INA851: Anti-aliasing for ADS127L11

Hi Joe,

The INA851 is essentially an instrumentation amplifier with differential outputs, providing high-impedance inputs. The circuit designer is able to select any passive RC filters at the input per the aliasing requirements as you will normally able to do when using a conventional instrumentation amplifier.  As you have mentioned, the INA851 allows to place a first order pole across the output stage feedback resistor as suggested on the datasheet.  This filter capacitor can be useful and make a noticeable difference on certain high-resolution SAR ADC applications to reduce bandwidth on the amplifier, and/or reduce broadband noise contributed by the amplifier on SAR ADCs that do not incorporate a digital filter, providing a trade-off between bandwidth and noise on SARs.  It can also help as an additional pole for anti-aliasing requirements.

There are considerations on how accurately you can tune the filter across the feedback.  The internal feedback resistors are approximately ~5kΩ nominal when the gain of the output stage is set to a unity gain of 1 V/V (when floating the G02+ and G02– pins), or the feedback resistors have a nominal value of approximately ~1kΩ when setting the output stage attenuating gain of 0.2 V/V (shorting those pins to the respective OUT+ and OUT– pins).  These internal feedback resistors are ratiometrically matched among themselves to provide accurate gain and low  gain drift error, but, the absolute resistance value may vary ~±20% with device, process variation and temperature. Hence, the designer needs to set the corner frequency of this feedback filter accounting for the internal resistor variation, keeping the pole outside the bandwidth of interest.

The final RC filter at the INA851 output or ADC input works primarily as a charge reservoir to filter the sampled input of the ADC. The charge reservoir reduces the instantaneous charge demand of the amplifier, maintaining low distortion and low gain error that otherwise can degrade because of incomplete amplifier settling. This last filter provides another pole, and also helps with aliasing, although its corner frequency is primarily optimized for ADC sample-and-hold drive purposes.

As shown on the ADS127L11 datasheet, when using a conventional discrete fully-differential amplifier (FDA), the user is able to select the discrete resistor values to configure the FDA as an active 2^nd or 3^rd order multi-feedback filter, and the FDA input and feedback resistor values are selected to provide the exact frequency poles required to achieve the high-order response characteristic.  The input resistors and feedback passive components of the FDA circuit on the ADS127L11 datasheet were carefully selected/tuned to provide a specific attenuation requirement.

In summary, the INA851 does allow you flexibility to select any passive RC filter at the input, and also add a pole on the output stage that can certainly help with antialiasing requirements.  However, this is a compact integrated instrumentation amplifier solution, incorporating internal resistors, an input stage and a output stage, similar to other instrumentation amplifiers and programmable gain amplifiers, and will not offer the same level of filtering tunability as a discrete FDA, where you can select resistor values and place the capacitors across the inputs while selecting the different passive components.  Depending on the frequency of stop band and attenuation requirement in your application, and how strict/abrupt is the antialiasing requirement or change from passband to stopband, you may or may not be able to use the INA851.   

What is your frequency bandwidth of interest? And what is the anti-aliasing attenuation level and frequency requirement?  In your application, is there a particular noise signal frequency of concern you are required to attenuate in this application?  I will look at  different possibilities, as well as to the suggestion of adding the capacitor across the output stage FDA+/- inputs.

Thank you and Regards.

Luis