I am constructing a hearing aid for my bachelor degree project. This hearing aid will consist of a microphone with a Nordic Semiconductor nRF2460 2.4 GHz transmitter, and a receiver of samë model with a speaker. This device should filter out any unwanted frequencies outside of speech, that is anything outside of 1-5 kHz. We want to sample at 12 kHz. I and my group are designing the PCB's and power supply ourselves. For simplicity we want mostly digital signals on our PCB's. Therefore we want to use the ADMP441 which has an ADC included with an I2S interface. Our transmitter and receiver (which are the same IC), support 16 bit I2S. The SSM2529 will be our DAC and amplifier stage, and also the filter stage.
The question is, can I use the ADMP441, which supports 24 bit, to send a 16 bit signal to my 16 bit compatible transmitter?
I think that there is some confusion about the clocks that we're talking about. It makes sense that the nRF2460 operates with a 256 x fs master clock, but that's probably not the rate of the CLK pin (the I2S port bit clock). Even if the nRF2460 must run at a 32 kHz sampling rate, as long as the CLK output is 64 x 32 kHz (2.048 MHz), then then ADMP441 will work directly with that I2S input port.
I would recommend that you look at the AD1938; this is the full codec version of the AD1934. This codec, as well as the AD1934 DAC, work well in the 32 kHz rate environment, using the PLL at 256 x Fs clock rate.
Thank you both for replying.
ColemanR: I've looked at the AD1938. When you say "full codec version", does this mean that I can use the AD1938 in two instances as an ADC on the transmitting side, and as DAC on the receiving end?
The AD1938 has 4 ADC channels and 8 DAC channels; the ADCs and DACs can be clocked from the same source (MCLK PLL, LRCLK PLL or PLL Bypass), or separate sources if that suits your needs. It is a very flexible part, allowing you to use the ports as though you have separate ADCs and DACs. I have reviewed designs where the ADCs are clocked from ALRCLK through the PLL and the DACs are clocked in PLL Bypass directly from a 512 x Fs MCLK. Please let me know if you have specific questions about your application.
Okay, I ordered 2 of the AD1938. I'm thinking of using a TDA2822M as a power amp for my 4 ohm speaker. I figured since the AD1938 requires filtering before audio output, I would filter out anything outside of speech frequencies after the DAC stage. At page 30 in the AD1938 datasheet you recommend what appears to be a Sallen-Key lowpass filter. I am however unsure of what the components to the right of node 1, at the output of the opamp, are for. Could you explain this?
Also, is it necessary to use an active filter (maybe it helps to reduce noise, create buffer for impedances)?