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opamp voltage converter question

Started by Tom Del Rosso March 27, 2013
A simple level converter.  It assumes the +4V supply can sink current, so 
why not just have 2 resistors between +4 and +10 and use the opamp as a 
buffer?


        +10V
          |
          |
          R
          |
          |
          |----------
          |          |
          |          |
          R          |
          |          |
          |          |
         GND         |
                     |
                     |
                     |---- +
                               >------- +7V out
   +4V -----R------------- -       |
                     |             |
                     |             |
                     |------2R-----|


-- 

Reply in group, but if emailing add one more
zero, and remove the last word. 


On Wed, 27 Mar 2013 22:56:14 -0400, "Tom Del Rosso"
<tomd_u1@verizon.net.invalid> wrote:

>A simple level converter. It assumes the +4V supply can sink current, so >why not just have 2 resistors between +4 and +10 and use the opamp as a >buffer? > > > +10V > | > | > R > | > | > |---------- > | | > | | > R | > | | > | | > GND | > | > | > |---- + > >------- +7V out > +4V -----R------------- - | > | | > | | > |------2R-----|
There are three different ways to do this with just two resistors. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
John Larkin wrote:
> On Wed, 27 Mar 2013 22:56:14 -0400, "Tom Del Rosso" > <tomd_u1@verizon.net.invalid> wrote: > > > A simple level converter. It assumes the +4V supply can sink > > current, so why not just have 2 resistors between +4 and +10 and > > use the opamp as a buffer? > > > > > > +10V > > | > > | > > R > > | > > | > > |---------- > > | | > > | | > > R | > > | | > > | | > > GND | > > | > > | > > |---- + > > >------- +7V out > > +4V -----R------------- - | > > | | > > | | > > |------2R-----| > > > > There are three different ways to do this with just two resistors.
I know what you mean. This was just someone's idea of an example but he should have thought of a better one. I wondered if there was any justification for using 4 resistors. -- Reply in group, but if emailing add one more zero, and remove the last word.
On Wed, 27 Mar 2013 22:56:14 -0400, Tom Del Rosso wrote:

> A simple level converter. It assumes the +4V supply can sink current, > so why not just have 2 resistors between +4 and +10 and use the opamp as > a buffer? > > > +10V > | > | > R > | > | > |---------- > | | > | | > R | > | | > | | > GND | > | > | > |---- + > >------- +7V out > +4V -----R------------- - | > | | > | | > |------2R-----|
The op-amp version has a -2 voltage gain for variations in the +4 line to the output. If you want that, you can't get it with passive components. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Tim Wescott wrote:
> On Wed, 27 Mar 2013 22:56:14 -0400, Tom Del Rosso wrote: > > > A simple level converter. It assumes the +4V supply can sink > > current, so why not just have 2 resistors between +4 and +10 and > > use the opamp as a buffer? > > > > > > +10V > > | > > | > > R > > | > > | > > |---------- > > | | > > | | > > R | > > | | > > | | > > GND | > > | > > | > > |---- + > > >------- +7V out > > +4V -----R------------- - | > > | | > > | | > > |------2R-----| > > The op-amp version has a -2 voltage gain for variations in the +4 > line to the output. If you want that, you can't get it with passive > components.
Yes, I know you can't get gain with passive components. :) I wasn't suggesting resistors with no opamp. I just wanted to know if the 4-resistor approach had any advantage over the obvious 2-resistor equivalent. In the OP I said 'use the opamp as a buffer', so do you mean that it would have no gain (unity gain) with that approach? -- Reply in group, but if emailing add one more zero, and remove the last word.
On Thu, 28 Mar 2013 15:59:35 -0400, Tom Del Rosso wrote:

> Tim Wescott wrote: >> On Wed, 27 Mar 2013 22:56:14 -0400, Tom Del Rosso wrote: >> >> > A simple level converter. It assumes the +4V supply can sink >> > current, so why not just have 2 resistors between +4 and +10 and use >> > the opamp as a buffer? >> > >> > >> > +10V >> > | >> > | >> > R >> > | >> > | >> > |---------- >> > | | >> > | | >> > R | >> > | | >> > | | >> > GND | >> > | >> > | >> > |---- + >> > >------- +7V out >> > +4V -----R------------- - | >> > | | >> > | | >> > |------2R-----| >> >> The op-amp version has a -2 voltage gain for variations in the +4 line >> to the output. If you want that, you can't get it with passive >> components. > > Yes, I know you can't get gain with passive components. :) > > I wasn't suggesting resistors with no opamp. I just wanted to know if > the 4-resistor approach had any advantage over the obvious 2-resistor > equivalent. > > In the OP I said 'use the opamp as a buffer', so do you mean that it > would have no gain (unity gain) with that approach?
If you use a pair of resistors to get 7V out of the 10V and the 4V, then when the 4V changes by dV, the output will change by significantly less than dV and in the same direction. If you use the setup you show and the 4V changes by dV, then the output will change by -2*dV. If what you need is to pick out and amplify some signal that's riding on the 4V, then the pictured circuit would be the way to go. You don't say what the circuit is from or what you want to use it for, so it's impossible to answer your "why not". My suggestion was just that -- one possible reason "why not", which you then have take and exercise your own brain cells to see if it makes sense or not. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Tim Wescott wrote:
> > If you use a pair of resistors to get 7V out of the 10V and the 4V, > then when the 4V changes by dV, the output will change by > significantly less than dV and in the same direction. > > If you use the setup you show and the 4V changes by dV, then the > output will change by -2*dV. If what you need is to pick out and > amplify some signal that's riding on the 4V, then the pictured > circuit would be the way to go.
Thanks.
> You don't say what the circuit is from or what you want to use it > for, so it's impossible to answer your "why not". My suggestion was > just that -- one possible reason "why not", which you then have take > and exercise your own brain cells to see if it makes sense or not.
It was somebody's example of a 'basic opamp circuit' so I wondered if there was any reason to do that other than making an imaginary circuit. -- Reply in group, but if emailing add one more zero, and remove the last word.