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>18bit ADC for measuring laser diode voltage

albert.toews
Prodigy 50 points
Other Parts Discussed in Thread: ADS1255, ADS1259, ADS1232, OPA350, OPA365, ADS1247

Hello,

I would like to measure the drop-voltage of a laser diode with an resolution of min. 18 bit. I was wondering, if I could use the ADS1255, in terms of common mode voltage and liniarity?

Greetings

Albert

  • 0
    TI__Guru**** 168605 points

    Hi Albert,

    We're on break for the Thanksgiving Holiday here in the US through Monday next week.  We'll get back to you on this question as soon as possible.

  • 0 in reply to Tom Hendrick
    TI__Guru** 112805 points

    Hi Albert,

    The ADS1255 will be outside the common mode input range with the buffer enabled, and the input impedance may cause some problems with your measurement in unbuffered mode.  The differential input impedance is about 150k ohm at a gain of 1.  The ADS1232 may be a possibility as the input impedance is higher when using a gain of 1 as compared to the ADS1255.  The ADS1259 has similar performance issues as the ADS1255, but by adjusting the modulator rate lower, you can increase the input impedance.

     Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Hey,

    thanks for the quick responce.

    I need at least 200SPS, so the ADS1232 is not a possibility for my project. But the ADS1255 may work! Is it possible to use two voltage followers to buffer the input to finally get a high input impedance? If yes, is the OPA365 or OPA350 suitable for that?

    Greetings

    Albert

  • 0 in reply to albert.toews
    TI__Guru** 112805 points

    Hi Albert,

    Yes, you could create an input buffer for higher impedance, but this will add a slight amount of noise and may add offset/gain issues.  One issue in particular is the ability to drive the output close to the rails.  You have stated that on the anode side of the diode the max voltage is 4.99V.  This would be within 10mV of the supply rail, and the output may not quite reach the desired level.

    I'm not an op amp guy, but hopefully one of them is monitoring this post and will jump in.  I believe that the OPA365 may be a good choice in terms of noise and offset at least. 

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Hey,

    thank you, for now. I think, I give it a try and see what I get!

    Greetings

    Albert

  • 0 in reply to albert.toews
    Prodigy 50 points

    Hey,

    one more question: Is it better to use the internal reference or a well buffered and filtered external reference for the best noise performance?

    Greetings

    Albert

  • 0 in reply to albert.toews
    TI__Guru** 112805 points

    Hi Albert,

    It is possible to find better references than the internal, but you can also get worse ones.  You have to compare the characteristics of noise and stability of the internal reference to the desired external reference.  One possibility of an external reference is the TI REF50xx family.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Hey,

    I try to use the ADS1259B to measure my diode voltage. But I can't communicate via SPI with the ADS1259 and XMEGA. I don't get any answer on DOUT (always 0x00).

    It's kind of frustrating.

    What I did:

    - RESET and CS are pulled up

    - using internal f_CLK (XTAL1 pulled low)

    - SPI communication looks good on the oscilloscope (SPI-Mode 1, sampling on falling edge)

    - I tried different SPI-CLK speed (10kHz up to 1MHz)

    - I changed the hardware (ADS1259B)

    - ADS1247 on the same SPI-Line works fine

    - I tried a different SPI Port-> same problem

    Does somebody have an idea, where the mistake is?

    Thanks a lot!

    Regards

    Albert