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A passive circuit that can average 2 voltages
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maalma



Joined: 01 Mar 2007
Posts: 33
Post30 Jun 2009 11:16   

passive average circuit
Hello all,

I have 3 input voltages VA, VB and VCM from these voltages I need a circuit that can give me in the output these values VCM+(VA+VB)/2 and VCM-(VA+VB)/2

How can I do this??
Thanks for your help

Last edited by maalma on 01 Jul 2009 13:51; edited 1 time in total
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kwkam



Joined: 25 Feb 2002
Posts: 299
Helped: 15
Location: Somewhere on earth
Post30 Jun 2009 16:06   

passive averaging circuit design
you can use opamp approach or V to I method.
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maalma



Joined: 01 Mar 2007
Posts: 33
Post30 Jun 2009 16:52   

passive averager circuit
alright, can you give me more details please
thanks for your reply Smile
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AdrianN



Joined: 28 Jun 2009
Posts: 20
Helped: 10
Location: California
Post02 Jul 2009 8:04   

Re: A passive circuit that can average 2 voltages
Here is a solution. See attached schematic. I used summing and differential amplifiers. For resistor calculations, take a look at these articles:

http://theartofelectronicsdesign.com/?p=26 Solving the Differential Amplifier - Part 1

and

http://theartofelectronicsdesign.com/?p=339 Solving the Summing Amplifier

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maalma



Joined: 01 Mar 2007
Posts: 33
Post02 Jul 2009 9:29   

Re: A passive circuit that can average 2 voltages
Thank you Adrian for your help Smile

i'll try your solution and i'll keep you informed if it works in my application
actually, resistors are very "noisy" and i'm using this in an ADC so the noise must be <1/2LSB=12mV.
i wonder if i can replace the resistors by capacitors??

thanks again
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Post02 Jul 2009 9:29   

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FvM



Joined: 22 Jan 2008
Posts: 5177
Helped: 768
Location: Bochum, Germany
Post02 Jul 2009 15:10   

A passive circuit that can average 2 voltages
Curiously, the question title requests a passive circuit, however all suggestion are about active solutions. That's plausible in so far, as a passive subtractor isn't feasible. Also SC is a possible solution, but not regarded as passive usually.
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AdrianN



Joined: 28 Jun 2009
Posts: 20
Helped: 10
Location: California
Post03 Jul 2009 18:42   

Re: A passive circuit that can average 2 voltages
I would not be worried about the resistor noise. Johnson noise on a 1kohm resistor is about 4nV/sqrt(Hz) at 25degC. Even if you have a bandwidth of 100MHz, the resistor noise would be 40uV. There are 10 resistors in this circuit and with a gain of 1 (or around) the noise generated by these resistors is roughly 40uV*sqrt(10) = 126uV, 40dB below your 12mV requirement.

Moreover, you can choose low noise resistors like metal-film or thin-film resistors, and choose a lower value resistor (hundreds of ohms) if the power consumption is not an issue.

I would be more worried about the flicker noise of the Op Amps you will use, but because the gain is low, that might not be an issue as well.

Therefore, use low noise resistors, low noise Op Amps, have a continuous plane ground to reduce rf interference, use surface mount parts, short traces, good decoupling, and reduce the bandwidth to the minimum required, and you should easily achieve below 12mV noise floor.
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maalma



Joined: 01 Mar 2007
Posts: 33
Post04 Jul 2009 9:33   

Re: A passive circuit that can average 2 voltages
thank you Adrian for these explanations Smile
can you please tell me more about a low noise opamp (i'm really a new commer in the analog design world)??
have a nice day
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AdrianN



Joined: 28 Jun 2009
Posts: 20
Helped: 10
Location: California
Post05 Jul 2009 0:25   

Re: A passive circuit that can average 2 voltages
Companies like Analog Devices, National Semi, Maxim Semi, etc. would post a list of Op Amps on their website and categorize them by Low Noise, High Speed, Low Power, Precision, etc.

As an example, here is a list of Op Amps from Analog Devices which are in the Low Noise category:

http://www.analog.com/en/amplifiers-and-comparators/operational-amplifiers-op-amps/products/index.html
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maalma



Joined: 01 Mar 2007
Posts: 33
Post05 Jul 2009 9:24   

Re: A passive circuit that can average 2 voltages
Hello Adrian,
Thanks for the link, but i'm not talking about devices, i'm talking about schematics to simulate with cadence.
anyway thanks for your help and welcom to EDA board Smile
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