Post on November 7, 2018
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.
Figure 1. Cost Effective SE to Diff Input Stage
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 “Ipower” noise voltages Vn1, Vn2, and Vn3 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 Vn1, Vn2, and Vn3 develop between “Input_A” and “Input_B” and is amplified by the TPA32xx amplifier.
This results in degraded low frequency audio performance.
Input Stage with Star Ground
To fix this issue of different THD+N results between stereo channels at 100 Hz is simply ground plane noise, G2, G3, and G4 input stage ground nodes were connected with separate traces directly to the TPA32xx amplifier analog ground reference, G1. With this star ground connection scheme, G1 G2 G3 and G4 are well coupled and share the same ground potential. Therefore, no noise voltage can be developed between the input stage grounds and the TPA32xx analog ground. Figure 3 shows this connection scheme.
Figure 3. SE to Diff Input Stage Star Ground
TPA32xx Ground Pin Reference
For reference, the pinout of the analog and power ground pins on the TPA32xx devices are listed below.
TPA32xx Class-D Amplifier Ground Pins
|
| Amplifier | Analog Ground Pins | Power Ground Pins |
| TPA3244 | 10, 11 | 25, 26, 33, 34, 41, 42 |
| TPA3245 | 12, 13 | 25, 26, 33, 34, 41, 42 |
| TPA3250 | 10, 11 | 25, 26, 33, 34, 41, 42 |
| TPA3251 | 12, 13 | 25, 26, 33, 34, 41, 42 |
| TPA3255 | 12, 13 | 25, 26, 33, 34, 41, 42 |
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