LM4675, LM4675SDBD, LM4675TLBD

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SNAS353C – AUGUST 2006 – REVISED MAY 2013

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

component on the outputs. The grounded capacitive filter elements 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 differential, necessitating a differential input on any analysis

instrument 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 high frequency

signals is questionable necessitating the on board measurement filter. When in doubt or 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 -0.25dB (3%). 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 outputs. 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.

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