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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.

  • Hello,

    I can offer youy a few options but suitability depends on the LED requirements.

    • You could use a DDS (like the AD9838 - single channel only) - and synchnoise multiple devices using the reset pin as shown in the AN-587 apps note. Different part but same Reset function. From a synchroninsing to the system this might make sense.
    • As the frequency requirements are quite slow you might be able to look at maybe 3 dual nanoDACs.AD5623 or AD5623R(reference option). Update rate of 280KSPS. Has LDAC pin. Buffer will be required.Synchonise option is /LDAC with /CONVST pin of ADC.
    • For higher speed (2.4MSPS) a dual AD5429 could be used - this is current output and will require a negative reference + I/V buffer.Synchonise option is /LDAC with /CONVST pin of ADC.

    Question comes down do what kind of signal you need to generate / how good a signal spectrally / how best to synchonise.

    Really need to be evaluated as a system on a bench to optimise selection.

    -Liam

  • The waveform is a sine wave.

    The peak current in the LED is 40mA.

    The light is to go through an optrode and converted to voltage by a photodetector and TIA. The voltage is converted by an ADC and an FFT is done on it. There will be six LEDs, each illuminates its own optrode. There will be some crostalk between channels that needs to be suppressed by having different frequencies on each channel. I am considering. I would like the ADC to have the same external buried zener voltage reference as the DDS so as to be one of the ways to suppress drift in the result. The ADC will be two AD7606s. For DDS  I am considering two four channel  AD9959s, even though these do not appear to have external references.

    I am not sure if 10 bits for a DDS is enough. I have started this thread about this question:

    http://groups.google.com/group/sci.electronics.design/browse_thread/thread/fe0cfc42098214bf?hl=en#

    From: LiamR analog@sgaur.hosted.jivesoftware.com

    Sent: Monday, January 23, 2012 2:01 AM

    To: Pflanze, Stephen

    Subject: New message: "Synchronizing DDS with ADC for FFT"

    Analog Devices EngineerZone<http://ez.analog.com/index.jspa>

    Synchronizing DDS with ADC for FFT

    reply from LiamR<http://ez.analog.com/people/LiamR> in Precision DACs - View the full discussion<http://ez.analog.com/message/40916#40916

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