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NCX2200GM,115 comparator behaviour at high frequencies

Question asked by Martin Moser on May 2, 2019

Dear Support

 

We're using the NCX2200GM,115 in a design and are facing some issues.

 

Current measured circuit is as follows (picture attached):

We're using the comparator as an inverting level shifter. Signal on the positive input (Signal_IN+) is generated with a resistor divider an works as switching threshold. Threshold with R1=4k7 and R2=1k is 0.58V. Signal on the negative input is a square wave 1MHz signal (1.8Vp).

 

We would expect the output of the comparator (Signal_Out) to toggle with 1MHz as well. But this does not happen. This is why we exchanged R2 with a potentiometer to change the switching threshold. 

 

Measurements show that when the threshold (Signal_IN+) is set to 0.68V an below, the output does not toggle (picture: tek00108_NCX2200_not_toggling_0.68V.png). When increasing the threshold the output starts toggling (tek00106_NCX2200_starting_toggling_0.68V.png), but duty cycle is not correct (ton to short). When further increasing the threshold (Signal_IN+), ton is increasing (tek00107_NCX2200_increasing_ton_0.97V.png). 

 

The attached pictures show the following signals/channels:

CH1: Signal_IN-

CH2: Signal_IN+

CH3: Signal_Out

 

We could not find anything explaining this behaviour in the datasheet, hence we have some questions:

 

Can you explain this behaviour to us? Common mode voltage VCM is given to be from VEE to VCC (Rail to Rail) in the datasheet.

 

What is the dependency between common mode voltage and frequency?

 

What is the behaviour at higher frequencies? The only graph showing showing some frequency-dependency is Fig. 6 in the datasheet and stops at 300kHz.

 

Does this comparator have a maximum frequency?

 

We would appreciate any kind of help, thanks.

 

Regards

Martin

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