# Howland current pump question

Started by May 11, 2015
```On Tuesday, 12 May 2015 14:27:13 UTC+10, whit3rd  wrote:
> On Monday, May 11, 2015 at 5:46:59 PM UTC-7, bitrex wrote:
> > John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
>
>
> > > 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.

Strong argument for using a precision resistor array on a common substrate. They aren't particularly expensive but you don't get a lot of choice in resistor values.

--
Bill Sloman, Sydney

```
```"whit3rd" <whit3rd@gmail.com> wrote in message
> 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.

Well, no; a little bit of negative conductance* might even be beneficial.

* "A little bit of negative resistance" would seem inaccurate, and "A
lotta bit of negative resistance" just doesn't sound right at all.

FWIW, just as negative resistance wraps around to positive resistance
around zero, the same happens at infinity, going out to infinite
resistance (positive to zero conductance) then back negative (negative
nonzero conductance).

Which is actually a really nice (physical) manifestation of some elegant
mathematics: analytical calculus often makes use of poles (singularities)
at infinity or zero, and executing loop integrals around them to make
certain solutions that wouldn't otherwise be possible (you can't integrate
1/x, along the real number line, through 0).  Those methods aren't really
important here, but the analytical nature of infinity as being a
wraparound point on a closed ring (when well-behaved) is quite nice.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

```
```On 5/11/2015 6:54 PM, bitrex 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?
>

Try moving the right end of R4 to ground.
```
```Le mardi 12 mai 2015 02:44:02 UTC+2, bitrex a &#2013265929;crit&#2013266080;:
> 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
> >
> >>
> >>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/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
> --
>
>
> http://usenet.sinaapp.com/

Your opamp is going to saturation +12V ... :-(

"Pas top" as we say in French.

Hab.
```
```On Tue, 12 May 2015 04:51:58 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"whit3rd" <whit3rd@gmail.com> wrote in message
>> 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.
>
>Well, no; a little bit of negative conductance* might even be beneficial.
>
>* "A little bit of negative resistance" would seem inaccurate, and "A
>lotta bit of negative resistance" just doesn't sound right at all.
>
>FWIW, just as negative resistance wraps around to positive resistance
>around zero, the same happens at infinity, going out to infinite
>resistance (positive to zero conductance) then back negative (negative
>nonzero conductance).
>
>Which is actually a really nice (physical) manifestation of some elegant
>mathematics: analytical calculus often makes use of poles (singularities)
>at infinity or zero, and executing loop integrals around them to make
>certain solutions that wouldn't otherwise be possible (you can't integrate
>1/x, along the real number line, through 0).  Those methods aren't really
>important here, but the analytical nature of infinity as being a
>wraparound point on a closed ring (when well-behaved) is quite nice.
>
>Tim

I recall that the OP wants to connect two current sources to opposite
corners of a bridge rectifier. That will have interesting dynamics.

--

John Larkin         Highland Technology, Inc
picosecond timing   laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```
```John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
> On Tue, 12 May 2015 04:51:58 -0500, "Tim Williams"
> <tiwill@seventransistorlabs.com> wrote:
>
>>"whit3rd" <whit3rd@gmail.com> wrote in message
>>> 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.
>>
>>Well, no; a little bit of negative conductance* might even be beneficial.
>>
>>* "A little bit of negative resistance" would seem inaccurate, and "A
>>lotta bit of negative resistance" just doesn't sound right at all.
>>
>>FWIW, just as negative resistance wraps around to positive resistance
>>around zero, the same happens at infinity, going out to infinite
>>resistance (positive to zero conductance) then back negative (negative
>>nonzero conductance).
>>
>>Which is actually a really nice (physical) manifestation of some elegant
>>mathematics: analytical calculus often makes use of poles (singularities)
>>at infinity or zero, and executing loop integrals around them to make
>>certain solutions that wouldn't otherwise be possible (you can't integrate
>>1/x, along the real number line, through 0).  Those methods aren't really
>>important here, but the analytical nature of infinity as being a
>>wraparound point on a closed ring (when well-behaved) is quite nice.
>>
>>Tim
>
> I recall that the OP wants to connect two current sources to opposite
> corners of a bridge rectifier. That will have interesting dynamics.
>
>

Two current source/sink that track each other well over
temperature, and are also voltage controllable.  And preferably a
setup where I could mirror one set of source/sinks into others
that are slaved to the first one to have multiple
bridges.

Figuring this out is kind of a pain in the ass.

