LM311: external resistor offset

Hello Keith,

The balance pins are connected to the emitter resistors in the front end diff pair.

The balance pot is placed across these pins and adjusted to slightly change the tail currents to correct the offset voltage by a few mV.

It would be possible to add a fixed offset by using a fixed resistor, but I would warn that each device will have slightly different currents and offset. Matching resistor values is what is critical for accuracy, not absolute resistor values, so there may be several percent variance in the actual internal resistor values (as long as they are "off" equally).

And there are differences between the currents in the different manufacturers (TI and ex. National [LM311-N], and the competitors such as ST and NJR) - so the resistor would need to be selected for that particular manufacturer, so 2nd sources would need to be re-calculated.

How much offset are they planning to add?

Hello,

For our application I don’t think it is important for us to have a fixed absolute offset. The problem we need to address is noise picked up by the PCB traces.

We are using the LM311 as a zero crossing detector and level shifter as the output is routed to a CPU. The minus input is connected to ground. The output of U4 has 50Hz (in our case) noise when there is no signal applied to the input.

We just need the offset to be high enough to filter the noise level. And add a safety margin for other parts. We don’t want to put a filter at the input as we do have to be able to measure 50Hz and 60Hz signals.

Thanks very much for your help, Keith

Hello Keith,

Can you provide a schematic? Either they need to add hysteresis or raise the threshold level if there is a inherent noise in the signal.

Also beware that the LM311 is not a ground sensing input and has a limited input range, so if running off a single supply, "GND" is outside the input range (VCC- + 0.5V to VCC+ - 1.5V). Are they using a split supply?

Hello Paul,

I emailed you the schematic separately.

Thank you, Keith