Bogus parts from Reputable Dealer?

[wraper]

Considering that when the good op amps are installed the whole project works as it should . So I think I have a pretty good idea of what I'm doing.

Frankly, you have no clue.

[Jwillis]

Considering that when the good op amps are installed the whole project works as it should . So I think I have a pretty good idea of what I'm doing.

Frankly, you have no clue.

I's that the only solution you can come up with is to attack me .I've given all the information  of what is occurring ,posted schematics and photos In as a reasonable way as I can and people attack me? 

I posted to find a solution to a problem.  To find a way to test the suspect chips and all you've done was give out nonsense and then attack me .

If you have something personal against me then then spit it out . Other wise why do you even reply to my post.

[Bud]

Do what Wrapper said in reply #22 to test your opamp when no external voltage source is connected to the Mosfets (no current through the Mosfets). Your problem is the inverting input of the opamp is effectively floating / isolated when no current flows through the Mosfet. Alternatively, connect a voltage source to your Mosfet drain to let current flow through the Mosfet, that will create voltage drop on the Source resistors into the inverting input of the opamp and everything should work. Wrapper provided you with a simulation that shows exact symptoms you experiencing when the Mosfets drain is disconnected.

[Cerebus]

The working circuit delivers a voltage from pin 6 to  the Mosfet  regardless whether theirs a voltage at the drain of the mosfet or not . it makes no difference . With the good op amps installed the whole project works when a voltage and current is supplied to the control board. The bad op amps lock the mosfets full on and can not be controlled.

The bad op amps deliver 9v continuously At pin 6 with 0 volts applied to pin 3 . With 0 volts applied to pin 3 on the bad chips there is still 250mV coming from pin 3.
Considering that when the good op amps are installed the whole project works as it should . So I think I have a pretty good idea of what I'm doing.

Can you please either (1) annotate your schematic with pin numbers or (2) refer to the signal names. Without digging out a datasheet we've no idea what signal is on what pin.

To enable sane discussion I've edited your quotes with pin numbers substituted for signal names and obvious typos corrected.

The bad op amps deliver 9v continuously at the output (pin 6)  with 0 volts applied to the non-inverting input (pin 3) .

Yup, the output at positive saturation just as I predicted. The op amp doesn't need to be bust for that to happen, I would expect it as long as there was some input offset voltage present because you've no effective feedback loop with no power applied to the mosfet sources. What's telling here is that you're not telling us what voltage is on pin 2 (the inverting input) at the same time. For this particular op amp V_out = (V_pin3 - V_pin2) * 100,000, so a 90 uV difference between the two inputs is enough to drive the output to the rail. The typical input offset voltage is specified as 2 mV, 22 times more than is needed to force saturation.

This MUST be a typo:

With 0 volts applied to the non-inverting input (pin3) on the bad chips there is still 250mV coming from the non-inverting input (pin 3).

If you mean the output instead of the non-inverting input then, again, with no effective feedback loop you cannot predict what voltage will be there.

Considering that when the good op amps are installed the whole project works as it should . So I think I have a pretty good idea of what I'm doing.

What constitutes "works as it should"? That is, what are the signal conditions that you are taking as "works"?

You keep claiming that you have "a pretty good idea of what I'm doing" or similar but all appearances suggest otherwise and that you're blaming "bad op amps" for a failure to understand how this circuit should work. Folks are trying to help, but we can't if all you do is say 'I know what I'm doing' when offered explanations and advice. Please convince us that you're not clueless and that you understand the basics of this circuit by answering one question: What is the transconductance of this whole circuit (from the control input to the source of the mosfets)?

Yes, this is a deliberate shibboleth and you should be able to answer it in 5 seconds if you know what you're about. If you can't then please listen to what people are saying.

[TerminalJack505]

I've attached a screenshot of your circuit running in a simulator under the conditions that you have described--so far as I can tell.

The simulation results should be pretty similar to what you would see in the real world.  What is it you're are seeing that is different?

mmm i have not seen this simulator before, whats it called?

That's the Tina simulator from DesignSoft.  You can get a free version of it from TI here: http://www.ti.com/tool/TINA-TI.

[Jwillis]

The circuit  appears to be working as it was intended to do so for now .And it may still need further work.But these are the observations I'm making .

With the 2 "good" op amps installed , when I apply 0 volts to the non inverting pin(3) as it is  drawn in the schematic . I get 0 volts at the out put pin (6). No current flows through the mosfets. When a variable DC voltage is applied to the non inverting pin (3) I get full variable range of current sink and both  Mosfets have an equal temperature. Which means that both Mosfets are sinking current equally .The circuit seems to work as it is intended to do so.

