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Howland current pump question

Started by bitrex May 11, 2015
Is it normal for the op amp inputs of the Howland pump to not be
 at the same voltage?  In all my experiments in LTSPICE with the
 "improved" topology and grounded resistive loads, this seems to
 have been the case.  The circuit obviously has both positive and
 negative feedback, but if the opamp output is not railed then I
 assume the negative feedback must be "winning."

But if this is normal, how can the analysis of the circuit's
 operation proceed from the ideal  negative feedback op amp
 assumption that both inputs are at the same potential?

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On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex
<bitrex@de.lete.earthlink.net> wrote:

> >Is it normal for the op amp inputs of the Howland pump to not be > at the same voltage? In all my experiments in LTSPICE with the > "improved" topology and grounded resistive loads, this seems to > have been the case. The circuit obviously has both positive and > negative feedback, but if the opamp output is not railed then I > assume the negative feedback must be "winning." > >But if this is normal, how can the analysis of the circuit's > operation proceed from the ideal negative feedback op amp > assumption that both inputs are at the same potential?
If the OpAmp inputs are not within the OpAmp offset voltage then something is very wrong. ...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, 11 May 2015 19:54:13 -0400 (EDT), bitrex
<bitrex@de.lete.earthlink.net> wrote:

> >Is it normal for the op amp inputs of the Howland pump to not be > at the same voltage?
No. LT Spice opamps generally have zero input offset voltage. A real opamp might have as much as a few millivolts. In all my experiments in LTSPICE with the
> "improved" topology and grounded resistive loads, this seems to > have been the case. The circuit obviously has both positive and > negative feedback, but if the opamp output is not railed then I > assume the negative feedback must be "winning." > >But if this is normal, how can the analysis of the circuit's > operation proceed from the ideal negative feedback op amp > assumption that both inputs are at the same potential?
The bigger problem with the Howland is resistor tolerances. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com>
 Wrote in message:
> On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> >>Is it normal for the op amp inputs of the Howland pump to not be >> at the same voltage? In all my experiments in LTSPICE with the >> "improved" topology and grounded resistive loads, this seems to >> have been the case. The circuit obviously has both positive and >> negative feedback, but if the opamp output is not railed then I >> assume the negative feedback must be "winning." >> >>But if this is normal, how can the analysis of the circuit's >> operation proceed from the ideal negative feedback op amp >> assumption that both inputs are at the same potential? > > If the OpAmp inputs are not within the OpAmp offset voltage then > something is very wrong. > > ...Jim Thompson
I will try to post a .asc file in a moment. -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com>
 Wrote in message:
> On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> >>Is it normal for the op amp inputs of the Howland pump to not be >> at the same voltage? In all my experiments in LTSPICE with the >> "improved" topology and grounded resistive loads, this seems to >> have been the case. The circuit obviously has both positive and >> negative feedback, but if the opamp output is not railed then I >> assume the negative feedback must be "winning." >> >>But if this is normal, how can the analysis of the circuit's >> operation proceed from the ideal negative feedback op amp >> assumption that both inputs are at the same potential? > > If the OpAmp inputs are not within the OpAmp offset voltage then > something is very wrong. > > ...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. >
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John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
> On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> >>Is it normal for the op amp inputs of the Howland pump to not be >> at the same voltage? > > No. LT Spice opamps generally have zero input offset voltage. A real > opamp might have as much as a few millivolts. > > In all my experiments in LTSPICE with the >> "improved" topology and grounded resistive loads, this seems to >> have been the case. The circuit obviously has both positive and >> negative feedback, but if the opamp output is not railed then I >> assume the negative feedback must be "winning." >> >>But if this is normal, how can the analysis of the circuit's >> operation proceed from the ideal negative feedback op amp >> assumption that both inputs are at the same potential? > > The bigger problem with the Howland is resistor tolerances.
How does the resistor toleraance/tempco affect temperature stability of the topology? I'm not concerned so much with absolutely accuracy as stability of whatever it is over some temperature range. Maybe this is the wrong way to go, but I'm enjoying experimenting as I'm not really familiar with this type of current source. -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
On Mon, 11 May 2015 17:08:28 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

