Categories: Electronics

Dpot pseudolog + log lookup desk = precise logarithmic acquire

That is the Microchip MCP41xxx digital potentiometer information sheet that features (on web page 15, their Determine 4-4) an fascinating software circuit comprising a Dpot managed amplifier with pseudologarithmic acquire settings. Nevertheless, as defined within the Microchip textual content, the positive aspects carried out 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 dimension within the acquire calculation will increase dramatically. This circuit is beneficial for positive aspects 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 practically 20% of obtainable decision. The straightforward modification proven in Determine 1 removes that limitation.

Determine 1 Two fastened resistors are added to sure the acquire vary to the beneficial limits whereas holding full 8-bit decision.

Wow the engineering world along with your distinctive design: Design Concepts Submission Information

This leads to the acquire vs code purple curve of Determine 2.

Determine 2 Considerably improved pseudologarithmic acquire curve from the easy modification proven in Determine 1.

Nevertheless, regardless of this enchancment, the important thing time period stays pseudologarithmic. It nonetheless isn’t an actual log perform and, the truth is, isn’t quantitatively even that shut, deviating by nearly 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 acquire vary settings to the Dpot codes wanted to logarithmically generate these positive aspects.

Let’s name the desk index variable J. Then for a 257-byte desk of (abs) positive aspects 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 so forth.

Inspection of the acquire expression in Determine 1 reveals that the Dpot decimal code N required for (abs) acquire 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 so forth.

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 positive aspects could be present in this excel sheet. The web end result, with pretty much as good log conformity as 8 bits will enable, is exhibited as Determine 4’s beautiful inexperienced line.

Determine 4 Absolutely the acquire [G_abs = 10^{(J/128 -1)}] versus decimal code (J).

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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