Using LM324 As a buffer

Mahesh,

I would expect the circuit with the +/-12V supply to perform better than the circuit with the single 12V supply. The reason is that as you approach the power supply rail the circuit will become nonlinear. When the dual supply configuration is outputting 0V it is not near any supply rail, but when the single supply configuration is outputting 0V it is near ground. I know that this is the opposite of what your measurements show, but I think you may have a connection or measurement issue.

Another thing to look at is the 1k pull up resistor. What are you using this for? I think you should probably eliminate this as it causes a 5mA current that will further degrade swing to the rail. In general current load will cause the output swing to degrade.

I suggest looking at the input and output limitations videos in the link below as it covers this kind if issue in detail.

https://training.ti.com/ti-precision-labs-op-amps-input-and-output-limitations

 Art

Thank you Mr Art,
Actually the 1k pull up is at load side, we have no control on it. The circuit here is used to feed analog voltage to load which will be having pull ups from 1k to 264k or no pull up. Can you please suggest accordingly?

A few comments / questions:

1. 1.      In general, a dual supply configuration should be better than the single supply configuration for swing to GND. So, your second circuit configuration should actually be better than the first. Since your measured results show worse performance, I suspect a possible measurement issue.
2. 2.      The LM324 may not be the best choice for this application. Are you at a point in development where you would consider other options? If so, what are your key design considerations?
3. 3.      Please post a full schematic of your design. I really would not expect the large error that you describe with the +/-12V supply. Perhaps we will uncover the issue by a detailed review of your schematic.

 

Art

To reiterate what Art said, the LM324 is not a good choice for low distortion applications.

The LM324 was originally designed for true single supply applications, with the load returned to V- ("GND" pin). The LM324 has a "unique" output stage that requires a minimum load to sink current. This is mentined in the LM324-N datasheet apps section 7.4:

"To reduce the power supply drain, the amplifiers have a class A output stage for small signal levels which converts to class B in a large signal mode. This allows the amplifiers to both source and sink large output currents.....

The output voltage needs to raise approximately 1 diode drop above ground to bias the on-chip vertical PNP transistor for output current sinking applications.

For ac applications, where the load is capacitively coupled to the output of the amplifier, a resistor should be used, from the output of the amplifier to ground to increase the class A bias current and prevent crossover distortion.

Where the load is directly coupled, as in dc applications, there is no crossover distortion."

And "DC Coupled" means the load is returned returned to V-, not half supply. Returning the load to mid-supply makes things worse. Your pull-up resistor is making things even more worse since it cannot sink current and you are counteracting the sourcing current!!

The zero crossover distortion of the LM324 (and LM358 and LM2902/4) is well know, and the "fix" is to place a pull-down resistor on the output large enough to pull >50uA and bias it to Class A. Google "LM324 crossover distortion" for a wealth of information..you are not the first to notice!

This does NOT apply to the "LMV" low voltage series devices. These have conventional, symmetrical R-R outputs and do not require pull-downs.

Regards,

LM324 interfacing.pdf

Thank you Mr. Art.

Please find detailed schematic in attachment.

The reading i mentioned in previous doc was at output of DAC and not opamp.

Whenever i interface opamp circuit with dual supply to DAC, DAC is giving -0.062v o/p for 0v i/p. But that is not the case for opamp with single supply.

You can suggest alternate part for this, we can implement it.

Thank you Paul.