TPA3003D2 datasheet(30/34 Pages) TI

30 / 34 page

TPA3003D2

SLOS406A − FEBRUARY 2003 − REVISED MARCH 2003

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APPLICATION INFORMATION

The transfer function for this circuit is shown in equation (10) where

ωO= REQCEQ, REQ = RFILTRANA and CEQ

= (CFILT + CANA). The filter frequency should be set above fMAX, the highest frequency of the measurement

bandwidth, to avoid attenuating the audio signal. Equation (11) provides this cutoff frequency, fC. The value of

RFILT must be chosen large enough to minimize current that is shunted from the load, yet small enough to

minimize the attenuation of the analyzer-input voltage through the voltage divider formed by RFILT and RANA.

A rule of thumb is that RFILT should be small (~100 Ω) for most measurements. This reduces the measurement

error to less than 1% for RANA ≥ 10 kΩ.

OUT

ANA

ANA)

FILT

) j w

C +

MAX

An exception occurs with the efficiency measurements, where RFILT must be increased by a factor of ten to

reduce the current shunted through the filter. CFILT must be decreased by a factor of ten to maintain the same

cutoff frequency. See Table 3 for the recommended filter component values.

Once fC is determined and RFILT is selected, the filter capacitance is calculated using equation (12). When the

calculated value is not available, it is better to choose a smaller capacitance value to keep fC above the minimum

desired value calculated in equation (11).

FILT +

FILT

Table 3 shows recommended values of RFILT and CFILT based on common component values. The value of fC

was originally calculated to be 28 kHz for an fMAX of 20 kHz. CFILT, however, was calculated to be 57000 pF,

but the nearest values of 56000 pF and 51000 pF were not available. A 47000 pF capacitor was used instead,

and fC is 34 kHz, which is above the desired value of 28 kHz.

Table 3. Typical RC Measurement Filter Values

MEASUREMENT

RFILT

CFILT

Efficiency

1 000

Ω

5 600 pF

All other measurements

100

Ω

56 000 pF

(10)

(11)

(12)

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