A short time again, I revealed a design concept that makes use of a single linear pot to manage the acquire of a excessive efficiency OP37 decompensated op-amp over an unusually huge (-30 dB to +60 dB) vary.
Determine 1 exhibits the circuit.
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Acquire = (R2ccw/(R1 + R2ccw))(R3/R2cw + 1). |
Determine 1 Grounded wiper makes R2 function each enter attenuator and output acquire set.
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Lately I began questioning whether or not a digital pot (Dpot) would work instead of Determine 1’s mechanical R2. Determine 2 exhibits what appeared like a possible Dpot topology.
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Acquire = (R2ds/(R1 + R2ds + Rw))(R3/(R2(1 – ds) + Rw) + 1) |
Determine 2 R2 has the identical operate 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 spite of everything. 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 exhibits the repair I ultimately resorted to.
Determine 3 Constructive and unfavorable suggestions loops round A2 mix to create lively unfavorable resistance = -R4.
A2 and its surrounding community are the premise of the trick. They generate an lively unfavorable 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 methodology for dialing out Rw is to put in writing the Dpot setting to zero, present a ~1v rms enter, then trim R4 for output null.
Right here’s some unfavorable resistance math. Be aware Vp# = voltage sign current at A2 pin #.
- Let Iw = wiper sign present, then
- Vp6 = Vp2 – R4*Iw
- Vp2 = Vp3 (unfavorable suggestions)
- Vp3 = Vp6/2 (optimistic 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!)
Acquire = (R2ds/(R1 + R2ds))(R3/(R2(1 – ds)) + 1)
Determine 4’s purple curve compares Determine 2’s habits with an (uncompensated) Rw = 150 Ω (believable for the Microchip Dpot illustrated), whereas the black curve exhibits 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 total vary of Dpot settings might 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 continues to be helpful.
Determine 4 Purple curve plots uncompensated Rw (~150 Ω), word the 20 dB loss from each ends of the span. Black curve plots the case the place Rw is compensated with unfavorable resistance (R4 = Rw = 150).
Determine 5 Acquire curve utilizing the mechanical pot is similar to Dpot with unfavorable 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 acquire 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 great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
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- Maintain Dpot pseudologarithmic acquire management on a leash
- Synthesize precision Dpot resistances that aren’t within the catalog
- Decreasing error of digital potentiometers
- Modify op-amp acquire from -30 dB to +60 dB with one linear pot
- Op-amp wipes out DPOT wiper resistance
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