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Understanding Opamp parameters

Understanding Opamp parameters

Understanding Opamp parameters

I am trying to understand which parameter of an opamp is effecting the performance of a simple buffer circuit.

I have attached circuit.

When using a TI lmc660 opamp my output is not the expected ~1.7 volts, but about 4 volts (close to upper limit for 660 opamp). If I use a Linear LTC2052 precision opamp, I get the expected 1.7 volt output.

The 2052 has much better specs than the 660, but I am trying to understand what spec/specs are not allowing the 660 to work. I don't think it is the offset, CMRR(maybe), or drift characteristics. I know it has to do with rejecting the 10Volt supply across the voltage divider, but I am not sure which spec is related to this. I wouldn't think the CMRR is playing a role since the voltage fed from the divider is lower then the supply voltage of the opamp?

Thanks in advance for helping me understand.

RE: Understanding Opamp parameters

A couple of things:
1) You should NEVER get 1.7V but 1.49V.

2) Maybe you've reversed the inputs on the poor LMC660?

3) Your power supply staging is possibly wrong having the 10V supply beat the 5V supply up, causing a latch-up when the 1.49V gets to the unpowered amp.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Understanding Opamp parameters

As Keith says (except for the 1.49 V (where did they come from, Keith?)

Is this a real circuit? Or is it a simulation? If it is a real circuit, anything could cause this. Wiring wrong, voltages way off, resistors not within tolerance, opamp doesn't feel well, parasitic capacitance and inductance in wires (if they are long), perhaps even a voltage drop in your instrument's reference connection (the "black" wire). Just check all these details.
The specsheet says that you can go up to V+ minus 1.5 V. So you should not expect more than 3.5 V even if there is a gross error in there. But nothing in the spec says that this should happen.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Understanding Opamp parameters

Did I screw up the divider ratio Gunnar?

Vout = (Vin x 100k)/(100k + 600k) = 1.43V

What am I doing wrong?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Understanding Opamp parameters

Your maths is fine, it's your eyes. tongue

I think the upper resistor is 500kΩ, not 600kΩ.

That would give 1.667V, which is close enough to the OP's expected ~1.7V.

RE: Understanding Opamp parameters

Ah.. Sure looks like a 6 to me, yep my eyes. With a magnifier app it is indeed 500k not 600k. Thanks Scotty.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Understanding Opamp parameters

And, suddenly, I can swear by my AVO Mk IV tube tester that it is 600 k! Who changed it?

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Understanding Opamp parameters

While the OP's schematic says one thing, their text says LMC660 and LTC2052; the LM324 only shows up in the schematic.

Nevertheless, the LMC660 also has insufficient drive capability to drive to the positive, or negative, rail with a 1K load. In fact, the LMC660 datasheet shows most parameters specified with a 1M load. LTC's datasheet is obtuse on this subject, but it looks to me that the LTC part has ever so slightly a better voltage swing, since they're willing to spec some things with a 2K load.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Understanding Opamp parameters

Geez IR; I think comprehension by all parties on this thread is jinxed in various ways.

The second line of the data sheet header brags about driving 600Ω loads with the LMC660

Keith Cress
kcress - http://www.flaminsystems.com

RE: Understanding Opamp parameters

600Ω is a standard analog (audio) impedance, although it is usually at low signal amplitudes. The datasheet seems a little economical with the facts.

RE: Understanding Opamp parameters

When debugging an op amp problem like this - USE AN OSCILLOSCOPE AND NOT A MULTIMETER! (yes, I yelled that, it's that important). Any unexpected oscillations can really give some really odd results on a multimeter. I say this because there is no capacitance shown in the circuit. While the datasheets show all is great at DC, capacitance can cause stability issues.

I once 'inherited' a incoming-test fixture at one job. A multimeter showed all 60 channels drifted in a similar fashion so the previous engineers accepted this as normal. The manufacturer said they saw no drift when they tested each unit. I found the opamps were not stable when the output cable (i.e. stray capacitance)) was connected. I changed to better op amps and we never saw drift again either.


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