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From: Robert Baer on 11 Mar 2010 01:20
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?
>
> Richard Holmes
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.
From: Richard Holmes on 11 Mar 2010 01:26
On Wed, 10 Mar 2010 20:05:21 -0600, John Fields
<jfields(a)austininstruments.com> wrote:

>Yes, use an inverting opamp as a current to voltage converter: (View in
>Courier)
>
> V+
> +----|----+
> | | [R]
>IN1>---[R1]--+-+---|-\ |
> | | >--+-->
>IN2>---[R2]--+ +--|+/
> . | |
> . GND V-
> .
> |
>INn>---[Rn]--+
>

So, assuming two input signals of 1Vpp and 2Vpp, and a single supply
op amp, the following circuit will work?

CA 3130

+12V
+-----|----+
| | |
| | [Trim pot] gain
>IN1 1Vpp>---[10K]--+-+---|-\ |
> |+12V | >--+----->
>IN2 2Vpp>---[4.7K]-+ +--|+/
. | |
[R2] |
|___|
|
GND
Not shown:

1. R1 between +12V and non-invert IP.
2. 120pf cap pins 1 + 7

Sorry, I don't have the ASCII circuit program.

Many thanks,

Richard Holmes
From: Phil Allison on 11 Mar 2010 01:54

"Richard Holmes"

> So, assuming two input signals of 1Vpp and 2Vpp, and a single supply
> op amp, the following circuit will work?
>
> CA 3130
>
> +12V
> +-----|----+
> | | |
> | | [Trim pot] gain
>>IN1 1Vpp>---[10K]--+-+---|-\ |
>> |+12V | >--+----->
>>IN2 2Vpp>---[4.7K]-+ +--|+/
> . | |
> [R2] |
> |___|
> |
> GND
> Not shown:
>
> 1. R1 between +12V and non-invert IP.
> 2. 120pf cap pins 1 + 7
>

** I see your pain.

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.



..... Phil




From: Rich Webb on 11 Mar 2010 08:22
On Thu, 11 Mar 2010 06:26:35 GMT, richardholmes(a)trentcor.com (Richard
Holmes) wrote:


>Sorry, I don't have the ASCII circuit program.

One of the most often used is over at
http://www.tech-chat.de/aacircuit.html

Hasn't been updated in a while but it's quite useful.

--
Rich Webb Norfolk, VA
From: Uwe Hercksen on 11 Mar 2010 08:49


Richard Holmes schrieb:

> So, assuming two input signals of 1Vpp and 2Vpp, and a single supply
> op amp, the following circuit will work?

Hello,

using bipolar input signals with a single supply op amp is not a good
idea, especially if you want bipolar output signals too.

Bye

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