Question about op amp errors

[king.oslo]

Hello there,

I have a couple of questions I found hard to get answered whilst searching the web:

1. If I model offset voltage as a voltage source in series with one of the inputs, how to I learn from the datasheet: in series with which input should it be modelled,and of which polarity is it?
2. I model offset current as a current source from one of the inputs. How do I determine which input? Is the polarity of the input current affected by the type of input transistor? Or does the polarity of the input current tell the engineer which input the bias should be modelled at, or both?

Thanks.M

[Kremmen]

Offset voltages and curents are by nature error or stray signals and there is no universal rule regarding their polarity. Often they can be ignored provided the input circuit provides a ground path preventing a capacitive integration of the offset magnitude.
When the offsets cannot be ignored a way must be provided to compensate or zero the offset signal. In such a case you usually cannot calculate it away due to its somewhat unpredictable nature.

[Bambur]

Hello there,

I have a couple of questions I found hard to get answered whilst searching the web:

1. If I model offset voltage as a voltage source in series with one of the inputs, how to I learn from the datasheet: in series with which input should it be modelled,and of which polarity is it?
2. I model offset current as a current source from one of the inputs. How do I determine which input? Is the polarity of the input current affected by the type of input transistor? Or does the polarity of the input current tell the engineer which input the bias should be modelled at, or both?

Thanks.M

Try looking at the op amp model in Fig 7-122 on page 144 of chapter 7 in Op Amp Applications Handbook http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html for some hints on modeling.

[king.oslo]

Thanks guys. I will check out the opamp handbook this evening. Where can I learn how to minimise op amp errors? For example, I imagine low resistance at the input minimise input current errors.

Thanks.M

[Kremmen]

This classic from Carter and Mancini has probably been linked a number of times on this forum but one more time for luck:
http://www.ti.com/lit/an/slod006b/slod006b.pdf
I can't recall if there is a chapter dedicated to errors specifically, but after reading&understanding the test, you won't really need one.

[king.oslo]

Thanks. I will read it tonight.M