Silicon Laboratories Si4700 User Manual page 4

AN232
3.2.2. Audio Amplifier
The EVB includes a high-output drive dual op-amp chip (U2) to buffer the audio outputs at the LOUT and ROUT
pins of the Si4700/01. U2 is located on the bottom layer of the daughter card and is not visible when the base
board and daughter card are mated. The LOUT and ROUT pins are ac-coupled to the inputs of the op-amps on U2.
The op-amps are connected in a unity-gain, noninverting configuration and biased at the middle of the audio power
supply. The outputs of the op-amps are in turn ac-coupled to the BNC connectors J10 and J11 on the base board.
The audio amplification network has been designed to drive resistances of 10 kΩ which is easily achievable on
most audio analyzers. The op-amps have enough drive capability to drive resistances much lower than 10 kΩ (e.g.
32 Ω headphones). In such cases, however, the lower end of the audio spectrum (up to 2.5 kHz) may get
attenuated. This is because the 3-dB points of the high-pass filters at the outputs of the op-amps move to higher
frequencies as the output resistance is decreased. Also note that the op-amps are not sufficiently protected against
short-circuit conditions. Hence, the audio outputs at J10 and J11 should not be connected to a mono input if the
Si4700/01 is configured to produce a stereo output.
4 Rev. 0.2