opa type comparator input range beyond supply rails?

[hsiangleung]

Are there any ways to realize such a operational amplifier type comparator with input range beyond the supply rail voltage,e.g, 4V input for 1.8V supply voltage? I only find some rail to rail implementations, but not beyond raii-to-rail. Can anyone give some suggestions and references. thanks a lot!

 

[jiripolivka]

Are there any ways to realize such a operational amplifier type comparator with input range beyond the supply rail voltage,e.g, 4V input for 1.8V supply voltage? I only find some rail to rail implementations, but not beyond raii-to-rail. Can anyone give some suggestions and references. thanks a lot!

If your input signal to the comparator is pulsed, with "zero" within the rails, and "one" beyond, then a simple voltage divider will make it work. Depends on setting the reference level in a comparator.
I remember using a LM339 with a negative input pulses (under the GND rail), and it worked within 200 mV negative.
Try to test our circuit. If it does not work, you will have to move your rails.

 

[hsiangleung]

hi jiripolivka, thanks for your reply! Actually the comparator requires an input common mode range beyond the supply rails. So both input terminal voltage may be greater than the supply rail. How can I make the design to meet such demands?

If your input signal to the comparator is pulsed, with "zero" within the rails, and "one" beyond, then a simple voltage divider will make it work. Depends on setting the reference level in a comparator.
I remember using a LM339 with a negative input pulses (under the GND rail), and it worked within 200 mV negative.
Try to test our circuit. If it does not work, you will have to move your rails.

 

[FvM]

The question has two aspects:
- you should to find/design a comparator circuit with the intended properties
- find out how it can be implemented in your available chip technology

It's effectively impossible to make high impedance comparator inputs with 2V beyond-rails input range. So you'll either implement on-chip voltage dividers or a current mirror input stage as used for high-side current sense amplifiers. Both solutions will draw a respective high input current.

 

[jiripolivka]

In a comparator, the TRANSITION between input voltage levels generates the output. Your design should make sure that the TRANSITION between both levels passes between actual rails. The end levels are not so important. If your rails are too narrow, you should redesign your circuit.

 

[KlausST]

Hi,

if the input impedacne is not the problem, then a voltage divider with reference to 0.6V (biased diode) may be a way.

Klaus

 

[FvM]

In a comparator, the TRANSITION between input voltage levels generates the output. Your design should make sure that the TRANSITION between both levels passes between actual rails. The end levels are not so important. If your rails are too narrow, you should redesign your circuit.

Good point, a specially designed comparator can work with one input beyond the rails if the other input stays within. Additionally requirement are however:
- no phase-reversal
- input protection designed for wide input range
- maximum voltage of the chip technology must be observed

 

[hsiangleung]

hi all,
your suggestions are really helpful and instructive, I'll try it. Thanks so much!