Digital pot can management achieve over a 90 dB span like an electromechanical

A short time again, I revealed a design concept that makes use of a single linear pot to manage the achieve of a excessive efficiency OP37 decompensated op-amp over an unusually broad (-30 dB to +60 dB) vary.

Determine 1 reveals the circuit.

Achieve = (R2ccw/(R1 + R2ccw))(R3/R2cw + 1).

Determine 1 Grounded wiper makes R2 function each enter attenuator and output achieve set.

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Not too long ago I began questioning whether or not a digital pot (Dpot) would work instead of Determine 1’s mechanical R2. Determine 2 reveals what appeared like a probable Dpot topology.

Achieve = (R2ds/(R1 + R2ds + Rw))(R3/(R2(1 – ds) + Rw) + 1)

Determine 2 R2 has the identical perform as in Determine 1 with DC bias from R4 R5 C2 to accommodate bipolar alerts. However what about Rw wiper resistance results?

On nearer inspection, it turned out to not be so very promising in any case. This is because of wiper resistance interfering with the isolation of the 2 halves of R2 that made the unique circuit work within the first place. Determine 3 reveals the repair I finally resorted to.

Determine 3 Optimistic and adverse suggestions loops round A2 mix to create lively adverse resistance = -R4.

A2 and its surrounding community are the idea of the trick. They generate an lively adverse resistance impact that subtracts from Rw and, if adjusted so R4 = Rw, can theoretically (the engineer’s least favourite phrase) cancel it out fully. 

A fast technique for dialing out Rw is to write down the Dpot setting to zero, present a ~1v rms enter, then trim R4 for output null.

Right here’s some adverse resistance math. Word Vp# = voltage sign current at A2 pin #. 

1. Let Iw = wiper sign present, then
2. Vp6 = Vp2 – R4*Iw
3. Vp2 = Vp3 (adverse suggestions)
4. Vp3 = Vp6/2 (constructive suggestions)
5. Vp6 = Vp6/2 – R4*Iw
6. Vp6 – Vp6/2 = Vp6/2 = -R4*Iw
7. Vp6 = -2*R4*Iw
8. If R4 = Rw, then IR4 = IRw
9. -2*R4*Iw = -(R4 + Rw)Iw
10. Vw = Vp6 + (Iw*R4 + Iw*Rw) = -Iw(R4 + Rw) + Iw(R4 + Rw)
11. Vw = 0 (Rw has been cancelled out!)

 Achieve = (R2ds/(R1 + R2ds))(R3/(R2(1 – ds)) + 1)

Determine 4’s crimson curve compares Determine 2’s conduct with an (uncompensated) Rw = 150 Ω (believable for the Microchip Dpot illustrated), whereas the black curve reveals what occurs if R4 = Rw = 150 Ω. Evaluate it to the efficiency of the unique (Determine 1) circuit utilizing a mechanical pot as proven in Determine 5.

After all, how excellent Rw cancellation over the complete vary of Dpot settings will be isn’t any higher than Rw match over the Dpot’s 257 totally different faucets on the 2.5v DC bias offered by R5R6. Typical matching inside a given pot’s resistor array appears good, however this isn’t the producer’s promise, which solely speaks to an element of +/-20% or so. However lowering Rw by an element of 5 remains to be helpful.

Determine 4 Crimson curve plots uncompensated Rw (~150 Ω), be aware the 20 dB loss from each ends of the span. Black curve plots the case the place Rw is compensated with adverse resistance (R4 = Rw = 150). 

Determine 5 Achieve curve utilizing the mechanical pot is equivalent to Dpot with adverse resistance Rw compensation.

Footnote: Subsequent to publishing the mechanical pot model of this concept, I discovered that Mr T. Frank Ritter had anticipated it by greater than 50 years in his “Controlling op amp achieve with one potentiometer,” revealed in “Electronics Designer’s Casebook”, 1972, McGraw Hill. 

So, I hereby supply a belated however enthusiastic tip of my hat to Mr. Ritter. I’ve at all times admired pioneers!

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.

 Associated Content material

• 8-bit PWM + 8-bit Dpot = 16-bit hybrid DAC
• Dpot pseudolog + log lookup desk = precise logarithmic achieve
• Hold Dpot pseudologarithmic achieve management on a leash
• Synthesize precision Dpot resistances that aren’t within the catalog
• Decreasing 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

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