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BTM521 DATASHEET

15

5.2.8

Audio Input Stage

The audio input stage of the module consists of a low noise input amplifier, which receives

its analogue input signal from pins MIC_AP and MIC_AN to a second–order ∑-∆ ADC

that outputs a 4Mbit/sec single-bit stream into the digital circuitry. The input can be

configured to be either single ended or fully differential. It can be programmed for either

microphone or line input and has a 3-bit digital gain setting of the input-amplifier in 3dB

steps to optimize it for the use of different microphones.

5.2.9

Microphone Input

5.2.10

Audio Output Stage

The output digital circuitry converts the signal from 16-bit per sample, linear PCM of

variable sampling frequency to a 2Mbits/sec multi-bit stream, which is fed into the analogue

output circuitry.

The output circuit comprises a digital to analogue converter with gain setting and output

amplifier. Its class-AB output-stage is capable of driving a signal on both channels of up to

2V

pk-pk- differential into a load of 16 . The output is available as a differential signal

between SPK_AP and SPK_AN for the left channel; and between SPK_BP and SPK_BN

for the right channel. The output is capable of driving a speaker directly if its impedance is

at least 8

if only one channel is connected or an external regulator is used.

The gain of the output stage is controlled by a 3-bit programmable resistive divider, which

sets the gain in steps of approximately 3dB.

The multi-bit stream from the digital circuitry is low pass filtered by a second order

bi-quad filter with a pole at 20kHz. The signal is then amplified in the fully differential

output stage, which has a gain bandwidth of typically 1MHz.

5.3

RF Interface

The module integrates a balun filter. The user can connect a 50ohms antenna directly to the

RF port.

5.4

General Purpose Analog IO

The general purpose analog IOs can be configured as ADC inputs by software. Do not

connect them if not use.

5.5

General Purpose Digital IO

There are nine general purpose digital IOs defined in the module. All these GPIOs can be

configured by software to realize various functions, such as button controls, LED displays

or interrupt signals to host controller, etc. Do not connect them if not use.