Audio Fading Circuit Using the MAX5456/MAX5457
While using digital potentiometers with AC signals, certain considerations need to be addressed. Firstly, it is important to understand how to bias an AC signal to within the DC supply range. Secondly, while the digital pot steps are usually designed to give equal-value resistive increments, as a byproduct of the process variation, the total end-to-end resistance can vary widely from part to part—as much as ±30% in some types of digital pots. (Refer to application note 161, "Using a DS1802 Push-Button Digital Potentiometer to Create an Audio Preamp with Attenuator" and application note 1828, "Audio Gain using Digital Potentiometers" for more information.)
For Larger Image
Figure 1. Circuit block diagram for implementing volume, balance, and fader control using three MAX5456/MAX5457 ICs.
For applications requiring fader control for multispeaker configurations, use additional MAX5456/MAX5467's to implement this design.
The MAX5457 switches between volume- and balance-control modes by toggling the active-low MODE input. An LED output indicates when the device is in volume- or balance-control mode. For the application circuit shown in Figure 1, U1 controls the balance and volume of the stereo input signal and outputs the left and right signals to the fader control formed by U2 and U3. Change the master volume with the active-low DN / active-low BAL0 or active-low UP / active-low BAL1 buttons of U1 in volume mode. To change the balance, toggle active-low MODE to switch to balance mode and use the active-low DN / active-low BAL0 or active-low UP / active-low BAL1.
U2 and U3 form the fader control for the left and right channels respectively. By adjusting the balance of U2, the left signal from U1 fades between L0 and L1. Similarly, by adjusting the balance of U3, the right signal from U1 fades between R0 and R1. Refer to the MAX5456/MAX5457 data sheet for volume- and balance-control operation. The volume-control function in U2 or U3 is typically not used, because U1 handles the volume adjustment. Adjusting volume in U2 and U3, however, can sometimes be useful to compensate for mismatches in the audio path.
The op amps preceding U1 in the application circuit amplify line-level or high-impedance input signals from a stereo audio source to drive the MAX5456/MAX5457. Given that the minimum end-to-end resistance of the MAX5456/MAX5457 ICs equal 7KΩ, the worst-case load impedance looking into the left or right input of U1 is 7KΩ (worst case) and 10KΩ (nominal). The two amplifiers on the outputs of U1 are used as buffers and limit the load to minimize THD. Finally, the four amplifiers on the outputs of U2 and U3 minimize wiper current and amplify the signal to drive the speakers.