Categories: Electronics

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 regulate the achieve of a excessive efficiency OP37 decompensated op-amp over an unusually huge (-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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Just lately 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 indicators. 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 ultimately resorted to.

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

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

A fast methodology 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 detrimental resistance math. Notice Vp# = voltage sign current at A2 pin #.

Let Iw = wiper sign present, then
Vp6 = Vp2 – R4*Iw
Vp2 = Vp3 (detrimental suggestions)
Vp3 = Vp6/2 (constructive suggestions)
Vp6 = Vp6/2 – R4*Iw
Vp6 – Vp6/2 = Vp6/2 = -R4*Iw
Vp6 = -2*R4*Iw
If R4 = Rw, then IR4 = IRw
-2*R4*Iw = -(R4 + Rw)Iw
Vw = Vp6 + (Iw*R4 + Iw*Rw) = -Iw(R4 + Rw) + Iw(R4 + Rw)
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. Examine it to the efficiency of the unique (Determine 1) circuit utilizing a mechanical pot as proven in Determine 5.

In fact, how good Rw cancellation over the total vary of Dpot settings might be isn’t any higher than Rw match over the Dpot’s 257 completely 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 decreasing Rw by an element of 5 remains to be helpful.

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

Determine 5 Achieve curve utilizing the mechanical pot is an identical to Dpot with detrimental 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 all the time admired pioneers!

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.

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