Stereo Gain Audio Trim Control Circuit
This circuit using the MAX5160L digital pot in a divider chain supplying the MAX4252 op amp with some positive feedback in addition to the usual negative feedback via the 100K and 50K resistors. The gain of this circuit can be shown to be:
AV = (1-Kn)/(Kp-Kn)
where Kn is the negative feedback fraction, Kp the positive feedback fractions (for the example in Figure 4, Kn = 100K/(100K + 50K) or 2/3, and Kp is variable).
When the MAX5160L wiper is positioned at the VREF terminal, the gain of the circuit is -0.5V/V, as there is no positive feedback contribution. When the wiper is at mid scale, Kp = 0.5, and the gain is now -2V/V. Hence, by using those 17 positions between VREF and midscale the gain can be varied over a ±6dB range. The 15 unused positions have been traded off for repeatability, as the gain value does not depend upon the digital pot resistance tolerance, as did the circuit of Figure 1. The gain tolerance is now only limited by the ±1% 100K/50K resistors, and the INL/DNL error of the MAX5160L (±4.6% max.).
An interesting point to note, the limit for stability in this circuit is reached when Kp ≥ 2/3, when the positive feedback fraction meets or exceeds the negative. The host processor controlling the MAX5160L should therefore prevent this situation occurring.
The circuit in Figure 5 shows an obvious appraoch to a 'traditional' style volume control using digital pots. All codes are valid, with settings ranging from 0dB to full attenuation. Table 1 shows the calculated attenuations based on the MAX5160L's 32 steps.