LM2903B: Lower Ibias than LM2903 (non-B)

Hi Hayashi,

the input bias current itself has nothing to do with the input impedance. The input impedance is the differential quotient (dU / dI) of input voltage change divided by input bias current change. See figures 6-21, 6-22 and 6-23 of datasheet. The "input impedance" at 25°C and 5V supply voltage can be estimated from figure 6-22 as

Rin =  2.5V / (2.75nA - 2.25nA) = 5GOhm

Keep in mind, though, that when having a bipolar comparator with such high input bias currents, where the input bias current is so much higher than the input bias current change with input voltage, datasheets do not talk in terms of input impedance but rather in terms of input bias currents. This is the reason why you do not find a specification of input impedance in this datasheet.

Hideki Hayashi1 said:

Customer concerns that high input impedance results in weaker against input noise

This is a widespread misunderstanding. If the input impedance is much higher than the source impedance, then it's mainly the source impedance what makes a circuit sensitive to noise. Or by other words, reducing the source impedance by a factor of 100 from 10k to 100R, as an example, improves the noise immunity much much more than decreasing the input impedance by a factor of 100 from 10G to 100M, if at all.

Kai

Thank you Kai, that is very well said. 

I hope this helps answer your questions, Hayashi.