[sdiy] Ofsetting LFO output

[Seb Francis]

I'm glad someone has a clearer head on than me today ;)

Seb


Steve Lenham wrote:

> If you build the circuit exactly as shown, the offset applied will be 
> more like 3.9V than 5V.
> Reason? The opamp inverting input is fixed at ground potential, so R5 
> is effectively in parallel with R3 and upsets the voltage divider ratio.
>
> To fix this you can either make R3 and R4 very small compared to R5 
> while keeping the ratio the same (making the R5 loading negligible) 
> or, better, take this loading effect into account when calculating the 
> resistor values.
>
> In the original circuit, the calculation is easy: we want a new value 
> for R3 which, when put in parallel with the 10K of R5, gives a 
> combined value of 5K. Changing R3 to 10K would achieve this.
>
> There is an even easier way, though: dump R3, R4 and R5 entirely and 
> simply connect a 24K resistor from -12V to the inverting opamp input. 
> The -12V sees an inverting amplifier with a gain of -10K/24K = -0.4, 
> and -12V * -0.4 = +5V. The input signal is then added to this offset 
> as before using R1.
>
> Hope this helps,
>
> Steve L.
>
> ----- Original Message ----- From: "Seb Francis" <seb at burnit.co.uk>
> To: "Mikael Mørup" <Mikael at mikmo.dk>
> Cc: <synth-diy at dropmix.xs4all.nl>
> Sent: Wednesday, June 01, 2005 1:25 PM
> Subject: Re: [sdiy] Ofsetting LFO output
>
>
> something like the attached circuit should do it ..
>
> it's inverting, so -5V in will give +10V out and +5V in will give 0V
> out, but this shouldn't matter for an LFO.
>
> R3+R4 are not 'real' resistor values, but anything in the same ratio
> will be fine .. even better would be to add a trimmer in series with one
> of these to get exactly the right offset
>
> seb
>
>
>
>
> Mikael Mørup wrote:
>
>> Hi list
>>
>> I have an LFO that puts out a -5V to +5V peak to peak
>> squarewave. For clocking another module i need a 0V to 10V
>> squarewave. is it possible to make a small circuit that
>> would "ofset" or whatever it's called, the LFO output ?
>>
>> My modules are running on +-12V.
>>
>> In my simple mind i am thinking something like an Opamp with
>> and ofset, but i don't know excatly how to design such a
>> thing.
>>
>> Any suggestions?
>>
>> A little schematic would be very nice :-)
>>
>> Thanks
>>
>> Mikael
>>
>>
>>
>>
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