Hi all
Just want to confirm a design choice. Basically in a circuit I have designed comparators and take cells as there inputs and determine if there is difference between cells (to allow for them to be balanced). The output of the comparators (differential op-amps) is 3V3 or under, I then decided to add butterworth filers after this I was wondering is this considered too much or is this a good idea.

Kind Regards
Sunny

 

Hi all
Just want to confirm a design choice. Basically in a circuit I have designed comparators and take cells as there inputs and determine if there is difference between cells (to allow for them to be balanced). The output of the comparators (differential op-amps) is 3V3 or under, I then decided to add butterworth filers after this I was wondering is this considered too much or is this a good idea.

Kind Regards
Sunny

That's too much.

 

Butterworth or other complex filters are usually only used when you need to filter a specific band of frequencies with good control of the filter bandwidth and rolloff rate.
If you want to filter out noise at the comparator, then just use a simple 1st order RC low-pass filter at the comparator inputs.
Make the RC time-constant as large as you can while still maintaining the desired response time for the comparators.

 

Why do you think you need a filter between the output of the comparator and the input to the microcontroller?

ak

 

Why do you think you need a filter between the output of the comparator and the input to the microcontroller?

ak

Hi my supervisor asked me to do this, I personally feel like it was way too much I just wanted to get a second opinion though

 

The output of the comparator is a digital signal (ON-OFF). So is compatible with the micro-controller input. The comparator is really a 1-bit analogue to digital converter. You need the filter on the input to the comparator - which is the analogue voltage from the battery. A simple R-C single stage will do with the time constant as long as possible.

This will remove noise and with an appropriate time constant, power line 50/60Hz hum from your signal. Best to use comparators not op-amps and have a very small amount of positive feedback to prevent oscillation when both input voltages are very close to equal.

Make sure the comparator output cannot go above 3.3V or you will damage the micro-controller.

 

If the concern is the noise burst that will happen as the battery voltage s l o w l y transitions across the detection level, the correct solution is adding hysteresis to the comparator to eliminate any possibility of noise, rather than adding a filter bandaid.

ak