With the "bad" op amps installed in the circuit  , When 0 volts is applied to the non inverting pin (3) . I get 9 V at the output pin (6) and the Mosfets go full on. and there is 250mV measured at the non inverting pin(3) when it should be 0V.

I've done the suggestions made and the observation remain the same as I described . 
The observation leads me to believe the suspected IC's seem to be  leaking internally. I could be wrong I'll admit that .

 But I want to test them separately out of the circuit to determine if there is indeed a problem with them. This was the intention from the beginning. A simple circuit to test the suspected chips .

[TerminalJack505]

It sounds like they probably are bad if what you say is true.  I could see you having this particular problem if you accidentally exchanged the two op amp inputs or if you were using different components (other than the op amp.) But if you are just swapping the op amps out on the breadboard and not changing anything else then they should all behave the same.

[ebastler]

It sounds like they probably are bad if what you say is true.  I could see you having this particular problem if you accidentally exchanged the two op amp inputs or if you were using different components (other than the op amp.) But if you are just swapping the op amps out on the breadboard and not changing anything else then they should all behave the same.

I am still not convinced. Having seen the wiring photos, the change in circuit behavior may just be due to having moved the wires around a bit when swapping ICs.

Jwillis — have you swapped back and forth between a known „good“ and a known „bad“ op-amp a few times to confirm that the problem is really tied to the op-amp? I.e. the problem reproducibly goes away when you put in the “good“ amp, and comes back when you install the “bad“ one?

Also, do you have an oscilloscope, and have you used it to inspect the circuit when it behaves badly? Your breadboard setup uses rather long wires and no decoupling on the power supply of the op-amp; so I still think that oscillations may be the root cause. (And that they come and go either with slight differences in the individual amps’ properties, or more likely with moving the wiring around.)

[Jwillis]

I am still not convinced. Having seen the wiring photos, the change in circuit behavior may just be due to having moved the wires around a bit when swapping ICs.

Jwillis — have you swapped back and forth between a known „good“ and a known „bad“ op-amp a few times to confirm that the problem is really tied to the op-amp? I.e. the problem reproducibly goes away when you put in the “good“ amp, and comes back when you install the “bad“ one?

yes I did  do that. I checked on the scope for any osculations with the good ones and I get a clean linear line on all T/D between 5 ms and 2us at .2V/D

I have not checked the bad ones at this time because it's getting pretty late and I have to figure a way of preventing the mosfets from burning out with them in the circuit. This is why I need to test them out of circuit.
I'll try again tomorrow .

I.m very sorry if I upset people .It's pretty clean that I have trouble with getting defensive when I think I'm interpreting  some comments as insults .I apologize.

[rstofer]

I think Wraper nailed it in Reply #22 (and #13 and #21).  I kept thinking that there would be some feedback caused by voltage drop across the 0.21 Ohm resistor caused by MOSFET current flow.  Lacking current flow, the op amp is running open loop, no feedback, with the inverting pin effectively grounded.  Any voltage on the non-inverting pin will cause the output to go high and stay there.

The idea of shorting out the capacitor as shown in Reply #22 seems like a good test.

You could also measure the Vcc current for the left op amp and then for the right op amp.  See if there is a difference.  Is the op amp on the right side of the schematic getting hot?

I think it is going to take some comparative testing of the two circuits to discover the problem.  If you bought your parts from an authorized distributor, I think you should give up on the idea that they are defective and start working on differential analysis.  Why would one side smoke the chips and the other side not.  Eliminate the MOSFETs and add the short proposed in #22 and see how the output follows changes in the input.  Measure power supply currents, all the possible tests you can think of.

The problem is not likely to be defective components.  More likely, you have a couple of op amps that have some internal difference, still within spec, that allows them to deal with whatever the fault really is.

If you had 1 op amp fail in a lot of 10, oh well.  It MIGHT happen.  But you're going through them like popcorn, the problem is not the supplier.

Circuits on the Internet often have problems.  If they are handouts from a seminar, you can bet the presenter said something like "Oh, by the way, there is an error in this schematic.  You need to ...".  Anybody who has ever gone to a seminar has heard something similar.  Sometimes it isn't a design issue but an operating condition.  Like with this circuit!  If there is no MOSFET current, there is no feedback.  Op amps don't like running open loop.

[rstofer]

So, I'm guessing you have caused static or electrical overstress damage in your experimenting.

I meant physically broken parts arrived, like inductors/transformers with chipped corners, capacitors with dented cans and chips with bent leads.

Bad chips that look good are rare, I think I've only encountered once, and it could be ESD or soldering.

