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INA851: Purpose of FDA block

Marvn Arcangel
Genius 10745 points
Part Number: INA851

Hi Team,

Can you please help with the inquiry below?

Looking at the INA851 block diagram, can you please explain what is the purpose of the FDA output stage?

I understand that a difference amp output is needed to achieve CMRR in a single-ended INA but for a differential output, what is gained by running the output of the gain/buffer stage through an FDA for unity gain? For attenuation (1.25k resistors switched in) I understand the need for the FDA. It seems to me that for unity gain, the FDA only adds noise to the output. Is this correct?

Thank you.

Regards,

Marvin

  • 0
    TI__Guru 54576 points

    HI Marvin,

    The INA851 is essentially an instrumentation amplifier with a fully-differential output; and provides high impedance inputs to measure differential signals at a wide common-mode voltage range offering high CMRR while driving fully-differential input ADCs.  

    The output FDA stage allows to drive fully-differential ADCs while keeping the output common-mode voltage fixed. Many fully-differential ADCs require a fixed common-mode voltage or require a common-mode to be centered to the middle of the supplies. The FDA output stage provides the VOCM pin, that sets the output common-mode voltage regardless of the input common-mode voltage. Also, when driving un-buffered SAR or Delta-Sigma ADCs, the FDA stage provides good drive capability ensuring the settling of the ADCs sample-and-hold providing good SNR and THD performance (up to 1-MSPS on high-resolution, un-buffered SARs).  In many cases, the ADC operates at a lower supply voltage than the instrumentation amplifier supply. The output stage offers the integrated clamping function, that limits the device output voltage preventing damage to the ADC when overdriven. 

    As you have mentioned, instrumentation amplifiers - also often true for single-ended output instrumentation amplifiers - offer the best noise performance when used in a gain configuration, where the output stage noise contribution is less significant. The INA851 is similar to traditional instrumentation amplifiers in this regard. In many cases the broadband noise can be filtered by the Delta-Sigma ADC's Digital Filter allowing for high-resolution results.  On SAR ADCs, the overall broadband noise can be limited by applying capacitors on the FDA feedback with a trade-off on the circuit bandwidth - access to the inverting and noninverting inputs of the FDA output stage is provided through the FDA_IN– and FDA_IN+ pins. The datasheet shows examples of performance while driving high-resolution, precision ADCs, where the INA851 provides a compact front-end solution with very good accuracy and low-noise.

    Kind Regards,

    Luis