Dual Opamp Chips: One as a voltage reference and other as comparator

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Hi,

I attach two circuit applications that I want to implement. One as voltage reference, and another as a comparator to inteface an optical speed sensor. I would like to use the MCP602, which are Single Supply CMOS Op Amps. The optical speed sensor circuit uses a LM393, which is specified in the datasheet as comparators. So I guess it has some extra features that make them good as comparators. Can I use the MCP602 for both purposes even though it doesn't specify that it could be use as comparator? At the end of the day, they are all opamps, but with different responses. What are your suggestions?
 

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crutschow

Joined Mar 14, 2008
26,340
As you say, switching speed is the main difference between a comparator and an op amp, so which you use will depend upon the maximum frequency of the speed sensor signal.
 
Last edited:

danadak

Joined Mar 10, 2018
4,057
The slew rate of the 602 is ~ 2.3 V/uS nominal, so ~ 4.6 uS for a 5V
swing. Using a rule of thumb that 25% of period should be max for
Tr and another 25% for Tf that equates, for square out, to ~ 18.4 uS
data rate, or ~ 50 Khz. If you want to go much faster than that I would
advise faster OpAmp or a comparator.

Regards, Dana.
 

danadak

Joined Mar 10, 2018
4,057
Does not compute.
For sure my ADD kicked in on this one thinking 2.3 was the slew time.

The OpAmp slews 2.3 V in 1 uS, so to slew 5 V its 5 / 2.3 / 1uS = 2.17 uS

So using 25% rule for Tr and Tf and Square wave we get 8.68 uS or

115 KHz.

Regards, Dana.
 

Gibson486

Joined Jul 20, 2012
331
You can use any op amp as a comparator. However, there are many dips in the road you will find. People have figured out these issues and made a specific op amp that you can buy for that very case. It fixed all the issues so you do not have to worry about them. It is kind of like getting rid of input offset voltage and current in an op amp. Yes, it can be an issue, but they newer generations of op amps kind of take care of this with fet inputs (ie choppers or whatever they call them) so you do not have to worry about it in most cases. It frustrates older engineers who had to live through it but that is how technology works.
 
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