And yes, I also have that touch ground before board habit, but I can't say for sure if that works reliably since CDM and MM can also kill chips.

I have a grounded mat that I use when I play with motherboards.  The rest of the time I am working on a Formica surface, no wrist strap, an ESD safe soldering iron and doing just about everything wrong.  Literally a train wreck in terms of ESD protection.

I have NEVER damaged a device!  I know it's just luck and the fact that I primarily mess with digital chips (Schottky protection) but I think it is the higher relative humidity where I live.  Lots of agriculture, just off the Delta (Central Valley of California), something like that.

I'm talking NEVER!  I know, someday I'm going to get bit!  Let's hope it is an inexpensive component.

[Cerebus]

yes I did  do that. I checked on the scope for any osculations with the good ones and I get a clean linear line on all T/D between 5 ms and 2us at .2V/D

You're watching them kissing? There are places you can get arrested for that sort of behaviour. 

[Jwillis]

When the op amps set up with the inverting pin set to ground  and no voltage at the inverting pin it acts like a comparitor with a 0V reference .  If the non inverting side goes above zero the output goes to V_dd. If the input goes to zero then so should the output go to zero. In an ideal op amp.
But because of imperfect manufacturing we don't get perfect op amps. The differential input transistors of real op-amps may not be exactly matched.So This causes the output to be zero at a non-zero value of differential input, called the input offset voltage.
 
For the TVL237X the input offset can be typically 2mV and as high as 4.5mV according to the Data sheet https://www.ti.com/lit/ds/symlink/tlv2370.pdf
 
Since i don't have the equipment necessary to do an proper offset voltage analysis I did what I could . I set up a test circuit like one of the pair in the schematic . Except i added a negative rail so I could inject a variable negative voltage into the non inverting side.The idea was to see much much negative voltage is required at the non inverting pin to get the out put to zero.  I tested all 10 of the op amps in this way.

The 2 that I considered as good showed  no voltage was required at  the non inverting pin to get the output to zero.
The remaining  8 op amps showed much different results.  2 showed results of -3.5mV  and  -5mV at  the non inverting pin to zero the output. They may be with in spec i don't know . Since these chip don't have an internal offset I'll have to do it externally.

The other remaining 6  showed even worse results ranging from -14mV  ,-35mV  , -16mV  , -52mv -215mv  to the worst one at - 618mv. to  zero the output . Now I may not be an expert  and the test is less than accurate but those figures seem pretty darn high to be with in spec.

[ebastler]

Did you also replace the GND supply rail with your negative supply? You are not supposed to pull an input below the negative supply rail on these op-amps. The absolute maximum rating (i.e. the potential damage threshold) is -0.2V, but functionality is only guaranteed down to 0V.

So I am not sure your large offset errors are meaningful, since they were probably measured in "no-man's land". If you want to measure the offset error, instead of pulling one input to negative voltages, I would suggest to pull the other one positive.

[wraper]

Please measure input offset voltage properly. First of all, it's as ebaster said. Secondly, "good opamps" most likely simply have input offset voltage of opposite polarity.

[rstofer]

Once again, Wraper has it spot on.  Input offset voltage is to be measured with Vin = Vdd/2 and Vout should also be Vdd/2 (+- offset voltage) according to the datasheet.
With 9V for Vdd and the ground pin at 0V, Wraper's circuit is exactly right.

Of course, the resistors could be replaced with a pot but I would add 100 Ohm resistors to each end such that the output voltage (input to non-inverting pin) could not reach either 9V or 0V.  In any event, the output should track the input as selected by the pot.

[rstofer]

The 2 that I considered as good showed  no voltage was required at  the non inverting pin to get the output to zero.

It's probably just my warped way of thinking but, to me, there is a huge difference between 0V and 'no voltage'.  I'm thinking that 'no voltage' means the pin isn't connected to anything.  0V, OTOH, is the value of a voltage.  Sure, it's zero but it is defined.  It's right there between a positive and a negative value for voltage.

To be clear, you got 0V output with no connection at the non-inverting pin?  Given the existence of an offset voltage, that would mean that the offset voltage is 0V and would be 'just luck'.

[Cerebus]

For measuring offset the best bet is a circuit like this:



It has the advantage of having gain that pulls what is going to be a small or very small voltage into a range that is easy to measure, it has the disadvantage of needing good matching between resistors for optimal common-mode rejection ratio.

[ebastler]

It's probably just my warped way of thinking but, to me, there is a huge difference between 0V and 'no voltage'.  I'm thinking that 'no voltage' means the pin isn't connected to anything.  0V, OTOH, is the value of a voltage.  Sure, it's zero but it is defined.  It's right there between a positive and a negative value for voltage.

