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Bogus parts from Reputable Dealer?

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Cerebus:

--- Quote from: wraper on January 20, 2020, 09:13:29 pm ---
--- Quote from: Cerebus on January 20, 2020, 09:05:00 pm ---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.

--- End quote ---
For sloppy op amp like one in question (4.5mV max offset @ 25oC), 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.

--- End quote ---

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:

--- Quote from: Cerebus on January 20, 2020, 10:32:03 pm ---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.

--- End quote ---
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.

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

--- Quote from: wraper on January 20, 2020, 10:48:03 pm ---... 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.

--- End quote ---

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:

--- Quote from: wraper on January 20, 2020, 07:01:16 pm ---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.

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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.

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