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OPA3S2859: A few questions

Patrick Coffey
Prodigy 165 points

Part Number: OPA3S2859
Other Parts Discussed in Thread: ADS1285, , REF7025

Our application is a ratiometric one, where the signal of interest is Sample/Reference.  We need to connect to photodiodes where the ratio of the two varies over a large dynamic range (six orders of magnitude).  Rather than using a voltage reference IC, the Reference channel is fed directly into the ADC, thereby measuring Sample/Reference directly. 

We are designing using the ADS1285.  Output from the ADS1285 will be ±2.5V centered on +2.5V DC. Data rates are less than 1K SPS, and we are using a single 5V analog supply (AVDD) based on the TI TPS7A2050.  Digital supply is 3.3V based on the TI TPS7A2033. 

Neither cost nor power is important, but we are trying to minimize noise wherever possible. 
 

We plan on using the OPA3S2859 as for several reasons:

·       It offers low input impedance to the ADS1285

·       Running both Sample and Reference through a single die will minimize noise.

·       Its selectable gain should help in spanning the dynamic range.

I have a few questions:

·       Have we chosen the best op-amp given our project definition?

·       Is there a simple way to bias the DC output of the OPA3S2859 to 2.5 AV?

·       I’m assuming that the answer to the previous question will involve a 2.5V reference.  I'm currently producing a common mode voltage of 2.5V using a voltage divider from a very clean AVDD.  Would using a REF5025 reduce noise?  Would a REF7025 further reduce noise past that?

·       Since both Sample and Reference will be using the same voltage reference in biasing the output, do I need to worry about crosstalk?

Thanks,

Patrick  

  • 0
    Guru 140200 points

    Hi Patrick,

    every photodiode application starts with the detector capacitance and the question: "Is the planned TIA stable with my detector capacitance?" So, I would run a "phase stability analysis" with the OPA3S2859 in combination with my detector capacitance, first.

    Keep in mind, that the ADS1285 is optimized for low signal frequencies, while the OPA3S2859 is optimized for high frequencies. Using a fast OPAmp in the TIA may mean a huge broadband noise at the input of slow ADC. So, either the TIA needs to have a big feedback capacitance for low pass filtering purpose (which may destabilize the OPAmp by eroding the phase margin) or you may need to insert a low pass filter between the OPAmp and ADC to prevent an increase of noise due to aliasing effects.

    Kai

  • 0
    TI__Mastermind 32615 points

    Hello Patrick,

      I agree with Kai; the OPA3S2859 might be too much of an overkill in bandwidth in respect with the ADC you picked. However, we do not know the photodiode capacitance and your gain requirements. That could play a role as Kai also mentioned: 

    1. What is the photodiode capacitance (total input capacitance to the amplifier)?
    2. Input current range (Above you mention 6 decades of magnitude between two diodes, but minimum and maximum value expected for each channel would be helpful as well) 

      The input common-mode of the amplifier can be set as a simple voltage reference at the non-inverting input of the device. If the power supply is clean, I do not think it is necessary to add a reference. Unless you expect ambient temperature to drastically change or have power concerns, and your design requires a strict tolerance. 

    Thank you,
    Sima