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Synchronizing DDS with ADC for FFT

I have six sensor channels that detect a gas by altering light transmissivity in detection chemistries. To suppress crosstalk the LED light sources are to be modulated at a different frequency for each channel. The amount of light passing through each detection chemistry in a channel is measured by the channel's own photodiode and transimpedance amplifer. The transimpedance amplifier's output at the LED's modulation frequency is extracted by an FFT.

For maximum rejection of crosstalk it is a requirement that the modulation frequencies of each channel be an integer multiple of 1/T where T is the length of time the FFT is done over. This time is tentitively 1 second so I need to set frequencies in multiples of 1 Hz. The modulations will be in the range of 300Hz to 1Khz.

I have decided to use a Blackfin DSP to do the FFT with an AD7606 ADC at 200kHz sampling rate and DMA. My problem is I need to synchronize 6 different frequencies with this ADC's CONVSTx's input using either 6 DDS chips or DACs. If a DAC it is preferable it have multiple outputs and for it to get its digital states from the DSP's memory by DMA so as not burden the DSP. Any suggestions on DDS or DAC chips to do this with are welcome.