LM258A: LM358A Vcm question!

人在江湖

Part Number: LM258A

The usage of the customer is the follower. The power supply for LM258A is VCC=20V, VGND = GND, PVin+= 18.2v.The customer's question is, if Vin+ is applied with a voltage of 18.2v in the full temperature range, will there be any problem?Is there a problem with Vin+ applying a higher voltage?What kind of problems will arise?Why is that?

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi,

section 7.5 of datasheet specifies the common mode input voltage range. Stay at least 1.5V away from the positive rail with your input voltages at room temperature and stay away 2.0V at maximum temperature range. So, your maximum input voltage would be 20V - 2.0V = 18.0V.

The scheme in section 9.2 of datasheet shows why: The LM258A is a very old OPAmp and is no input rail-to-rail OPAamp. The input signals sees two pn-junctions in series to the positive rail as result from the two PNP input transistors. Increasing the input voltage would make the input transistors turn-off. The OPAamp would no longer work properly then.

Keep in mind that also the output voltage swing ot LM258A is limited. See section 7.5 of datasheet.

One remedy could be to use an RRIO (input / output rail-to-rail) OPAmp instead of the LM258A.

Kai

0 Paul Goedeke over 5 years ago in reply to kai klaas69

TI__Mastermind 30600 points

Kai has provided some excellent advice. It is always advisable to make sure you're not violating the input common mode limitations. I recommend the TI Precision Labs video on this topic: training.ti.com/ti-precision-labs-op-amps-input-and-output-limitations-1

Best,
Paul

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LM258A: LM358A Vcm question!