IC voltage comparator

[Arul Raju]

Hi,
We were using IC voltage comparator LF311H, now this part is obsolete with no direct replacement. I've been informed that LM311N is similar to the part.
Later i've compared both spec sheets to know the difference between these two parts.
1. Input offset current 6nA in the new part; 5pA in the old part.
2. Input bias current 100 nA in the new part; 25 in the old part.
3. Strobe ON current is 2.0mA in the new part; 3.0mA in the old part.
4. Input offset voltage 10 was 15 mV in the old spec.

Those above changes are affect my existing circuit if i use the new part available.
Please someone help me.

Thanks,
Arul

 

[KrisBlueNZ]

Hi Arul and welcome to Electronics Point

Thank you for giving a thorough problem description and a schematic in your first post. This is VERY helpful!

The LF311 and LM311 are very similar. The main difference is that the LF311 has FET inputs, which have a much lower input current than the bipolar transistors used in the LM311. In this application, the inverting input (pin 3) is driven from a very low-impedance source, so input bias currents are not significant. But the schematic you posted does not show what the non-inverting input, pin 2, is driven from. What is connected to the right side of R15?

The Strobe feature is not used in that circuit (pins 5 and 6 are just connected together), and there is no offset adjustment, so the input offset voltage is obviously not very critical.

So I'm confident that the LM311 will work fine in your circuit, with no changes. The only question is what is driving R15. If that signal has a high impedance, the input current might be a problem. Can you post some more of the schematic?

The LF311 seems to have been manufactured only in a round metal package. The LM311 is available in other packages too. Make sure you order the right one!

 

[Arul Raju]

Non inverting input side image attached

Hello Kris,

Thank a lot for your post towards my thread.
It was great to know about the circuit.My apologies for an insufficient input, i've attached R15 right side circuit.
Please look at it for me and let me know the input bias current is sufficient for this circuit or not?
Great site this is...I missed it so far..will utilize it for the coming days.

Thanks again Kris.

Regards,
Arul

 

[KrisBlueNZ]

OK, R15 is being driven directly from the output of an op-amp - a very low-impedance source.

So I'm confident you can use an LM311 to replace the LF311 in that circuit.