Rheostats
Rheostats are easy and ubiquitous circuit parts, normally comprising a potentiometer related as an adjustable two terminal resistor. The supply of guide pots with resistances spanning ohms to megohms makes the optimum selection of nominal resistance simple. However when an software requires a digital potentiometer (Dpot), the issue might be difficult.
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Dpots are solely obtainable in a resistance vary that’s slender in comparison with guide pots. Additionally they sometimes endure from problematically excessive wiper resistance and resistance tolerance. These limitations conspire to make Dpots a tough medium for implementing precision rheostats. Current EDN design thought (DI) articles have addressed these points with quite a lot of methods and topologies:
- Op-amp wipes out DPOT wiper resistance
- Synthesize precision Dpot resistances that aren’t within the catalog
- Synthesize precision bipolar Dpot rheostats
- A category of programmable rheostats
Whereas every of those designs corrects a number of complaints on the prolonged listing of digital rheostat shortcomings, none fixes all of them and a few introduce problems of their very own. Examples embody crossover distortion, unreduced sensitivity to resistance tolerances, resolution-reducing nonlinearity of the programmed resistance, and simply plain previous complexity.
The design
Determine 1’s circuit isn’t an ideal resolution both. But it surely does synthesize an correct programmed resistance equal to reference resistor R1 linearly multiplied by U1’s Rbw/Rab digital setting (the ratio between the terminal B to wiper resistance and complete component resistance).
Determine 1 A precision digital rheostat that synthesizes an correct programmed resistance equal to reference resistor R1 linearly multiplied by U1’s Rbw/Rab.
Right here’s the way it works.
R = (Va – Vb)/Ia
R = R1/(Uncooked/Rbw + 1) = R1 Rbw/Rab
Rab = Uncooked + Rbw = sometimes 5k to 10k
The place R is the programmed artificial resistance, R1 is the reference resistor, Uncooked is the resistance between terminal A and wiper terminal, Rbw is the resistance between B and wiper terminal, and Rab is the overall component resistance.
U1 works in “voltage divider” (pot) mode to set the acquire of inverting amplifier A2. Pot mode makes acquire insensitive to each U1’s wiper resistance (Rw) and Rab. They actually don’t matter a lot—see Determine 4-4 within the Microchip MCP41XXX/42XXX datasheet.
Turning the crank on Determine 1’s design equation math, we get:
Ga2 = Uncooked/Rbw
The place Ga2 is A2’s acquire. Additional,
Voltage throughout R1 = (Va – Vb) + Ga2(Va – Vb) = (Uncooked/Rbw + 1)(Va – Vb) = Rab/Rbw(Va – Vb)
Present by means of R1 = Ia = Rab/Rbw(Va – Vb)/R1
Then, since R = (Va – Vb)/Ia:
R = R1*Rbw/Rab
Va is evenly loaded by A1’s ~10 picoamp (pA) enter bias, so R1 can vary from lots of of ohms as much as a number of megohms as the appliance could dictate. It needs to be precision, definitely 1% or higher; then, programming and the maths above takes over.
Determine 2 plots the linear relationship between R and Rbw.
Determine 2 Linear relationship between R and Rbw displaying the circuit synthesizes an correct programmed resistance equal to reference resistor R1 linearly multiplied by U1’s Rbw/Rab.
A compensation capacitor (C1) most likely isn’t mandatory for the elements choice proven in Determine 1 for A2 and U1. But when a quicker amplifier or a better resistance Dpot is chosen, then 10 pF to twenty pF would most likely be prudent.
In the meantime, I believe it will be truthful to say this design appears to be like aggressive with its friends. However earlier I described it as imperfect. Apart from being a single-terminal topology (like two others on the listing), the place else does it fall wanting being a whole resolution to the best digital rheostat (Digistat) downside?
Shortcomings
Right here’s the place: As Determine 3 exhibits, when the programmed worth for R goes down, A2’s acquire (Ga2) should go up. Studying the graph from proper to left, we see acquire rising reasonably as R declines by 75% from R1 to R1/4 the place, Rbw/Rab = 64/256 and acquire = 3, however then it takes off. This tends to magnify errors like enter offset, finite GBW and different op-amp nonidealities whereas creating the potential for early A2 saturation at comparatively low sign ranges.
Determine 3 Graphs for Ga2 (pink) and R/R1 (black) versus Rbw/Rab on the x-axis. When the programmed worth for R goes down, Ga2 should go up.
The severity of the impression of those results on utility of the design, whether or not gentle, critical, or deadly, will depend upon how low you want go in R/R1 and different specifics of the appliance. So, it’s definitely not excellent, however perhaps it’s nonetheless helpful someplace.
Two-terminal design
And about that single terminal downside. You probably have an software that completely requires a two-terminal programmable resistance, you may contemplate Determine 4. Relying on the exterior circuitry, it may not oscillate.
Determine 4 Duplicate and cross-connect Determine 1’s circuitry to get a two-terminal programmable resistance.
In closing…
Because of frequent contributor Christopher R. Paul for his intelligent improvements and stimulating discussions on this matter, I’d probably by no means have give you this design with out his assist. Extra thanks go to editor Aalyia Shaukat for her intelligent creation of this DI part that makes enjoyable teamwork like this potential within the first place. This text would positively by no means have occurred with out her assist.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- Op-amp wipes out DPOT wiper resistance
- Synthesize precision Dpot resistances that aren’t within the catalog
- Synthesize precision bipolar Dpot rheostats
- A category of programmable rheostats
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