A foolish easy and ubiquitous circuit community is a variable resistance consisting of the collection connection of a manually adjusted rheostat-connected pot and stuck resistor proven in Determine 1.
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Rmax = Rs + Rr |
Determine 1 Traditional variable resistance with the collection connection of a manually adjusted rheostat-connected pot and a hard and fast resistor.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
The provision of pots and resistors spanning ohms to megohms makes optimum selections of Determine 1’s part values apparent and simple. But when an utility requires utilizing a digital potentiometer (Dpot), the state of affairs will get extra—ahem—attention-grabbing.
Dpots are solely accessible in a comparatively slim vary of resistance in comparison with guide pots. In addition they undergo from bigger wiper resistances and wider tolerances. These limitations make them a doubtful alternative for implementing precision rheostats if Determine 1’s basic passive topology is solely relied upon. Determine 2 gives an energetic and extra Dpot-friendly various.
Right here’s the way it works.
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Rmax = (Rab-1 + Rp-1)-1 |
Determine 2 Artificial Dpot evades issues utilizing FET shunt, precision fastened resistors, and op-amp.
Regardless of the actual fact we’re implementing a variable resistance, Dpot U1 is operated in potentiometer mode. So, its resistance tolerance (+/-20% for the MCP41xx collection) has little destructive impact. The precision of Rs and Rp dominate. Likewise, Dpot wiper resistance is rendered purely educational by the pA enter present and T ohms enter impedance of A1. A1 and Q1 are linked as a programmable present supply. Its output is proportional to the Va – Vb voltage differential, thus forming a exact programmable resistance. This relationship makes present Iab linearly proportional to N.
Design equations are for applicable resistances ranging from specified Rab, Rmax, and Rmin are:
- Rab > Rmax
- Rp = (Rmax-1 – Rab-1)-1
- Rs = (Rmin-1 – Rab-1 – Rp-1)-1
Determine 3 reveals a typical design instance for Rmax = 20k, Rmin = 1k.
Determine 3 Artificial rheostat design instance the place Rmax = 20k and Rmin = 1k.
Determine 4 plots R and present per (Va – Vb volts) as features of N.
Determine 4 Efficiency of Determine 3’s circuit with values proven, the linear relationship between N and Ia conserves the Dpot’s restricted 8-bit decision.
Be aware the precisely linear relationship between N and Ia present which does job of conserving Dpot restricted 8-bit decision.
A query arises: What if the required Rmax is bigger than the Rab resistance of accessible Dpots? Determine 5 gives a sensible (though admittedly considerably busy) resolution that may simply implement an correct Rmax extending far into the multi-megohm vary.
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Rmax = Rp |
Determine 5 Two buffer amps take away Rab from Rmax equation, permitting an for an Rmax extending far into the megaohms.
One other (stickier) query is: What occurs if the polarity of Va – Vb is topic to reversal? Determine 1 can accommodate this with no second thought, nevertheless it’s a big downside for this design thought.
I’ll should get again to you on that one!
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.
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The publish Synthesize precision Dpot resistances that aren’t within the catalog appeared first on EDN.
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