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? -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Howland current pump question
Started by ●May 11, 2015
Reply by ●May 11, 20152015-05-11
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.
Reply by ●May 11, 20152015-05-11
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
Reply by ●May 11, 20152015-05-11
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 ThompsonI will try to post a .asc file in a moment. -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●May 11, 20152015-05-11
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. >Version 4 SHEET 1 904 680 WIRE 576 -32 416 -32 WIRE 848 -32 656 -32 WIRE -128 -16 -128 -48 WIRE 0 -16 0 -48 WIRE 688 96 688 64 WIRE -128 112 -128 64 WIRE 0 112 0 64 WIRE 272 128 176 128 WIRE 416 128 416 -32 WIRE 416 128 352 128 WIRE 496 128 416 128 WIRE 624 144 560 144 WIRE 496 160 416 160 WIRE 176 176 176 128 WIRE 272 272 176 272 WIRE 416 272 416 160 WIRE 416 272 352 272 WIRE 496 272 416 272 WIRE 688 272 688 192 WIRE 688 272 576 272 WIRE 720 272 688 272 WIRE 848 272 848 -32 WIRE 848 272 800 272 WIRE 176 336 176 272 WIRE 848 336 848 272 WIRE 176 496 176 416 WIRE 848 496 848 416 FLAG 528 112 Vcc FLAG 528 176 Vee FLAG 176 176 0 FLAG 688 64 Vcc FLAG 848 496 0 FLAG -128 112 0 FLAG 0 112 0 FLAG -128 -48 Vcc FLAG 0 -48 Vee FLAG 176 496 0 SYMBOL npn 624 96 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL res 816 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL res 832 320 R0 SYMATTR InstName R6 SYMATTR Value 1k SYMBOL voltage -128 -32 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL voltage 0 -32 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value -12 SYMBOL res 368 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 47k SYMBOL res 368 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 47k SYMBOL res 672 -48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 47k SYMBOL res 592 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 47k SYMBOL voltage 176 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value 5 SYMBOL Opamps\\LT1014 528 80 R0 SYMATTR InstName U1 TEXT -106 176 Left 2 !.tran 0.1 -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●May 11, 20152015-05-11
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/
Reply by ●May 12, 20152015-05-12
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
Reply by ●May 12, 20152015-05-12
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
Reply by ●May 12, 20152015-05-12
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.
Reply by ●May 12, 20152015-05-12
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.
