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From: Richard Holmes on 10 Mar 2010 19:44
Can anyone give me some pointers for mixing DC-20KHz analog signals
from different sources? For example, generated noise, sinewaves, etc.

I have read about it and tried various circuits, resistor networks, a
summing op amp, but it does not appear so straightforward. Adding a
signal causes a voltage drop in the sum, etc.

As a novice, there is obviously something I am overlooking. Any
adfvice would be appreciated.

Is there a sure-fire cirucit I can use for everything in this type of
application?

Richard Holmes
From: Phil Allison on 10 Mar 2010 20:07

"Richard Holmes"
>
> Can anyone give me some pointers for mixing DC-20KHz analog signals
> from different sources? For example, generated noise, sinewaves, etc.
>
> I have read about it and tried various circuits, resistor networks, a
> summing op amp, but it does not appear so straightforward. Adding a
> signal causes a voltage drop in the sum, etc.


** Adding a signal source affects a resistor summing network - just adding
a signal does not.

A "summing" op-amp is not affected by either.

Eg: http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm



...... Phil





From: John Fields on 10 Mar 2010 21:05
On Thu, 11 Mar 2010 00:44:06 GMT, richardholmes(a)trentcor.com (Richard
Holmes) wrote:

>Can anyone give me some pointers for mixing DC-20KHz analog signals
>from different sources? For example, generated noise, sinewaves, etc.
>
>I have read about it and tried various circuits, resistor networks, a
>summing op amp, but it does not appear so straightforward. Adding a
>signal causes a voltage drop in the sum, etc.
>
>As a novice, there is obviously something I am overlooking. Any
>adfvice would be appreciated.
>
>Is there a sure-fire cirucit I can use for everything in this type of
>application?

---
Yes, use an inverting opamp as a current to voltage converter: (View in
Courier)

V+
+----|----+
| | [R]
IN1>---[R1]--+-+---|-\ |
| | >--+-->
IN2>---[R2]--+ +--|+/
. | |
. GND V-
.
|
INn>---[Rn]--+


JF
From: Jon Slaughter on 10 Mar 2010 21:58
Richard Holmes wrote:
> Can anyone give me some pointers for mixing DC-20KHz analog signals
> from different sources? For example, generated noise, sinewaves, etc.
>
> I have read about it and tried various circuits, resistor networks, a
> summing op amp, but it does not appear so straightforward. Adding a
> signal causes a voltage drop in the sum, etc.
>
> As a novice, there is obviously something I am overlooking. Any
> adfvice would be appreciated.
>
> Is there a sure-fire cirucit I can use for everything in this type of
> application?
>

S1--R1---+
S2--R2---+----Vo
S3--R3---+

Where + is all connected is the basic idea. The problem is that you cannot
draw any current from Vo without distorting the results. This is called
loading the output. You can, for example, connect a scope probe to Vo and
expect to see S1 + S2 + S3.

Now because we don't want to "load the output" we have to one of any number
of methods to allow us to effectively load the output(i.e., still gives us
Vo but let us draw a current from it without changing Vo).

To do this you must use some type of buffering/amplification technique. A
simple op amp buffer will work but will not provide amplification. You'll
essentially get Vo out of the buffer but be able to drive larger loads than
the resistor network will.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar.html

The op amp buffer is at:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html#c1

So you connect Vo to Vin of the buffer and then Vo of the buffer is your Vo
but you can now drive larger loads(up to the op amps spec).

You can also implement buffers and amplifiers with bjt's and fets with
effectively the same result(op amps are more precise).

Here is what you are trying to accomplish:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html#c1


Make sure you get the resistor values right. If you have


S1--R1---+
S2--R2---+----Vo
S3--R3---+

Then the sum is

(R1*S1 + R2*S2 + S3*R3)/(R1 + R2 + R3)

which is known as a weighted sum. Generally you'll want R1=R2=R3. Also, The
R's should not be too large. Something in the 10k's should work. Nothing
below 1k in general.

From: krw on 11 Mar 2010 00:03
On Wed, 10 Mar 2010 20:05:21 -0600, John Fields
<jfields(a)austininstruments.com> wrote:

>On Thu, 11 Mar 2010 00:44:06 GMT, richardholmes(a)trentcor.com (Richard
>Holmes) wrote:
>
>>Can anyone give me some pointers for mixing DC-20KHz analog signals
>>from different sources? For example, generated noise, sinewaves, etc.
>>
>>I have read about it and tried various circuits, resistor networks, a
>>summing op amp, but it does not appear so straightforward. Adding a
>>signal causes a voltage drop in the sum, etc.
>>
>>As a novice, there is obviously something I am overlooking. Any
>>adfvice would be appreciated.
>>
>>Is there a sure-fire cirucit I can use for everything in this type of
>>application?
>
>---
>Yes, use an inverting opamp as a current to voltage converter: (View in
>Courier)
>
> V+
> +----|----+
> | | [R]
>IN1>---[R1]--+-+---|-\ |
> | | >--+-->
>IN2>---[R2]--+ +--|+/
> . | |
> . GND V-
> .
> |
>INn>---[Rn]--+
>

It should be noted that the output signal will be the sum of the INn (where
'n' is the number of the input) times the ratio of R/Rn. It's a very powerful
circuit.
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