# LM358 op amp parameters...differential amp

Discussion in 'General Electronics Discussion' started by eem2am, Nov 3, 2012.

1. ### eem2am

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Aug 3, 2009
Hello,

The attached is a differential op amp. It uses the LTC1006, but in the actual circuit, it will use the LM358.

Which of the parameters in the LM358 datasheet have the most effect on the accuracy of the output of this circuit?

Also, how do you use the parameter(s) to come up with a maximum error value in the output voltage?

[please note that there is a common mode voltage of 3.6V]

LM358 DATASHEET

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
Input offset voltages and currents are typically the main issues.

Input offset currents will be less of an issue because your input is relatively low in impedance and the resistance seen by both inputs is similar.

Input offset voltage will introduce a small error. It will be small because the gain is quite low. (3mV * 10 = 30mV output error)

The pull-down of the output seems superfluous.

Yoyr circuit diagram appears to show the current source charging the battery. Is that correct? Or is it supposed to be a load?

3. ### eem2am

429
0
Aug 3, 2009
Thanks,

The 3V6 battery is just there to give the common mode voltage, as in the real circuit, there is a battery of 3.6V.

I thought that this woul dcause problems, due to the common mode voltage, but as you say, its the input offset voltage of the LM358 thats the worst feature.?

(attached is more like the real circuit, but it woul duse LM358 and not LT1006)

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4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
What is the biggest problem is really determined by what aspects of the circuit's operation you believe are most critical.

Have you posted this circuit in another thread? Because I'm sure I have commented earlier that it looks a little over engineered.