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OPA227: OPA227 Bias Current over Temperature

slocke
Prodigy 100 points
Part Number: OPA227
Other Parts Discussed in Thread: OPA277

In the datasheet for the OPAx227 parts, Figure 15 shows the typical bias current change over temperature. I'd like to clarify my interpretation of this graph, as these typical bias currents (+/-0.5nA) are quite different than the typical bias currents given in the table (+/-2.5nA).  The reason I ask is that a similar figure for the OPAx277 (Figure 11) shows something more like what I would expect (the "graph typicals" match the "table typicals" for that particular device).

So, regarding the 227, which of these interpretations of Figure 15 is correct:

This graph shows the ACTUAL bias current of a typical part over temperature.  

Or, alternatively:

This is how much the bias current will typically CHANGE by over temperature (additive to a typical value in the +/-2.5nA range).

(Or, possibly, some other interpretation I'm missing).

I'm aware that the graph represents typical data and I should not count on it for exact analysis, however I'm just trying to get a feel for what kind of shift in bias current I might expectover a ~20C range.  Is it also safe to assume the offset current could change by a similar amount?

  • 0
    TI__Guru 60570 points

    Slocke,

    It's difficult to talk about TYPICAL input bias when it may be positve or negative due to internal input bias cancellation circuitry.  OPA277 shows a family of IB graphs while OPA227 show a single graph over temperature - how do you show +/-2.5nA at 25C using a single graph.

    The only thing you can draw from the OPA227 graph is that the IB may typically change by +/-1nA over 165 deg C - think of this as one standard deviation variation (one-sigma).

    Thus 2-sigma IB temp variation may be within +/-2nA (95.4% of units), 3-sigma variation may be within +/-3nA (99.7%), etc. but the IB value may be anywhere between +/-10nA specified in the datasheet. 

  • 0 in reply to Marek Lis
    Prodigy 100 points

    Ok, thanks for the quick reply.  This cleared things up.  Effectively, if I was to screen parts, I would see a typical slope (~1 sigma, as you said) of about +/-1nA over a 165C range. 

    For the record, I much prefer the the presentation in the OPA277 sheet....  The OPA227 sheet raised questions for me because the single unit chosen to demonstrate a temperature change just so happened to have a very small input bias current even though most units will be farther from "0" (given that the typical spread is +/-2.5nA).  Having been bitten by datasheet errata in the past, things like that tend to raise red flags.  I would have been less tentative if a "typical" unit with a room temp offset of 1.5nA was graphed over temperature.