Page 36

EDNE MARCH 2013

PDM SOURCE 1 MODULATOR CLOCK DATA OUTPUT PDM SOURCE 2 MODULATOR CLOCK DATA OUTPUT designer must take care to ensure that the outputs of one device will will output data in the format that the inputs of another are expecting. PDM data connections PDM data connections are becoming more common in portable audio applications, such as cell phones and tablet computers. PDM is an advantage in size-constrained applications because it allows audio signals to be routed around noisy circuitry, such as LCD screens, without having to deal with the interference issues that analog audio signals may have. With PDM, up to two audio channels can be transmitted with only two signal lines. Figure 4 shows a system diagram with two PDM sources driving a common data line into a receiver. The system master generates a clock that can be used by two slave devices, which use alternate edges of the clock to output their data on a common signal line. The data is modulated at a 64× rate, resulting in a clock that is typically between 1 and 3.2 MHz. The bandwidth of the audio signal increases as the clock rate increases, so lower-frequency clocks are used in systems that can trade off a reduced bandwidth for lower power consumption. A PDM-based architecture differs from I2S and TDM in that the decimation filter is in the receiving IC rather than the transmitting IC. The output of the source is the raw highsample rate modulated data, such as the output of a sigmadelta modulator, rather than decimated data, as it is in I2S. A PDM-based architecture reduces the complexity in the source device and often makes use of decimation filters that are already present in a codec’s ADCs. With this approach, system designers not only can use audio codecs that they may already be using but also can take advantage of a digital data connection’s reduced sensitivity to interference. Further, decimation filters may be more PDM RECEIVER (PROCESSOR, CODEC, AMPLIFIER) CLOCK DATA INPUT EDNMS4447 Fig 4.eps DIANE efficiently implemented in the finer silicon geometries used for fabricating a codec or processor, rather than in the processes used for the microphone ICs. Codecs, DSPs, and amplifiers have had I2S ports for years, but until now a system’s input devices, such as microphones, have had either analog or PDM outputs. As digital interfaces are pushed further toward the ends of the DECIMATION FILTER AND CLOCK GENERATOR Figure 4 In this system diagram, two PDM sources drive a common data line into a receiver. signal chain, new ICs will be needed to support the new system architectures. Microphones, such as the Analog Devices ADMP441 MEMS microphone, that have an integrated I2S interface make it easier for designers to build this component into systems in which PDM microphones are not easily used or analog interfaces are not desired. Only a subset of audio codecs accepts a PDM input, and very few audio processors outside of those specifically designed for mobile phones and tablets natively accept this type of data stream. In some designs, an I2S output microphone could completely eliminate the need for any analog front-end circuits because many designs may have only an ADC and PGA to support a microphone input to the processor. An example of such a system is a wireless microphone with a digital transmitter. The wireless transmitter SOC may not have a built-in ADC, so using an I2S output microphone enables completely digital connections between the transducer and transmitter. I2S, TDM, and PDM audio interfaces each have their advantages and applications for which they are best suited. As more audio ICs transition from analog to digital interfaces, system designers and architects will need to understand which interface would be most appropriate for a particular design. With a digital signal chain from microphone to DSP to amplifier, analog signals can be pushed completely off of the PCB and exist only in the acoustic domain.EDN Acknowledgment This article originally appeared on EDN’s sister site, Planet Analog, at www.eetimes.com/4374249. Author’S biographY Jerad Lewis is an applications engineer for MEMS microphones at Analog Devices. He joined the company in 2001 after having earned his bachelor’s of science degree in electrical engineering from Pennsylvania State University (University Park, PA). Since then, Lewis has supported different audio ICs, such as converters, SigmaDSPs, and MEMS microphones. 36 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
To see the actual publication please follow the link above