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LOG114: What ADC is suggested to digitize the signal from the LOG114

PATRICK OHARA
Genius 3205 points
Part Number: LOG114
Other Parts Discussed in Thread: TPS60241, ADS8321

Hello,

I'm struggling to wrap my head around which ADC to use for the digitization of the photodiode signal that comes from the LOG114. I presume that I'll need a very high resolution ADC to get resolution across the entire 8 decades of amplifier range, but I'm not sure which architecture and resolution to choose. The implementation is used to measure scattered light from smooth surfaces and spectral reflectance of relatively high intensity - thus the need for 8 decades of range. Please provide a suggestion and a EVM that I could attach to the LOG114 EVM would be great.

Thank you,

Patrick

  • 0
    TI__Expert 7540 points
    Hi Patrick,

    Due to the logarithmic response of the amplifier, you may not need as much resolution. 8 decades span about 3V of output from the LOG114, so using a 3.3V ADC, you'd end up with ~7445 LSBs per decade of input current. A 24 bit ADC would up this to ~1.9 million LSBs per decade. Asking around, to get the desired performance out of a higher resolution ADC may require an additional amplifier between the LOG114 and the ADC in order to reduce settling time to an acceptable level.

    As for ADC architecture, that's going to be somewhat dependent on required sampling rate and resolution. In the 16-20 bit range, both SAR and Delta-Sigma ADCs are available. Above 20 bits, it's exclusively delta-sigma.
  • 0 in reply to Alex Davis
    Genius 3205 points
    Hi Alex,

    Thanks very much for the thoughtful reply.

    The application involves modulating a laser at 1kHz (50% duty cycle) and detecting the scattered light in up to 5 scatter detectors simultaneously. Some detectors (very far off axis) will see modest intensity while others (near the specularly reflected beam) will have pretty high intensity. I intend to use a dual +/- 5VDC supply (TPS60241). I'm in the process of calculating the expected current generated on each detector now. In light of this modest data rate and ~7445 LSBs maybe a traditional 16 bit SAR ADC might be a good choice.

    I'd appreciate your suggestion as to a suitable ADC among the many that TI offers. Also any reference documents showing the LOG Amp connected to an ADC would be helpful.

    Thanks again,

    Patrick
  • 0 in reply to Alex Davis
    Genius 3205 points
    Hi Alex,

    I'm thinking of using the ADS8321 to digitize the output of the LOG114. I have two questions. Can I use the 2.5V reference on the LOG114 to feed the ADS8321? Does the ADS8321 have a daisy chain feature for multiple sensor acquisition? Any comments on pairing the LOG114 and ADS8321 would be appreciated.

    Thanks,

    Patrick
  • 0 in reply to PATRICK OHARA
    TI__Expert 7540 points
    Hi Patrick,
    It doesn't look like the ADS8321 can be daisy-chained, since it only has a data out pin. There shouldn't be any issue with using the LOG114's onboard reference as an ADC reference, though you may want to use one of the uncommitted amplifiers in the LOG114 as a buffer to be safe.

    You'll probably want to use one of the other uncommitted amplifiers as the actual output driver. This will allow you to add compensation (SAR inputs can be tricky loads to drive), and scale and offset the LOG114's bipolar output signal to a range your ADC can handle.