1). In Lm339, when I connected inv terminal with +5V & non-inv terminal at 0v with 10K resistance in path of both. When I measured voltage at inv terminal its bit less than 5V & voltage at non-inv terminal is bit greater than 0v or 6-10mV. Why is it so?
2). How to select resistances b/w "inv & volatge" & "non-inv terminal & voltage" , so that current consumption is minimum & performance is also good.
3). Its an open collector device, so here also should I keep Rinv & Rn-inv equal.
The INV input may measure lower than 5V because the meter you used likely has a 10M input impedance. Taking the measurement lowers INV by 10K/(10M+10K)*5V or 5mV.Becuase the NON-INV input is much lower than the INV input, double the input bias current will flow out of the INV pin only. This is 2*25nA = 50nA. This 50nA flows through the 10k resistor and meter. 50nA * 10k|10M = 0.5mV. This calculation is lower than you observed. Be sure to compare the NON-INV voltage with the grounded side of the 10k resistor to verify that is truly 0.000V.
The input resistor do not change the input bias current which is negligible compared to the LM339 ICC current.For lowest input offset voltage, the resistance seen by both inputs should be equal.
Regards,Ronald MichallickLinear Applications
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1. I have attached a ckt below having R1 & R2. How does input bias current changes if R1=R2=0 ohm or R1=R2=100K or R1=100K & R2=200K.
I think input bias current remain same, but as you mentioned it may get doubled.
2. The error produced, do it increase the input offset voltage. Can you shed more light on it. I read that base resistance produces error voltage
vin= Ib1*Rb - Ib2*Rb.
How does it relates to input offset voltage.
1) The input resistor does not change the current. the current in R1 should be ~50nA, current in R2 should be ~0nA. (with equal input both will be 25nA)
2) The device offset does not change but application offset has an additional error per your formula. So the application offset is increased.
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