That is the Microchip MCP41xxx digital potentiometer knowledge sheet that features (on web page 15, their Determine 4-4) an attention-grabbing software circuit comprising a Dpot managed amplifier with pseudologarithmic achieve settings. Nonetheless, as defined within the Microchip textual content, the beneficial properties applied by this circuit begin altering radically because the management setting of the pot approaches 0 or 256. As Microchip places it: “Because the wiper approaches both terminal, the step measurement within the achieve calculation will increase dramatically. This circuit is beneficial for beneficial properties between 0.1 and 10 V/V.”
That’s good recommendation. Sadly, following it could successfully throw away some 48 of the 256 8-bit pot settings, amounting to a lack of almost 20% of obtainable decision. The straightforward modification proven in Determine 1 removes that limitation.
Determine 1 Two fastened resistors are added to certain the achieve vary to the beneficial limits whereas preserving full 8-bit decision.
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This ends in the achieve vs code purple curve of Determine 2.
Determine 2 Considerably improved pseudologarithmic achieve curve from the easy modification proven in Determine 1.
Nonetheless, regardless of this enchancment, the important thing time period stays pseudologarithmic. It nonetheless isn’t an actual log operate and, actually, isn’t quantitatively even that shut, deviating by virtually an element of two in locations. Can we do higher? Sure!
The straightforward (software program) trick is to arrange a 257-byte logarithmic lookup desk that interprets the 0.1 to 10.0 achieve vary settings to the Dpot codes wanted to logarithmically generate these beneficial properties.
Let’s name the desk index variable J. Then for a 257-byte desk of (abs) beneficial properties G from 0.1 to 10.0 inclusive,
J(G) = (128 LOG10(abs(G)) + 128)
…examples…
J(0.1) = 0,
J(0.5) = 89,
J(1.0) = 128,
J(10.0) = 256,
and many others.
Inspection of the achieve expression in Determine 1 reveals that the Dpot decimal code N required for (abs) achieve G is:
N(G) = (284.4G – 28.4)/(G + 1)
…thus…
N(.1) = (28.4 – 28.4)/(.1 + 1) = 0/1.1 = 0,
N(.5) = (142 – 28.4)/(.5 + 1) = 114/1.5 = 76,
N(1.0) = (284.4 – 28.4)/(1 + 1) = (256)/2 = 128,
N(10.0) = (2844 – 28.4)/(10 + 1) = 2816/11 = 256,
and many others.
Determine 3 summarizes the ensuing relationship between G, J, and N.
Determine 3 The Dpot settings [N(J)] versus log desk indices [J(G)], summarizing the connection between G, J, and N.
The desk of log beneficial properties could be present in this excel sheet. The web end result, with nearly as good log conformity as 8 bits will enable, is exhibited as Determine 4’s beautiful inexperienced line.
Determine 4 Absolutely the achieve [Gabs = 10(J/128 -1)] versus decimal code (J).
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- Preserve Dpot pseudologarithmic achieve management on a leash
- Synthesize precision Dpot resistances that aren’t within the catalog
- Decreasing error of digital potentiometers
- Alter op-amp achieve from -30 dB to +60 dB with one linear pot
- Op-amp wipes out DPOT wiper resistance
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