TPA32xx Analog Input Grounding for Single Ended Inputs
TPA32xx Analog Input Grounding for Single Ended Inputs
With high power Class-D audio amplifiers, layout is often critical for achieving maximum power and optimal performance. For this reason, a ground plane on two layers is recommended to provide a solid ground reference for the amplifier and to minimize ground voltage differentials across the die of the monolithic amplifier. However, as found with the TPA32xx family of amplifiers, referencing the analog input grounds directly to the ground plane can cause issues, especially when using single ended op-amps.
NOTE
This applications report discusses the grounding recommendations for analog inputs only. It is always recommended to use a ground plane for high power and switching nodes with all Class-D amplifiers.
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1 Single Ended to Differential Input Stage
Figure 1 shows the basic setup for a Single-Ended (SE) to Differential (Diff) input stage used with the TPA32xx amplifier. In this use case, the amplifier is configured for BTL output and requires “Input_A” and “Input_B” to be driven differentially. A pair of inverting Op-amps is used where the Op-amp gain is set to
–1. By cascading the operational-amplifiers as shown, the analog signal from the RCA jack labeled “SE Input” appears on both “Input_A” and “Input_B” of the TPA32xx amplifier out of phase with each other.
These operational-amplifiers also run from a single 12 V supply rail to reduce cost. In order to pass analog signals that swing below ground, the operational-amplifiers are biased to the mid rail (6 V) using a resistor divider and filter. Then DC blocking capacitors are used to isolate the Op-amp bias voltage from a ground referenced analog input.
This cost effective SE to Diff converter works quite well; however, since this design is not using a truly differential operational-amplifier stage, there will always be some imbalance between “Input_A” and “Input_B” with this configuration and less than ideal common mode (CM) rejection.
