# [sdiy] level shifting

Ray Wilson raywilson at comcast.net
Fri Jan 9 14:52:26 CET 2009

```You might find this tool helpful for testing level shifting with an op amp.
http://www.musicfromouterspace.com/ElectronicTools/electroToolbox/formulas.html

Cheers

Ray

----- Original Message -----
From: "Dan Snazelle" <subjectivity at hotmail.com>
To: <dixon at interchange.ubc.ca>; "sdiy" <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, January 08, 2009 4:19 PM
Subject: [sdiy] level shifting

>
> "> Dan,
>>
>> Level shifting with inverting opamps is a little bit tricky"
>
>
> yeah well lucky for me i am terrible at math.
>
> i need to figure out how to do this on a calculator.
>
> one thing...is it any easier with a non inverting amp?
>
> thanks for the help.
>
>
>
>
>
>
>
>
>
>
> ----------------------------------------
>> Date: Thu, 8 Jan 2009 14:12:31 -0800
>> From: dixon at interchange.ubc.ca
>> Subject: RE: [sdiy] variable width pulse wave for JEFT sample and hold
>> To: subjectivity at hotmail.com; synth-diy at dropmix.xs4all.nl
>>
>> Dan,
>>
>> Level shifting with inverting opamps is a little bit tricky. It is not as
>> intuitive as one would hope. For example, having a +5V reference voltage
>> at
>> the (+) input terminal does not necessarily shift the output by +5V!
>>
>> The best way to figure this out is actually to analyze the opamp circuit.
>> Draw the opamp, the input resistor and the feedback resistor, both taken
>> as
>> R. Apply the input voltage V_in to the input resistor. Now, assume a
>> reference voltage V_ref at the input terminals. What is the output
>> voltage
>> at a given input voltage?
>>
>> Well, the current through the input resistor is I = (V_in - V_ref)/R. As
>> no
>> current flows through the input terminals, the current through the
>> feedback
>> resistor is also I = (V_in - V_ref)/R = (V_ref - V_out)/R. Rearranging,
>> V_out = 2*V_ref - V_in. Hence, to get a +5V offset, V_ref must be +2.5V,
>> and the voltage divider resistors must have the ratio 5:1 for a +15V rail
>> voltage, or 19:5 for a +12V rail voltage.
>>
>> If you have a summing amp with two inputs, each with R-valued input
>> resistors, then things are different. Assuming both are feeding V_in,
>> then
>> the net current through both inputs (and through the feedback resistor)
>> is I
>> = 2*(V_in - V_ref)/R = (V_ref - V_out)/R. Hence, in this case, V_out =
>> 3*V_ref - 2*V_in. Now, 2*V_in is what you want from the summation of the
>> two inputs. However, 3*V_ref is the offset. Hence, to get an output
>> offset
>> of +5V, the reference voltage must be +1.67V, and now the voltage divider
>> resistors must have the ratio 8:1 for a +15V rail voltage, or 31:5 for a
>> +12V rail voltage.
>>
>> Clear as mud? This stymied me too, until I realized that this is one of
>> those situations in electronics where you really must do the maths!
>>
>> Cheers,
>> Dave
>>
>>
>> David G. Dixon
>> Professor
>> Department of Materials Engineering
>> University of British Columbia
>> Vancouver, B.C. V6T 1Z4
>>
>> Tel 1-604-822-3679
>> Fax 1-604-822-3619
>>
>> "PERFECTA FINGAMUS SERVIAT NATURA"
>>
>> The information in this email and in any attachments is confidential and
>> intended solely for the attention and use of the named addressee(s). It
>> must not be disclosed to any person without the writer's authority. If
>> you
>> are not the intended recipient, or a person responsible for delivering it
>> to
>> the intended recipient, you are not authorized to and must not disclose,
>> copy, distribute, or retain this message or any part of it.
>>
>> -----Original Message-----
>> From: Dan Snazelle [mailto:subjectivity at hotmail.com]
>> Sent: Thursday, January 08, 2009 11:26 AM
>> To: dixon at interchange.ubc.ca; synth-diy at dropmix.xs4all.nl
>> Subject: RE: [sdiy] variable width pulse wave for JEFT sample and hold
>>
>>
>>
>> This statement brings up an issue i still am trying to learn..
>> how to level shift signals and voltages
>> (or voltage shift or offset)
>>
>> it seems you are saying to do something like this:
>>
>> input the square wave into a 102 cap with a 10k resistor to grnd. then
>> the
>> level shift: plug the signal into the inverting
>> input of an opamp, which on the non inverting input has a voltage divider
>> (
>> 2 to 1 would that mean a 100k to + and 50k to gnd?
>>
>>
>> i would LOVE to be able to figure out this SHIFTING thing.
>>
>> does anyone know of a chapter in any of the common opamp books that talks
>> about it? is it technically called voltage shifting? or voltage offset?
>>
>> thanks
>>
>>
>> I keep on learning new junk everyday
>>> Why not just use an RC high-pass filter with a time constant of about 10
>> us
>>> to generate spikes from the square (0.001 uF and 10 k will do nicely),
>>> and
>>> then shift the pulses with a unity-gain inverting opamp with a voltage
>>> 2:1
>>> divider off the positive input between -15V and 0V so that the output DC
>>> rides at about -5V and the spikes go up to 0V to trigger the JFET
>>> (which,
>> I
>>> presume, is npn and therefore is full off at about -3.5V and full on at
>>>
>>>
>>> David G. Dixon
>>> Professor
>>> Department of Materials Engineering
>>> University of British Columbia
>>> Vancouver, B.C. V6T 1Z4
>>>
>>> Tel 1-604-822-3679
>>> Fax 1-604-822-3619
>>>
>>> "PERFECTA FINGAMUS SERVIAT NATURA"
>>>
>>> The information in this email and in any attachments is confidential and
>>> intended solely for the attention and use of the named addressee(s). It
>>> must not be disclosed to any person without the writer's authority. If
>> you
>>> are not the intended recipient, or a person responsible for delivering
>>> it
>> to
>>> the intended recipient, you are not authorized to and must not disclose,
>>> copy, distribute, or retain this message or any part of it.
>>>
>>> -----Original Message-----
>>> From: synth-diy-bounces at dropmix.xs4all.nl
>>> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Dan Snazelle
>>> Sent: Tuesday, January 06, 2009 9:14 AM
>>> To: synth-diy at dropmix.xs4all.nl
>>> Subject: [sdiy] variable width pulse wave for JEFT sample and hold
>>>
>>>
>>> i am trying to find a way to get a variable pulse wave from a square so
>> that
>>> i can successfully clock a sample and hold built around a JFET (pn4391)
>>>
>>> i am having no luck with a regular square wave.
>>>
>>> thanks
>>>
>>>
>>>
>>>
>>>
>>>
>>> --------------------------------------------
>>> check out various dan music at:
>>>
>>> http://www.myspace.com/lossnyc
>>>
>>>
>>> http://www.soundclick.com/lossnyc.htm
>>>
>>>
>>> http://www.indie911.com/dan-snazelle
>>> (or for techno) http://www.myspace.com/snazelle
>>>
>>>
>>>
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>
>>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

```