Active biasing improves stereo performance

Comparing active and passive biasing circuits for digital potentiometers

BY BRIAN C. WADELL
Maxim Integrated Products
Boston, MA
http//www.maxim-ic.com

The use of digital potentiometers and volume-control ICs to replace mechanical pots enables a wider variety of user-interface features, but consumer-product designers are also pushed to reduce costs at every turn. To achieve the best audio performance you must pay careful attention to circuit details, which sometimes requires a tradeoff between performance and cost. The following discussion compares active and passive biasing circuits for digital potentiometers, considers their performance, and provides equations for making the right tradeoffs in your design.

Volume controls and digital potentiometers

Figure 1 illustrates the circuitry in question. We focus on single-supply applications, which are common in products powered by batteries and wall adapters. In a single-supply application, all parts of the system are powered between VDD and ground, and the signals swing between VDD and ground. You can use capacitors between the stages, or eliminate them altogether for reasons of cost and performance.

Fig. 1. The wiper buffer reduces distortion by reducing current through the switch array internal to the digital potentiometer.

A digital-potentiometer IC (outlined in red) is an array of resistors and switches under the control of logic that emulates the sliding contact wiper of a mechanical potentiometer. A volume-control IC (outlined in blue) is a digital-potentiometer IC that incorporates two other circuits very important for best audio performance — an op-amp buffer for the wiper, and a bias generator for producing VBIAS .

Passive bias circuit

To control costs while using a digital potentiometer, you can generate the bias voltage with a passive resistive divider (see Fig. 2a ). Equal resistor values set VBIAS at the midpoint between VDD and ground. To reduce the ac impedance and noise at VBIAS , you should also add a bypass capacitor (C2 , Fig. 2b ).

Fig. 2. Passive VBIAS : basic (a), with noise suppression (b).

Let’s consider how the chosen circuit values affect audio performance. Source impedance for the bias circuit, sometimes called the stiffness of the supply, is calculated using Equation (1):

Z2 =(R2 x 1/sC2 )/(R2 + 1/sC2 ) =

R2 /(R2 x sC2 + 1) Equation (1)

Zin = (Z1 x Z2 )/(Z1 + Z2 ) =

[R1 x R2 /(R2 x sC2 + 1)]/[R1 + R2 /(R2 x sC2 + 1) =

R1 x R2 /(R1 x R2 x sC2 + R1 + R2 ) Equation (2)

At dc (s = 0), note that equation 2 reduces to the impedance of R1 and R2 in parallel. We now insert this impedance into the volume-control circuit (see Fig. 3 ).

Fig. 3. Circuit model for the analysis.

Note that with the wiper at the low (L) end of the digital potentiometer, the finite source impedance in the bias network allows a signal to appear on the L terminal of the digital potentiometer. Yet, setting the wiper to L is supposed to mute the audio channel! Instead of no signal, as desired, we see a divided-down version of the source voltage, VIN . For example, with two 10-kΩ bias resistors and a 40-kΩ pot with wiper at the L position, we see an output voltage of

VOUT = 5 kΩ/(40 kΩ + 5 kΩ) x VIN

which is only −19 dB below full scale (dBFS ) at dc.

What happens if we include C2 in the analysis? Substituting 0.01 µF for C2 in equation 2, the result is shown in Fig. 4 . We had almost no effect below 1 kHz, and the effect at 20 kHz is to reduce the impedance to just 785 Ω. We achieve a mute attenuation of just –22 dB, as shown in Fig. 5 . The capacitor can be raised to 10 µF (large). At 100 Hz we now have a mute attenuation of just −36 dB. This is not even close to the ultimate performance of the volume control, nor is it close to a reasonable specification for mute.

Fig. 4. Output impedance of the bias network, for C2 values of 0.01, 10, and 100 μF.

Although we analyzed this problem with the wiper sitting at the mute position (the L terminal), it’s apparent that a finite source impedance for VBIAS affects all potentiometer settings. As you approach the low end of the potentiometer, that effect increases inaccuracy in the attenuation curve.

Fig. 5. Mute attenuation for the bias netowork, for C2 values of 0.01 μF, 10 μF, and 100 μF.

How to fix the problem

We need to reduce the impedance, and increase the muting to a level of 90 dB or so. To reach 90 dB, the source impedance for VBIAS must be in the range of single-digit ohms. You can reduce this impedance by using smaller values for R1 and R2 , at the expense of higher dc current, but that approach isn’t practical. Obviously, with this passive circuit, we must depend on C2 to get the impedance very low before entering the audio band. To achieve 95 dB at 100 Hz, for example, you find that the capacitor value is again unrealistic. For values of 10 kΩ, 10 kΩ, and (a somewhat large) 100 µF, the attenuation is still only moderately effective. The solution to this problem is an active circuit.

Before discussing the active circuit, consider a stereo design. For left and right signals sharing a passive bias generator, we not only have the problems of muting and feedthrough, but the additional problem of crosstalk as well. Crosstalk is the leakage of signal from the left (L) channel into the right (R) channel, and vice versa. The circuit for a shared bias line appears as in Fig. 6 .

Fig. 6. Stereo volume control, sharing VBIAS .

Finite bias impedance creates a signal voltage on the L terminal in response to an input on the H terminal, and the shared bias ensures signals at both VOUT pins from an input on either the left or the right. This in-channel signal appears as poor muting attenuation (a failure to follow the attenuation curve). The opposite-channel signal appears as crosstalk or a loss of stereo separation.

Active bias

The solution to all of these problems is to provide a very stiff (low-impedance) source for VBIAS , as shown in Fig. 7 . The divided-down VDD is buffered by an op amp whose closed-loop output impedance at dc is a fraction of an ohm. With careful design, you can then achieve muting attenuation in the 90-dB range.

Fig. 7. A unity-gain buffer provides a low source impedance for the VBIAS generator.

A test board allows measurements that compare the passive and active approaches. Figure 8 illustrates the typical performance achieved for both passive and active circuits, using a test board that includes a dual audio-taper digital potentiometer for stereo applications (MAX5457). The passive circuit includes two 1-kΩ resistors bypassed by 4.7 µF, which produces a pole at 68 Hz (calculated), and a continuous battery drain of 2.5 mA.

Fig. 8. Full-scale, mute response for passive biasing (middle trace) and mute response with active biasing (bottom trace).

The active circuit can have higher-value resistors, because its only load is the non-inverting buffer. Thus, two 100-kΩ resistors hardly affect the active-bias curve, and their continuous battery drain is just 25 µA.

Integrate it

As we have seen, passive circuits add cost and size while yielding poor performance. An active circuit that includes a buffered version of the resistor divider works well, but adds the overhead of an op amp. A better alternative is a volume-control IC like the MAX5486 shown in Fig. 9 , which integrates an op amp with the digital potentiometer.

By choosing various members of this IC family, you can implement a direct interface to a microprocessor, pushbuttons, rotary encoders, or even infrared remote controls. Synchronized zero-crossing wiper movements and other features optimize these devices for audio applications.

Fig. 9. This volume-control IC (MAX5486) includes the buffered VBIAS voltage and wiper buffers required for audio applications.