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OPA2333-HT: driving a sigma delta DC ADS1278-HT

John Smith
Intellectual 410 points
Part Number: OPA2333-HT
Other Parts Discussed in Thread: ADS1278-HT, , OPA211, OPA2333, THS4521, ADS1278, OPA2333P

Hi There

i'm designing a high temperature PCB using two ADS1278-HT daisy chained together (16 channels) in frame sync mode, TDM mode, 1MHz clock. Low frequency input signal 0-8Hz, 0-5V centred around 2.5V. System level accuracy is 16 bits

due to PCB space issues i'd like to drive the ADS1278-HT directly from the OPA2333-HT.

My understanding is that delta sigma ADCs over sample at a rate related to the ADC input clock and thus the internal switched capacitor network can be switched quite quickly even for a low bandwidth application

As mentioned above i'd like to use the OPA2333-HT because of its great DC performance, wide input common mode, wide output swing. The concern is its very low bandwidth 350kHz.

Even with this limitation i think its suitable for my application - ADC clock is 1MHz, ADC set up is low speed, with a divide by 40 clkdiv. Thus i think the the actual switch capacitor of the ADC is thus 25kHz (1MHz/40) ??

so even with the 350kHz bandwidth of the OPA2333-Ht it should be fine to drive the ADS1278-HT - if somebody could confirm?

Note: i wasn't really sure if this is an ADC or OP amp question so a similar question is also posted on the ADC forum

https://e2e.ti.com/support/data-converters/f/73/t/869955

Note: I've looked at the HEAT evaluation schematics and op amps such as the OPA211 or THS4521 seem to be used to drive the ADC - the reason i'm trying to avoid these is mainly due to input common mode problems. I could use them but i's also need opa2333 for input common mode and level shifting reasons and PCB real estate is a big problem thus i'd like to use only the dual packed OPA2333-Ht if possible

kind regards

Martin

  • 0
    TI__Guru 51326 points

    Hi Martin,

    We could look into this detail for you.  You are correct, the low bandwidth of the OPA2333-HT could potentially be a limitation; however, as you have mentioned, since you are setting the ADS1278 with fclk=1-MHz, in the Low-Speed mode with a lower modulator frequency of  fmod=1-MHz/40, this may be possible. I will need to verify this on simulation.

    Per your post on the ADC forum, it appears you require operation at +165-C, hence you require the High Temp grade devices.  

    Is the OPA2333 set-up in buffer, or gain configuration, or inverting configuration?  What voltage supplies, the input voltage common-mode requirement, input signal amplitude, and what reference voltage is used for the ADS1278? Is the input signal a DC or AC signal? if AC, what is the frequency range?  If you have a preliminary schematic showing the OPA2333-HT circuit, this would be helpful and we could look into this, verify that the circuit works, and optimize the RC filter between the OPA2333-HT and ADS1278-HT.

    Thank you and Kind Regards,

    Luis 

  • 0 in reply to Luis Chioye
    Intellectual 410 points

    Hi Luis

    yes i can only consider -HT parts (i would use a combination of opa2333 and THS4521 but PCB space is a problem and i like the opa2333 DC specs)

    The plan is to use the op amp in a inverting (but could be non inverting) with a gain of of around 1.5, buffered reference at the + terminal of 2.5V, 0-5V supply rails, input signal is 0.5V to 4.5V centred around 2.5V

    something like the Tina file below

    kind regards

    Martin

    mag test circuit opa2333.TSC

  • 0 in reply to John Smith
    TI__Guru 51326 points

    HI Martin,

    I will look into in detail this evening / tomorrow.  I expect I will get back to you within the next couple of days.

    Many Thanks,

    Kind Regards,

    Luis

  • 0 in reply to John Smith
    Guru 48195 points

    Hey Martin, 

    Thanks for the TINA file, those always answer a lot of questions. Your 10Hz stimulus is well within the OPA2333 capability. I was thinking of reminding you the THS4521 FDA can do all the single to diff with output CM control all in one stage, but, it will be far worse on noise at that lower frequency vs. the Chopper OPA2333. The FDA has both 1/f and popcorn issues that the chopper would not. Best to stick with the the OPA2333-HT I would think. 

  • 0 in reply to Michael Steffes
    TI__Guru 51326 points

    Hi Martin,

    As we discussed on the other thread, using the ADS1278 in Low-Speed mode, with fclk=1-MHz, the output data rate is Data Rate (SPS)=1MHz/2560=~390-SPS. The normalized frequency response of the digital filter scales with the modulator frequency.  Since the device is used with fclock=1-MHz, in low-speed mode, with clk-div-1, the output data rate is 390-Hz, and the BW of  the digital filter is approximately ~(0.453)*(390Hz) or 176-Hz.

    The modulator samples at 25-kHz and the OPA2333P is optimal to drive the ADS1278 and settle within a few 10's of micro-volts.  A possible circuit is below using the OPA2333P in dual-feedback configuration.  Using a COG grade capacitor in the range of either of 18-nF (or 15-nF) across the inputs of the ADS1278 provides a relatively small settling error in the order of a few 10s of microvolts. 

    The noise contribution of the OPA2333P circuit at the 176-Hz (ADS1278 digital filter BW) is approximately 1.6uV-RMS.  The ADS1278 offers a typ SNR of 107-dB, or an equivalent ADC input referred noise of approximately 7.895uV-RMS when using a 2.5-V reference.  Therefore, the combined ADS1278+OPA2333P noise performance will be around 8uV-RMS typ (Note: Noise of reference circuit or extrinsic noise is not included on the analysis). 

    I checked the inverting and non-inverting stage for stability, providing ~80 degrees of Phase Margin (>> 45-degrees).  I recommend using COG grade capacitors across the inputs of the ADS1278, and at the feedback of the OPA2333P circuit as shown below.

    Attached is a zip file including a transient simulation of the OPA2333P stage driving an approximate model of the ADS1278 at low-speed mode (Master CLK=1-MHz, based on datasheet parameters), the OPA2333P stage noise simulation and stability analysis.  Please let me know if you have any questions or if you need me to adjust the circuit. 

    Thank you,

    Kind Regards,

    Luis Chioye

     ADS1278_OPA2333P_low_speed_mode.zip

  • 0 in reply to Luis Chioye
    Intellectual 410 points

    Hi Luis

    many thanks for your fantastic answer, exactly what i was after, funny enough i was trying something similar but with a 8881 SAR as i couldn't find any TINA delta sigma ADC models

    But what you've supplied here is great so i'm going to go with the OPA2333-HT parts and use your ADC model for some other sensor inputs i need to look at

    Kind Regards

    Martin

  • 0 in reply to John Smith
    TI__Guru 51326 points

    HI Martin,

    Thank you.  Please let us know if you have any questions.

    Kind Regards,

    Luis