Offset voltage of LM358

[npelov]

@Rufus So I only need to buffer it through one opamp and make sure the range also includes it's voltage offset. Then I'll have a good Vos correction because the output impedance of an opamp is quite low. I guess the other way is to calculate Rg to add the divider value ... or choose divider in a way to exactly match Rg (and short Rg).


@megajocke 100 dBis 10^5? But Rufus said that 20db is a factor of 100, 100dB is 10^10, so who's right?

Also, looking at LM358 datasheet on page 8, figure 5, there is a graph for A_VOL to frequency relation. if you get DC (<1Hz) you'll see that the value is 110 dB, which is ... 10^11? Well at least between 10^10 and 10^12.  This means that open loop gain is ~ 100 000 000 000.
So can you tell me the difference between the value of A_VOL which is given in dB and Large Signal Voltage Gain which is given as V/mV (or V/uV) - for LM358 it's 25V/mV at room temp or 15 at full temp. range. I thougt it's the same thing, but looks like it's not.

[Rufus]

@Rufus So I only need to buffer it through one opamp and make sure the range also includes it's voltage offset.
Then I'll have a good Vos correction because the output impedance of an opamp is quite low.

Yes but you will have Vos and bias drift in the buffer so it is probably better to still have a divider after the buffer. The divider can replace Rg. A 1k and 10R divider for Rg is only 1% different from a straight 10R.

@megajocke 100 dBis 10^5? But Rufus said that 20db is a factor of 100, 100dB is 10^10, so who's right?

Also, looking at LM358 datasheet on page 8, figure 5, there is a graph for A_VOL to frequency relation. if you get DC (<1Hz) you'll see that the value is 110 dB, which is ... 10^11?

100db is actually 10^9 - my sloppiness. The graph is showing gain change with supply voltage. If the absolute values are supposed to be realistic they look ridiculously large. The data sheet does not specify any maximum Avol but some typicals at 100V/mV or 10^5 or 60db.

dbs are just 10 * log(ratio). The confusion with power is due to power being proportional to V^2 so x10 voltage (+10db) produces x100 power (+20db).

[npelov]

Does input offset voltage vary when gain is changed? I mean input offset voltage, not multiplied offset on output. I ask because I want to have 2 gain resistors - one for "normal usage" - let's say gain = 100 and one for offser zero (to set gain to 1000 or more). So if I zero offset voltage with gain = 1000 to <=0.1uV will the offset voltage stay the same if I set gain to 100:
1. using Rgain
2. using both Rgain and Rfb

Let's say that Ibias and Ioffset are corrected.

I'm mostly asking about opamps with offset zero inputs like OP07. It's quite hard to measure offset in microvolts.