Texas Instruments TPA3003D2 User Manual

Page 30

TPA3003D2

SLOS406A − FEBRUARY 2003 − REVISED MARCH 2003

www.ti.com

APPLICATION INFORMATION

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

= R

, R

= R

FILT



ANA

and C

= (C

FILT

+ C

ANA

). The filter frequency should be set above f

MAX

, the highest frequency of the measurement

bandwidth, to avoid attenuating the audio signal. Equation (11) provides this cutoff frequency, f

. The value of

FILT

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 R

FILT

and R

ANA

A rule of thumb is that R

FILT

should be small (~100

Ω

) for most measurements. This reduces the measurement

error to less than 1% for R

ANA

≥

10 k

Ω

OUT

ANA

)

FILT

)

MAX

An exception occurs with the efficiency measurements, where R

FILT

must be increased by a factor of ten to

reduce the current shunted through the filter. C

FILT

must be decreased by a factor of ten to maintain the same

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

Once f

is determined and R

FILT

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 f

above the minimum

desired value calculated in equation (11).

FILT

Table 3 shows recommended values of R

FILT

and C

FILT

based on common component values. The value of f

was originally calculated to be 28 kHz for an f

MAX

of 20 kHz. C

FILT

, 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 f

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

Table 3. Typical RC Measurement Filter Values

MEASUREMENT

FILT

Efficiency

1 000

Ω

5 600 pF

All other measurements

100

Ω

56 000 pF

(10)

(11)

(12)