LM361: LM361M/NOPB Input voltage

Hello Ryoji,

The input common mode range looks good. Typically the input range for the input is 2V below V+ and 1V above V- (+13 to -14).

However, the differential input voltage (voltage between the input pins) is limited to 5V because of the unclamped input stage. Voltages greater than 6V between the inputs can damage the input devices (due to reverse base-emitter breakdown - see Q1 and Q2 on page 7).

Input 1 looks okay since ±0.35V is well below ±2.5V (assuming you are adjusting the OFFSET line to around GND level)

Input 2 is greatly exceeding the differential limit.

The ±5.04 (10.08Vpp) is much greater than the ±2.5V (5Vpp) differential limit. So the INPUT 1 needs to be attenuated by slightly more than 2 otherwise eventual damage to the input pair will occur. Use R54 and add another resistor to GND from INPUT2 pin to create a voltage divider. The resistor values should be fairly low (~1k or less) to avoid adding RC delay. Don't forget the ±0.35V counts in the differential range, too.

It may "seem" to work now as drawn, but you may find the offset shifting with eventual failures. Add the input 2 attenuation and you should be fine.

Hello Paul,

Thank you for your support.

Datasheet mentioned that Differential limit is up to ±5V.  

But you mentioned ±2.5V differential limit as following. Could you please explain it?

 > The ±5.04 (10.08Vpp) is much greater than the ±2.5V (5Vpp) differential limit. 

Best regards,

Hello Ryoji,

The inputs MUST NOT be more than 6V apart at any time. The recommended maximum is 5V apart.

Input 1 is between +350mV and -350mV, near ground. This means input 2 should not go more than 5V beyond that, or  +4.65 to -4.65V single ended (due to the out-of-phase  +/-350mV hysteresis feedback)

So when you say your "input" signal is ±5.04V, are you saying the signal at pin 2 (referred to GND) is +5.04V to -5.04V?

If +5.04 to -5.04, then you will be slightly outside the specified differential input range (5.39V total). You are still within abs-max, but 400mV beyond recommend. I don't anticipate that causing any issues, but...you are operating slightly outside the recommended range.

Maximum speed is achieved after 50mVpp, you are WAY above that, so a slight reduction in the input signal level will not make a difference in the prop delay.

Also see AN-87 for a little more info on the LM361:

 AN-87 Comparing the High Speed Comparators (Rev. B)