Understanding Opamp parameters
Understanding Opamp parameters
(OP)
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.
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: Understanding Opamp parameters
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
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Understanding Opamp parameters
Vout = (Vin x 100k)/(100k + 600k) = 1.43V
What am I doing wrong?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Understanding Opamp parameters
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: Understanding Opamp parameters
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Understanding Opamp parameters
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
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RE: Understanding Opamp parameters
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
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
RE: Understanding Opamp parameters
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.
Z