--

http://usenet.sinaapp.com/
```
```On Tue, 12 May 2015 12:55:06 -0400 (EDT), bitrex

[snip]
>
>Two current source/sink that track each other well over
> temperature, and are also voltage controllable.  And preferably a
> setup where I could mirror one set of source/sinks into others
> that are slaved to the first one to have multiple
> bridges.
>
>Figuring this out is kind of a pain in the ass.

Specs?

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| 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 Tuesday, May 12, 2015 at 12:55:36 PM UTC-4, bitrex wrote:
> John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
> > On Tue, 12 May 2015 04:51:58 -0500, "Tim Williams"
> > <tiwill@seventransistorlabs.com> wrote:
> >
> >>"whit3rd" <whit3rd@gmail.com> wrote in message
> >>> 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.
> >>
> >>Well, no; a little bit of negative conductance* might even be beneficial.
> >>
> >>* "A little bit of negative resistance" would seem inaccurate, and "A
> >>lotta bit of negative resistance" just doesn't sound right at all.
> >>
> >>FWIW, just as negative resistance wraps around to positive resistance
> >>around zero, the same happens at infinity, going out to infinite
> >>resistance (positive to zero conductance) then back negative (negative
> >>nonzero conductance).
> >>
> >>Which is actually a really nice (physical) manifestation of some elegant
> >>mathematics: analytical calculus often makes use of poles (singularities)
> >>at infinity or zero, and executing loop integrals around them to make
> >>certain solutions that wouldn't otherwise be possible (you can't integrate
> >>1/x, along the real number line, through 0).  Those methods aren't really
> >>important here, but the analytical nature of infinity as being a
> >>wraparound point on a closed ring (when well-behaved) is quite nice.
> >>
> >>Tim
> >
> > I recall that the OP wants to connect two current sources to opposite
> > corners of a bridge rectifier. That will have interesting dynamics.
> >
> >
>
> Two current source/sink that track each other well over
>  temperature, and are also voltage controllable.  And preferably a
>  setup where I could mirror one set of source/sinks into others
>  that are slaved to the first one to have multiple
>  bridges.
>
> Figuring this out is kind of a pain in the ass.
>
And currents are in the range of opamps? (~10 -30 mA)
How much compliance voltage?

George H.
> --
>
>
> http://usenet.sinaapp.com/

```
```On 12/05/2015 16:30, John Larkin wrote:
> I recall that the OP wants to connect two current sources to opposite
> corners of a bridge rectifier. That will have interesting dynamics.
>

When this was done to drive a so-called vario-losser diode bridge
automatic volume control network back in the 1968 (all descrete)
Kudelski Nagra4 I seem to recall one current source was the master and
the other slaved by servo action to maintain the bridge midpoint voltage.

piglet

```
```On Tue, 12 May 2015 12:55:06 -0400 (EDT), bitrex

>John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
>> On Tue, 12 May 2015 04:51:58 -0500, "Tim Williams"
>> <tiwill@seventransistorlabs.com> wrote:
>>
>>>"whit3rd" <whit3rd@gmail.com> wrote in message
>>>> 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.
>>>
>>>Well, no; a little bit of negative conductance* might even be beneficial.
>>>
>>>* "A little bit of negative resistance" would seem inaccurate, and "A
>>>lotta bit of negative resistance" just doesn't sound right at all.
>>>
>>>FWIW, just as negative resistance wraps around to positive resistance
>>>around zero, the same happens at infinity, going out to infinite
>>>resistance (positive to zero conductance) then back negative (negative
>>>nonzero conductance).
>>>
>>>Which is actually a really nice (physical) manifestation of some elegant
>>>mathematics: analytical calculus often makes use of poles (singularities)
>>>at infinity or zero, and executing loop integrals around them to make
>>>certain solutions that wouldn't otherwise be possible (you can't integrate
>>>1/x, along the real number line, through 0).  Those methods aren't really
>>>important here, but the analytical nature of infinity as being a
>>>wraparound point on a closed ring (when well-behaved) is quite nice.
>>>
>>>Tim
>>
>> I recall that the OP wants to connect two current sources to opposite
>> corners of a bridge rectifier. That will have interesting dynamics.
>>
>>
>
>Two current source/sink that track each other well over
> temperature, and are also voltage controllable.  And preferably a
> setup where I could mirror one set of source/sinks into others
> that are slaved to the first one to have multiple
> bridges.
>
>Figuring this out is kind of a pain in the ass.

Dueling current sources are inherently unstable, but I don't know what
else the BR is connected to.

Why not just resistors to adjustable V+ and V-? With some junction
drop compensation maybe. That is easy and stable.

But I don't have the big picture of what you're trying to do.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```