection diodes conduct current when the inputs > are over-driven. This may disturb the slewing behavior of unity-gain follower applications, but will not damage the > operational amplifier. > > What the heck does inputs spitting out DC voltages mean?
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 negative 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.)
On 11/04/2016 02:28 PM, George Herold wrote:
> Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > > A circuit fragment looks like this, > > > Vin----RRRRR-----+----more of the same... > | > R1 > | > +--|>|---+ > | |\ | > +-|-\ | > | >--+ > V ref--------------|+/ > |/ > > I've found the OPA277's to be a little "delicate" in the past, is there > something bad about the diode in the feedback path? > > I'm going to try a different opamp type. > > Oh, I also use the similar OPA227... faster more Vos. > In the spec sheet for the opa227 they recommend a minimum resistance > of 500 ohms in the feedback path. > > George H. >
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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Fri, 4 Nov 2016 11:28:14 -0700 (PDT), the renowned George Herold
<gherold@teachspin.com> wrote:

>Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > >A circuit fragment looks like this, > > >Vin----RRRRR-----+----more of the same... > | > R1 > | > +--|>|---+ > | |\ | > +-|-\ | > | >--+ >V ref--------------|+/ > |/ > >I've found the OPA277's to be a little "delicate" in the past, is there >something bad about the diode in the feedback path? > >I'm going to try a different opamp type. > >Oh, I also use the similar OPA227... faster more Vos. >In the spec sheet for the opa227 they recommend a minimum resistance >of 500 ohms in the feedback path. > >George H.
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
On Friday, November 4, 2016 at 3:53:05 PM UTC-4, Phil Hobbs wrote:
> On 11/04/2016 02:28 PM, George Herold wrote: > > Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > > > > A circuit fragment looks like this, > > > > > > Vin----RRRRR-----+----more of the same... > > | > > R1 > > | > > +--|>|---+ > > | |\ | > > +-|-\ | > > | >--+ > > V ref--------------|+/ > > |/ > > > > I've found the OPA277's to be a little "delicate" in the past, is there > > something bad about the diode in the feedback path? > > > > I'm going to try a different opamp type. > > > > Oh, I also use the similar OPA227... faster more Vos. > > In the spec sheet for the opa227 they recommend a minimum resistance > > of 500 ohms in the feedback path. > > > > George H. > > > > 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.
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.
> > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
On Friday, November 4, 2016 at 6:26:37 PM UTC-4, Spehro Pefhany wrote:
> On Fri, 4 Nov 2016 11:28:14 -0700 (PDT), the renowned George Herold > <gherold@teachspin.com> wrote: > > >Hi all, I'm prototyping this square root circuit. (I had a thread 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 diodes to fail. (All of a sudden the inputs are spitting out DC voltages.) > > > >A circuit fragment looks like this, > > > > > >Vin----RRRRR-----+----more of the same... > > | > > R1 > > | > > +--|>|---+ > > | |\ | > > +-|-\ | > > | >--+ > >V ref--------------|+/ > > |/ > > > >I've found the OPA277's to be a little "delicate" in the past, is there > >something bad about the diode in the feedback path? > > > >I'm going to try a different opamp type. > > > >Oh, I also use the similar OPA227... faster more Vos. > >In the spec sheet for the opa227 they recommend a minimum resistance > >of 500 ohms in the feedback path. > > > >George H. > > Ummm. maybe a series resistor in that Vref path would be a good idea.
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.
> > --sp > > > -- > Best regards, > Spehro Pefhany > Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
On Fri, 4 Nov 2016 15:59:14 -0700 (PDT), the renowned George Herold
<gherold@teachspin.com> wrote:

> >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
On Friday, November 4, 2016 at 7:06:19 PM UTC-4, Spehro Pefhany wrote:
> On Fri, 4 Nov 2016 15:59:14 -0700 (PDT), the renowned George Herold > <gherold@teachspin.com> wrote: > > > > >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.
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 <~0.1% level for reasons I've never understood. I've now got a circuit that can kill 'em in a few strokes. (oh I should add that these are the 8 pin dip duals, 2277, all the twos and sevens get confusing.) George H.
> > --sp > > -- > Best regards, > Spehro Pefhany > Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8