Little help on TL064 opamp

[giovanni82]

Hello guys
this is my first post so I' ll keep it simple

I' m working with the TI TL064 (see the attached schematic) but in stand-by (when SENSEU is 0V) i get an output voltage of 1.3V instead of 2.65V ( that is what i'm expecting).

This is a simple non inverting opamp but i don't understand what i'm missing...

Any suggestion? is the TL064 not the right part to do the job or is there something wrong in my circuit?

Thank you very much

 

[thylacine1975]

'morning Giovanni,

Your circuit looks fine - the problem is the TL064 isn't suited to the job. Specifically, the devices "input common mode voltage range" specification doesn't extend down low enough for your application.

The data sheet characterises the device for +/- 15V supply rails [which is always a warning sign when you're intending to use the device in a single-supply configuration like yours] and lists the input common mode voltage range as +/- 11V (worst case). This means the device won't tolerate it's inputs being within 4V of the supply rails if you want it to behave like an ideal op-amp should (where feedback can maintain 0V difference between the inverting/non-inverting inputs). In short, U3 won't operate as intended if either input goes outside the range 4 -11V!

Which sucks

The simple solution is... find a better op-amp. "Rail-to-rail input/output/operation" are the keywords widely boasted by manufacturers advertising friendlier common mode voltage ranges. Alternatively, look for words like "common mode range includes V- [or Vss or GND]" or "optimised for single supply operation". The LM324 is a (cheap) bipolar part of similar vintage to the TL0xx family that is often used for this attribute (assuming it's other characteristics such as offset/bias current/drifts etc are useful to you). Good luck!

 

[KlausST]

Hi,

Just a comment to opamp datasheet values...
I find the "dual supply" information in opamp datasheet misleading (as we can see here).

Usually this makes sense where devices have a dedicated gnd pin. (Opamps usually don't ).
I don't see any restriction for a +/-15V specified opamp to operate on 0V/30V and vice versa,.

In opposite to that the split supply information on some analog swtches/muxes makes sense, because they have dedicated gnd pins additionally to the two supply pins. Usually the logic input levels are referenced to gnd, where the analog levels depend on suuply rails.

Klaus

 

[crutschow]

,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
I find the "dual supply" information in opamp datasheet misleading (as we can see here).

Usually this makes sense where devices have a dedicated gnd pin. (Opamps usually don't ).
I don't see any restriction for a +/-15V specified opamp to operate on 0V/30V and vice versa,.
.........................

The restriction is there, just not explicit, in the voltage limits for the input common-mode voltage and the output voltage swing. If those two values don't go to the minus rail, then you can't operate it from a single supply if you have a signal that goes to zero and/or want the output to go to ground.

 

[Audioguru]

I notice that the resistor values are VERY low. The TL064 and LM324 are low power opamps but the low resistor values use fairly high power.

I agree that an LM324 will work fine in this circuit since its minimum supply is 3V and its inputs work when they have a voltage as low as the negative supply which is 0V in this circuit.
An LM324 has four opamps in a 14-pins package. An LM358 has only two of the same opamps in an 8-pins package.

 

[Borber]

Common practice, almost a rule, for classic OPA and single supply is to use virtual ground at half supply voltage.