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# LM2902KAV: The output voltage is different between calculated value and measured value on the application circuit

Part Number: LM2902KAV
Other Parts Discussed in Thread: LM2902, LM2904B

Dear Specialists,

My customer is evaluating LM2902KAV and has a question about the output voltage between calculated value and measured value.

I would be grateful if you could advise.

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When using the LM2902KAVQPWR as a DC input amplification with a single power supply, the output was different from the calculated value.
Is this the correct behavior? Please tell me the reason.

In the circuit used (could you please see attached Excel file), the operational amplifier in the red frame was set to LM2902 for measurement.
Then, I checked the output voltage (Vo) when the resistor Rx (blue frame in the circuit diagram) was changed (12kΩ, 13kΩ, 14kΩ, open).
Vo at the time of Rx change is as follows, and the red line part is different from the theory before replacement.

On the other hand, when another company's operational amplifier (uPC324) was used, the values were almost as calculated.

I understand that the offset voltage and bias current do not reach 0V, but it does not match the calculation and I cannot understand the behavior of sticking around 0.5V.

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Best regards,

Shinichi

• Hello Shinichi-san,

The difference is a result of the maximum current sink at low output voltage. V- is high as 2.8V so output sink current can be as high as 2.8V / (39k +10k) = 57uA.

More information in application note. Design Guidelines for Devices with LM324/LM358 Cores Section 3.1

To get a lower voltage, decrease the sink current by increasing resistance of feedback resistors. Also consider using LM2904B which has a stronger sink and 50uA electrical characteristic table value.

• Hi Ron,