>On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex ><bitrex@de.lete.earthlink.net> wrote: > >> >>Is it normal for the op amp inputs of the Howland pump to not be >> at the same voltage? > >No. LT Spice opamps generally have zero input offset voltage. A real >opamp might have as much as a few millivolts. > > In all my experiments in LTSPICE with the >> "improved" topology and grounded resistive loads, this seems to >> have been the case. The circuit obviously has both positive and >> negative feedback, but if the opamp output is not railed then I >> assume the negative feedback must be "winning." >> >>But if this is normal, how can the analysis of the circuit's >> operation proceed from the ideal negative feedback op amp >> assumption that both inputs are at the same potential? > >The bigger problem with the Howland is resistor tolerances.
It's been a while since I played with the Howland, but I remember the Ratios determine the accuracy. Then you can worry about the tolerances. Cheers
On Mon, 11 May 2015 20:46:30 -0400 (EDT), bitrex
<bitrex@de.lete.earthlink.net> wrote:

>John Larkin <jlarkin@highlandtechnology.com> Wrote in message: >> On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> >>>Is it normal for the op amp inputs of the Howland pump to not be >>> at the same voltage? >> >> No. LT Spice opamps generally have zero input offset voltage. A real >> opamp might have as much as a few millivolts. >> >> In all my experiments in LTSPICE with the >>> "improved" topology and grounded resistive loads, this seems to >>> have been the case. The circuit obviously has both positive and >>> negative feedback, but if the opamp output is not railed then I >>> assume the negative feedback must be "winning." >>> >>>But if this is normal, how can the analysis of the circuit's >>> operation proceed from the ideal negative feedback op amp >>> assumption that both inputs are at the same potential? >> >> The bigger problem with the Howland is resistor tolerances. > >How does the resistor toleraance/tempco affect temperature > stability of the topology? I'm not concerned so much with > absolutely accuracy as stability of whatever it is over some > temperature range. > >Maybe this is the wrong way to go, but I'm enjoying experimenting > as I'm not really familiar with this type of current > source.
Here's a discussion. http://www.ti.com/lit/an/snoa474a/snoa474a.pdf -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Mon, 11 May 2015 20:46:30 -0400 (EDT), bitrex
<bitrex@de.lete.earthlink.net> wrote:

>John Larkin <jlarkin@highlandtechnology.com> Wrote in message: >> On Mon, 11 May 2015 19:54:13 -0400 (EDT), bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> >>>Is it normal for the op amp inputs of the Howland pump to not be >>> at the same voltage? >> >> No. LT Spice opamps generally have zero input offset voltage. A real >> opamp might have as much as a few millivolts. >> >> In all my experiments in LTSPICE with the >>> "improved" topology and grounded resistive loads, this seems to >>> have been the case. The circuit obviously has both positive and >>> negative feedback, but if the opamp output is not railed then I >>> assume the negative feedback must be "winning." >>> >>>But if this is normal, how can the analysis of the circuit's >>> operation proceed from the ideal negative feedback op amp >>> assumption that both inputs are at the same potential? >> >> The bigger problem with the Howland is resistor tolerances. > >How does the resistor toleraance/tempco affect temperature > stability of the topology? I'm not concerned so much with > absolutely accuracy as stability of whatever it is over some > temperature range. > >Maybe this is the wrong way to go, but I'm enjoying experimenting > as I'm not really familiar with this type of current > source.
Do the math, ignore the Tulane cockroach. ...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 Monday, May 11, 2015 at 5:46:59 PM UTC-7, bitrex wrote:
> John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
[about Howland current sources]
> > The bigger problem with the Howland is resistor tolerances. > > How does the resistor toleraance/tempco affect temperature > stability of the topology?
Topologies don't have stability. The Howland circuit achieves infinite output impedance (is a current source) because the output impedance is computed by dividing by a quantity that is a difference of two nearly-equal things. If they are nearly-equal and the quantity is positive, the output impedance is large. If they are nearly-equal and the quantity is negative, the output impedance is negative. That's an OOPS event. Here's the downside to a Howland pump: it depends critically on the small differences, which means it depends critically on small thermal differences, too. Parts-per-million in the resistors, then you take a difference, and it's billions of parts-per-million of the near-zero that you are depending on.