To be clear, you got 0V output with no connection at the non-inverting pin?  Given the existence of an offset voltage, that would mean that the offset voltage is 0V and would be 'just luck'.

I had the same thought, but then recalled the input voltage divider in the schematic Jwillis had posted earlier. I do hope that the voltage divider was present, and hence a pulldown to 0V.

[wraper]

For measuring offset the best bet is a circuit like this:

It has the advantage of having gain that pulls what is going to be a small or very small voltage into a range that is easy to measure, it has the disadvantage of needing good matching between resistors for optimal common-mode rejection ratio.

For sloppy op amp like one in question (4.5mV max offset @ 25^oC), it does not matter. Unless you only have multimeter which does not have mV range. Even crappy 830 series multimeter with 200 mV range would be enough for evaluating if it's in spec using follower circuit I posted.

[Cerebus]

For measuring offset the best bet is a circuit like this:

It has the advantage of having gain that pulls what is going to be a small or very small voltage into a range that is easy to measure, it has the disadvantage of needing good matching between resistors for optimal common-mode rejection ratio.

For sloppy op amp like one in question (4.5mV max offset @ 25^oC), it does not matter. Unless you only have multimeter which does not have mV range. Even crappy 830 series multimeter with 200 mV range would be enough for evaluating if it's in spec using follower circuit I posted.

Well exactly, there's the problem, 4.5mV max/200mV = 2.25% max of a 200 mV range which isn't a great place to be measuring something. Following the usual don't measure below 10% of range rule you'd change range there if you could.  A more reliable measurement  for the cost of two extra resistors (perhaps 4 extra resistors if you don't have a split supply handy) seems cheap to me. You don't need to go the whole gain of 1001 - indeed in the case in point (9V rail, 4.5mV offset) that'd get you into trouble - but a gain of 11, 101 or 'whatever pairs of resistors I have on hand' is still an improvement.

[wraper]

Well exactly, there's the problem, 4.5mV max/200mV = 2.25% max of a 200 mV range which isn't a great place to be measuring something. Following the usual don't measure below 10% of range rule you'd change range there if you could.  A more reliable measurement  for the cost of two extra resistors (perhaps 4 extra resistors if you don't have a split supply handy) seems cheap to me. You don't need to go the whole gain of 1001 - indeed in the case in point (9V rail, 4.5mV offset) that'd get you into trouble - but a gain of 11, 101 or 'whatever pairs of resistors I have on hand' is still an improvement.

It's for evaluating if opamp is within spec. Not precisely characterizing it, in which case you won't use cheapest multimeter anyway. Even if measurement is 20% off, it does not matter in this case. Also with particular resistor values gain is so high that output will clip to power rails at max offset spec.

Following the usual don't measure below 10% of range rule

There is nothing wrong with measuring at the bottom of the range with digital instruments as long as you understand specifications and limitations.
EDIT, and BTW, this particular circuit with meter in series with input is not suitable for CMOS op amps like TLV2371 due to very high input impedance. You are basically connecting voltmeter in series with tiny capacitor.

[Cerebus]

... and BTW, this particular circuit with meter in series with input is not suitable for CMOS op amps like TLV2371 due to very high input impedance. You are basically connecting voltmeter in series with tiny capacitor.

Erm, that's a voltage source symbol, symbolising the offset voltage. Why would you even believe that was a meter? There would be no point of adding gain to then just go and read the unamplified signal at the input.

[rstofer]

Measuring the offset voltage may be an interesting experiment but at this point I think it would be more helpful to know if the op amp did anything at all.  For that, the simple voltage follower with the fixed resistors replaced by a potentiometer (plus the end resistors I mentioned above) would be a more useful test.

I suppose if the device is destroyed, the offset voltage might be out of range but that's only one manifestation of a smoked op amp and perhaps it will be apparent that the op amp is toast without having to get into details.

I'm going to vote for the simple follower...  I can measure the offset voltage at any point in the range of the potentiometer and I can watch the output track the input.

[Jwillis]

Please measure input offset voltage properly. First of all, it's as ebaster said. Secondly, "good opamps" most likely simply have input offset voltage of opposite polarity.

Before I get started on this experiment . Do I need a split supply , single supply or no supply .? I'm guessing VDD must be 9V  but I want to get this right . I want to be sure as to probe orientation . The  voltage at pin 3 must be exactly 4.5V ? I'll try to find exact or as close as I can for the resistors.
I would like to do this right so I give you the right measurements.