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Howland current pump question

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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Reply to
bitrex
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If the OpAmp inputs are not within the OpAmp offset voltage then something is very wrong. ...Jim Thompson

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| 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 to
Jim Thompson

I will try to post a .asc file in a moment.

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Reply to
bitrex

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

The bigger problem with the Howland is resistor tolerances.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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Reply to
bitrex

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.

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Reply to
bitrex

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 to
Martin Riddle

Here's a discussion.

formatting link 

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

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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 to
Jim Thompson

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.

Reply to
whit3rd

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.

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Bill Sloman, Sydney
Reply to
Bill Sloman

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

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

Try moving the right end of R4 to ground.

Reply to
John S

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

"Pas top" as we say in French.

Hab.

Reply to
habib.bouaziz

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

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

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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.

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Reply to
bitrex
[snip]

Specs? ...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

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

And currents are in the range of opamps? (~10 -30 mA) How much compliance voltage?

George H.

Reply to
George Herold

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

Reply to
piglet

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.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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