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.

Note: Only one channel of TPA32xx Class-D Amplifier is shown

Figure 1. Cost Effective SE to Diff Input Stage

2 Input Stage with Ground Plane

Now suppose that all of the ground points for the input stage are tied to the ground plane at the most convenient locations, as typically done in PCB layout (Figure 2).

At high output power, the ground plane used for the TPA32xx amplifier will start to see heavy current flow and switching noise. This is generally worse at lower audio frequencies where the sustained amplitude of the audio signal peaks is longer when compared to higher frequency signals. This can cause lots of noise on the ground plane.

Figure 2. SE to Diff Input Stage Ground Plane Noise

Due to the heavy current flow into the ground plane as represented by the arrow “I_power” noise voltages V_n1, V_n2, and V_n3 develop across the ground plane. Therefore, the ground nodes for the input stage G2, G3, and G4 will all be at different potentials relative to the TPA32xx amplifier analog ground reference G1.

Since our Op-amp input stage is not truly differential with poor CM rejection, signal imbalance is inevitable. It is likely that a differential noise voltage due to V_n1, V_n2, and V_n3 develop between “Input_A” and “Input_B” and is amplified by the TPA32xx amplifier.

This results in degraded low frequency audio performance.