LM4666 datasheet(13/15 Pages) NSC

13 / 15 page

Application Information (Continued)

REFERENCE DESIGN BOARD SCHEMATIC

In addition to the minimal parts required for the application

circuit, a measurement filter is provided on the evaluation

circuit board so that conventional audio measurements can

be conveniently made without additional equipment. This is a

balanced input, grounded differential output low pass filter

with a 3dB frequency of approximately 35kHz and an on

board termination resistor of 300

Ω (see schematic). Note

that the capacitive load elements are returned to ground.

This is not optimal for common mode rejection purposes, but

due to the independent pulse format at each output there is

a significant amount of high frequency common mode com-

ponent on the outputs. The grounded capacitive filter ele-

ments attenuate this component at the board to reduce the

high frequency CMRR requirement placed on the analysis

instruments.

Even with the grounded filter the audio signal is still differ-

ential necessitating a differential input on any analysis instru-

ment connected to it. Most lab instruments that feature BNC

connectors on their inputs are NOT differential responding

because the ring of the BNC is usually grounded.

The commonly used Audio Precision analyzer is differential

but its ability to accurately reject fast pulses of 160ns width is

questionable necessitating the on board measurement filter.

When the signal needs to be single-ended, use an audio

signal transformer to convert the differential output to a

single ended output. Depending on the audio transformer’s

characteristics, there may be some attenuation of the audio

signal which needs to be taken into account for correct

measurement of performance.

Measurements made at the output of the measurement filter

suffer attenuation relative to the primary, unfiltered outputs

even at audio frequencies. This is due to the resistance of

the inductors interacting with the termination resistor (300

Ω)

and is typically about -0.35dB (4%). In other words, the

voltage levels and corresponding power levels indicated

through the measurement filter are slightly lower than those

that actually occur at the load placed on the unfiltered out-

puts. This small loss in the filter for measurement gives a

lower output power reading than what is really occurring on

the unfiltered outputs and its load.

The AUX-0025 Switching Amplifier Measurement Filter from

Audio Precision may be used instead of the on board mea-

surement filter. The AUX-0025 filter should be connected to

the high current direct outputs on the evaluation board and in

series with the measurement equipment. Attaching oscillo-

scope probes on the outputs of the AUX-0025 filter will

display the audio waveforms. The AUX-0025 filter may also

be connected to the on board filter without any adverse

effects.

20055801

FIGURE 4.

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