That is the Microchip MCP41xxx digital potentiometer information 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. Nevertheless, as defined within the Microchip textual content, the positive factors 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 dimension within the achieve calculation will increase dramatically. This circuit is beneficial for positive factors 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 achieve vary to the beneficial limits whereas retaining full 8-bit decision.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
This ends in the achieve vs code crimson curve of Determine 2.
Determine 2 Considerably improved pseudologarithmic achieve curve from the straightforward 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, 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 organize 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 positive factors.
Let’s name the desk index variable J. Then for a 257-byte desk of (abs) positive factors 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 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 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 factors could be present in this excel sheet. The web outcome, with nearly as good log conformity as 8 bits will enable, is exhibited as Determine 4’s pretty 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
- Maintain Dpot pseudologarithmic achieve management on a leash
- Synthesize precision Dpot resistances that aren’t within the catalog
- Lowering error of digital potentiometers
- Regulate op-amp achieve from -30 dB to +60 dB with one linear pot
- Op-amp wipes out DPOT wiper resistance
googletag.cmd.push(perform() { googletag.show(‘div-gpt-ad-native’); });
–>
The submit Dpot pseudolog + log lookup desk = precise logarithmic achieve appeared first on EDN.
