summing amplifire resistor selection

[aruna1]

hi
I'm making a non inverting summing amplifier using LM324. I will be adding 5V inputs (supplied using micro controller pins to get voltages 5,10,15,20 from summer) since all the resistor values are equal, is there any procedure to select a proper resistor value ? see the attachment
thank you.

 

[ericgibbs]

hi,
Look at this link for the maths.
http://www.ee.nmt.edu/~thomas/ee321/op-amp-std-ninv.html

 

[Nigel Goodwin]

I must say, I don't like that as a circuit, and probably wouldn't consider using it. I'd either use a pair of opamps, one as a proper virtual earth mixer, the second as an inverter - or (most likely) use a single opamp as a virtual earth mixer and do the inverting in the software.

 

[aruna1]

hi,
Look at this link for the maths.
http://www.ee.nmt.edu/~thomas/ee321/op-amp-std-ninv.html

Thanks I'll chk the link

I must say, I don't like that as a circuit, and probably wouldn't consider using it. I'd either use a pair of opamps, one as a proper virtual earth mixer, the second as an inverter - or (most likely) use a single opamp as a virtual earth mixer and do the inverting in the software.

MMMMMM
I didnt understand what you said. not even a bit

and I dont have access to dual supply

can you please explain? I'm not familiar with such circuit

 

[ericgibbs]

Basically for the non invert, make the gain equal to the number of inputs and keep all the input resistors the same, if all the input voltages are the same.

 

[aruna1]

Basically for the non invert, make the gain equal to the number of inputs and keep all the input resistors the same, if all the input voltages are the same.

Ya I know this, thats why I used 3 10k resistors to set gain. 4 inputs so gain is four (3+1). my question is what is the proper resistor value? is 10k resistors are ok or should i use all 1k resistors or any other resistor value?
thank you

PS: link you posted show how to get the equation, which I already aware.

 

[ericgibbs]

OK,
As its a PIC with a low output impedance output, I would have chosen 2.2K thru 10K.

How fast are the pins being switched.?

EDIT:
Ideally the driving source impedance should be close to zero to give the best summing accuracy.

 

[aruna1]

OK,
As its a PIC with a low output impedance output, I would have chosen 2.2K thru 10K.

How fast are the pins being switched.?

pins are not switched fast. most of the time if one pin s switched on it will stay on for rest of time

 

[ericgibbs]

It should work OK with 10K's.

 

[aruna1]

I'm still wondering what nigel said. It doesnt make any sence to me.

anyway off topic question:

can I use a gyrator circuit to make LC low pass filter for a SMPS?

 

[Nigel Goodwin]

I'm still wondering what nigel said. It doesnt make any sence to me.

A proper summer uses inverting mode, as this gives a virtual earth and hence no interaction between the inputs.

Although as you're simply switching logic levels, it's not so much of a problem.

anyway off topic question:

can I use a gyrator circuit to make LC low pass filter for a SMPS?

No, a gyrator is a low level signal device, not a power device.

 

[ericgibbs]

The NI summing OPA amplifier as posted by the OP is standard practice and in common use.
It requires only one OPA, a single supply rail and common same value resistors on the inputs.

 

[MrAl]

Hi there guys,

The way i look at it is that there are advantages and disadvantages for each method, non inverting or inverting.
The inverting does have the advantage of one end of each input resistor going to virtual ground, the non inverting has advantage of only needing one power supply.
I think which one is best is determined by other things in the system dont you?

 

[ericgibbs]

hi Al
Its a simple high/low input from 4 output pins on a PIC, say an ideal swing of 0V to +5V.
And as a NI summer only requires only one supply voltage, I fail to see how it can best determined by other things in the system.??

IMHO the solution the OP has posted for his application, is the only logical choice.
Eric

 

[MrAl]

Hi Eric,

I was speaking more generally. In other words, what you mention *are* the other things in the system.

 

[ericgibbs]

Hi Eric,
I was speaking more generally. In other words, what you mention *are* the other things in the system.

hi Al,
Not wishing to be picky, its just that the OP posted a NI 'summer' image , also the Title included the word 'summing' and so did his text.

I 'try' to answer posts in a way specific to the question being asked, otherwise we seem to get involved in a discussion that spirals away from a simple explanation.

In this case, all the OP asked: 'are my resistor values OK.?'

Eric

 

[MrAl]

Hi Eric,

Oh ok maybe there's a slight problem with the terminology being used? ie 'op amp mixer' circuit is almost the same as an 'op amp summer' circuit.
Just a guess though, maybe it's not this...apologies if it's not.