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Design >> Analog Design >> Passive adder/subractor possibility
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Message started by eternity on Feb 25^th, 2014, 1:55am

Title: Passive adder/subractor possibility
Post by eternity on Feb 25^th, 2014, 1:55am

Hi all,
I would like to know about the ways to do the voltage addition or subraction without using an opamp.

If you know some ways, please feel free to share it here

Thanks

Title: Re: Passive adder/subractor possibility
Post by raja.cedt on Feb 25^th, 2014, 7:21am

Hi,
Potential divider would help for addition.

Tx,
Raj.

Title: Re: Passive adder/subractor possibility
Post by eternity on Feb 25^th, 2014, 7:33am

Thanks for the reply
i am more interested in voltage subraction than addition
potential divider will not help me with this scenario i guess

thanks

Title: Re: Passive adder/subractor possibility
Post by carlgrace on Feb 25^th, 2014, 7:51am

You can do it with capacitors. Charge up two caps in parallel, then put them in series. Charge in conserved and voila! You have an adder. Flip some of the capacitors around before you put them in series and voila! You have a subtractor.

Their are lots of practical problems with this technique though. The principal issue is charge sharing with unavoidable parasitics. This means your addition will not be accurate (and will be hard to predict).

That's why op amps are used in this type of circuit. Virtual grounds are very nice things.

Title: Re: Passive adder/subractor possibility
Post by carlgrace on Feb 25^th, 2014, 8:05am