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OpAmp Voter

Started by SRob...@SpamSucks.net September 8, 2014
I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and =
pass the highest of the signals, without diode losses.  Using a microproces=
sor is out of the question.  Cost is a object and actives should be limited=
 to devices that can be second sourced.=20

This is for converting a RGB command signal to an intensity signal.

Steve=20

=20
On Mon, 8 Sep 2014 13:16:11 -0700 (PDT), "SRoberts@SpamSucks.net"
<sroberts6328@gmail.com> wrote:

>I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and pass the highest of the signals, without diode losses. Using a microprocessor is out of the question. Cost is a object and actives should be limited to devices that can be second sourced. > >This is for converting a RGB command signal to an intensity signal. > >Steve > >
Three opamps, each with a diode in its output. Each acts like a follower, but all three get their nfb from the output node. Needs a pulldown from output to ground or V-, too. Classic process control "high select." -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 08/09/2014 21:16, SRoberts@SpamSucks.net wrote:
> I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and pass the highest of the signals, without diode losses. Using a microprocessor is out of the question. Cost is a object and actives should be limited to devices that can be second sourced. > > This is for converting a RGB command signal to an intensity signal. > > Steve
You're welcome. Cheers -- Syd
On Monday, September 8, 2014 9:35:34 PM UTC+1, John Larkin wrote:
> On Mon, 8 Sep 2014 13:16:11 -0700 (PDT), "SRoberts@SpamSucks.net" > <sroberts6328> wrote:
> >I have three DC-5Khz, 0-5 V positive going signals. I'd like to select a=
nd pass the highest of the signals, without diode losses. Using a micropro= cessor is out of the question. Cost is a object and actives should be limi= ted to devices that can be second sourced.=20
> > > >This is for converting a RGB command signal to an intensity signal.
> Three opamps, each with a diode in its output. Each acts like a > follower, but all three get their nfb from the output node. Needs a > pulldown from output to ground or V-, too. > Classic process control "high select."
Yup. A less precise option is putting each input thru a diode, with the out= put of the diodes going thru an opamp with a diode in the nfb path to add a= diode drop to the output. An extra resistor to supply rail needed to keep = the diode biased. 4 diodes 1 opamp versus 3 diodes 3 opamps. The performanc= e difference between this and the above should be fairly obvious. NT
On Mon, 8 Sep 2014 13:16:11 -0700 (PDT), "SRoberts@SpamSucks.net"
<sroberts6328@gmail.com> wrote:

>I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and pass the highest of the signals, without diode losses. Using a microprocessor is out of the question. Cost is a object and actives should be limited to devices that can be second sourced. > >This is for converting a RGB command signal to an intensity signal. > >Steve > >
Just add another input to this circuit I posted last November... bitrexCircuitChallenge_2013_11_07.pdf on the S.E.D/Schematics Page of my website. Probably best if you used a quad-OpAmp... three for inputs, the resistor to ground (or to negative rail) to ensure "Who's on first"
>:-}
Fourth OpAmp as the buffer. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Mon, 08 Sep 2014 13:35:34 -0700, the renowned John Larkin
<jlarkin@highlandtechnology.com> wrote:

>On Mon, 8 Sep 2014 13:16:11 -0700 (PDT), "SRoberts@SpamSucks.net" ><sroberts6328@gmail.com> wrote: > >>I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and pass the highest of the signals, without diode losses. Using a microprocessor is out of the question. Cost is a object and actives should be limited to devices that can be second sourced. >> >>This is for converting a RGB command signal to an intensity signal. >> >>Steve >> >> > >Three opamps, each with a diode in its output. Each acts like a >follower, but all three get their nfb from the output node. Needs a >pulldown from output to ground or V-, too. > >Classic process control "high select."
With something like this tacked on it will work better at 5kHz: http://www.speff.com/max.png It will still be a bit glitchy where the levels cross. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
On Mon, 08 Sep 2014 18:43:58 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Mon, 08 Sep 2014 13:35:34 -0700, the renowned John Larkin ><jlarkin@highlandtechnology.com> wrote: > >>On Mon, 8 Sep 2014 13:16:11 -0700 (PDT), "SRoberts@SpamSucks.net" >><sroberts6328@gmail.com> wrote: >> >>>I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and pass the highest of the signals, without diode losses. Using a microprocessor is out of the question. Cost is a object and actives should be limited to devices that can be second sourced. >>> >>>This is for converting a RGB command signal to an intensity signal. >>> >>>Steve >>> >>> >> >>Three opamps, each with a diode in its output. Each acts like a >>follower, but all three get their nfb from the output node. Needs a >>pulldown from output to ground or V-, too. >> >>Classic process control "high select." > > >With something like this tacked on it will work better at 5kHz: > >http://www.speff.com/max.png > >It will still be a bit glitchy where the levels cross. > > > >Best regards, >Spehro Pefhany
Two of the three amps will be railed negative at any time, so your circuit doesn't really need R2. I'd go for schottky diodes and faster amps, meself. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
At 5kHz, any of the suggestions above will do.

I also have one that works at high frequencies, without suffering from 
glitches or integrator wind-up.

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"SRoberts@SpamSucks.net" <sroberts6328@gmail.com> wrote in message 
news:7a4c4f48-9a12-427c-8328-57ca8f9a8ad5@googlegroups.com...
I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and 
pass the highest of the signals, without diode losses.  Using a 
microprocessor is out of the question.  Cost is a object and actives 
should be limited to devices that can be second sourced.

This is for converting a RGB command signal to an intensity signal.

Steve



On Mon, 08 Sep 2014 16:16:50 -0700, the renowned John Larkin
<jlarkin@highlandtechnology.com> wrote:

>On Mon, 08 Sep 2014 18:43:58 -0400, Spehro Pefhany ><speffSNIP@interlogDOTyou.knowwhat> wrote: > >>On Mon, 08 Sep 2014 13:35:34 -0700, the renowned John Larkin >><jlarkin@highlandtechnology.com> wrote: >> >>>On Mon, 8 Sep 2014 13:16:11 -0700 (PDT), "SRoberts@SpamSucks.net" >>><sroberts6328@gmail.com> wrote: >>> >>>>I have three DC-5Khz, 0-5 V positive going signals. I'd like to select and pass the highest of the signals, without diode losses. Using a microprocessor is out of the question. Cost is a object and actives should be limited to devices that can be second sourced. >>>> >>>>This is for converting a RGB command signal to an intensity signal. >>>> >>>>Steve >>>> >>>> >>> >>>Three opamps, each with a diode in its output. Each acts like a >>>follower, but all three get their nfb from the output node. Needs a >>>pulldown from output to ground or V-, too. >>> >>>Classic process control "high select." >> >> >>With something like this tacked on it will work better at 5kHz: >> >>http://www.speff.com/max.png >> >>It will still be a bit glitchy where the levels cross. >> >> >> >>Best regards, >>Spehro Pefhany > >Two of the three amps will be railed negative at any time, so your >circuit doesn't really need R2. > >I'd go for schottky diodes and faster amps, meself.
The reverse diodes + 10K resistors keep the op-amps from railing. Some op-amps take a long time to recover from being railed (as I think you have measured). Using Schottky diodes as you suggest would reduce the volts they have to slew. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
On Mon, 8 Sep 2014 18:29:24 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>At 5kHz, any of the suggestions above will do. > >I also have one that works at high frequencies, without suffering from >glitches or integrator wind-up. > >Tim
No, No, don't show us that one. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com