![](http://datasheet.mmic.net.cn/130000/TPA2005D1GQYRQ1_datasheet_5021599/TPA2005D1GQYRQ1_11.png)
Input Resistors (RI)
Gain + 2
150 kW
R
I
(1)
Decoupling Capacitor (CS)
Input Capacitors (CI)
fc +
1
2p R
I
C
I
(2)
C
I +
1
2p R
I
fc
(3)
www.ti.com............................................................................................................................................. SLOS474B – AUGUST 2005 – REVISED DECEMBER 2008
The input resistors (RI) set the gain of the amplifier according to equation Equation 1. Resistor matching is very important in fully differential amplifiers. The balance of the output on the
reference voltage depends on matched ratios of the resistors. CMRR, PSRR, and cancellation of the second
harmonic distortion diminish if resistor mismatch occurs. Therefore, it is recommended to use 1% tolerance
resistors, or better, to keep the performance optimized. Matching is more important than overall tolerance.
Resistor arrays with 1% matching can be used with a tolerance greater than 1%.
Place the input resistors very close to the TPA2005D1 to limit noise injection on the high-impedance nodes.
For optimal performance, the gain should be set to 2 V/V or lower. Lower gain allows the TPA2005D1 to operate
at its best and keeps a high voltage at the input, making the inputs less susceptible to noise.
The TPA2005D1 is a high-performance class-D audio amplifier that requires adequate power-supply decoupling
to ensure the efficiency is high and total harmonic distortion (THD) is low. For higher frequency transients,
spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR) ceramic capacitor, typically
1
F, placed as close as possible to the device V
DD lead, works best. Placing this decoupling capacitor close to
the TPA2005D1 is very important for the efficiency of the class-D amplifier, because any resistance or
inductance in the trace between the device and the capacitor can cause a loss in efficiency. For filtering
lower-frequency noise signals, a 10-
F, or greater, capacitor placed near the audio power amplifier also helps,
but it is not required in most applications because of the high PSRR of this device.
The TPA2005D1 does not require input coupling capacitors if the design uses a differential source that is biased
from 0.5 V to VDD – 0.8 V (shown in Figure 26). If the input signal is not biased within the recommended common-mode input range, if needing to use the input as a high pass filter (shown in
Figure 27), or if using a
single-ended source (shown in
Figure 28), input coupling capacitors are required.
The input capacitors and input resistors form a high-pass filter with the corner frequency, fc, determined in
The value of the input capacitor is important to consider, as it directly affects the bass (low frequency)
performance of the circuit. Speakers in wireless phones usually cannot respond well to low frequencies, so the
corner frequency can be set to block low frequencies in this application.
Equation 3 is reconfigured to solve for the input coupling capacitance.
If the corner frequency is within the audio band, the capacitors should have a tolerance of ±10% or better,
because any mismatch in capacitance causes an impedance mismatch at the corner frequency and below.
For a flat low-frequency response, use large input coupling capacitors (1
F). However, in a GSM phone the
ground signal is fluctuating at 217 Hz, but the signal from the codec does not have the same 217-Hz fluctuation.
The difference between the two signals is amplified, sent to the speaker, and heard as a 217-Hz hum.
Copyright 2005–2008, Texas Instruments Incorporated
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