A typical drawback for digital potentiometers (DigiPots) is the impact of wiper resistance, which produces fairly noticeable non-linearities of regulation at each ends of the resistance vary. These results additionally result in an elevated tempco in these areas, for the reason that excessive wiper resistance temperature coefficient will dominate.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
Determine 1 is reproduced from Determine 4-6 on the datasheet for the “Digital Potentiometer MCP41XXX_42XXX” (Microchip, DS11195C, web page 15). Absolutely the achieve response is kind of typical, so it’s repeated as a great illustration. The circuit makes use of the DigiPot as a achieve management component, the inexperienced line is added to point out the best achieve management.
Determine 1 Acquire versus decimal code for inverting and differential amplifier circuits (black line) reproduced from Digital Potentiometer MCP41XXX/42XXX and very best achieve management (inexperienced line). Supply: Microchip
Now, let’s see how we are able to scale back the affect of the wiper resistance on each ends of the resistive component.
An answer proven in Determine 2 exploits the truth that wiper resistance of a DigiPot isn’t associated with its nominal whole resistance. The thought is kind of easy and easy: two DigiPots from the identical chip are linked abreast. (Each DigiPots have to be programmed with the identical code.) As proven in Determine 1, absolutely the error—induced by a non-zero wiper resistance (r)—will get decrease by worth of r/2.
Determine 2 An answer that reduces the errors of digital potentiometers by exploiting the truth that the wiper resistance of the DigiPot will not be associated to its nominal whole resistance. Supply: Peter Demchenko
The answer is extra appropriate for non-single DigiPots, since they’ll assure a suitable resistor-matching.
The answer could also be additionally helpful for the Rheostat mode tempco and Rheostat INL error, the place each may be diminished.
Whereas the wiper resistance of a DigiPot isn’t associated to its nominal resistance, the wiper resistance might improve when its nominal will increase; this may destroy the benefits of the circuit, so it is very important watch out.
To take care of the identical whole resistance, each DigiPots ought to have 2x the nominal whole resistance, which can be troublesome to make sure typically, for the reason that assortment of nominal values is quite restricted.
Word additionally that the bigger nominal worth of the DigiPot can result in some discount of the efficient frequency vary.
—Peter Demchenko studied math on the College of Vilnius and has labored in software program growth.
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