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
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.
formatting link
The op amp buffer is at:
formatting link
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:
formatting link
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.
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.
You must have done something wrong in the use of a summing op amp; you can add more inputs without any effect at the output o the other signals - ASSUMING the op-amp is in the linear range AND the frequencies aer well within the gain-bandwidth.
A summing amp is inherently an * inverting stage* - ie positive inputs generate negative outputs.
If the input signals are DC coupled, ground referenced and swing positive or both ways - then you MUST use a suitable op-amp with dual supplies.
The schem you posted will only work with AC coupled input signals - ie there is a cap between the sources and each input and the ouput will have a +6 volt fixed bias.
Hmm, probably a silly idea with a mistake. But if you could reference the signals to some voltage half way between the supply voltages (call it common). You could then use the summing op-amp circuit. Then invert back about the common point and remove the DC offset at the end... sounds like a lot of trouble just to add two voltages with single supply opamp.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.