OPA333: ECG applications for MSP432

Hi Yoon,

The MSP432P411x incorporates a SAR analog to digital converter. In general, SAR ADCs are not low impedance inputs and need to be driven with an amplifier that has enough bandwidth (or low output impedance over frequency) to be able to completely charge the MSP432's internal ADC sample-and-hold capacitor. The ADC’s sample-and-hold capacitor voltage has to settle within the Least Significant Bit (LSB) resolution of the ADC during the acquisition period, in order to obtain accurate results. The SAR ADCs acquisition time is inversely proportional to the sampling rate.

The ECG design above has the OPA333 buffer stage (U2A, U2B), in a dual-feedback configuration, optimized to drive the ADS8881 18-B SAR ADC at a sampling rate of 10-kSPS.  As described on section 2.6 of the design document, all passive components are carefully selected for noise, stability and transient settling while driving the ADS8881 at 10-kSPS; and the circuit was optimized using simulation.

If you use the OPA333 on the design, depending on how fast is your sampling rate (or depending on the duration of the available acquisition period), you will most likely need a separate ADC input driver buffer stage to drive the ADC as shown on the example above.  The RC kickback filter and all passive feedback components will need to be tuned for stability and transient settling accounting for the specific ADC acquisition time, resolution, and sample-and-hold structure.  

Regarding your question, it is possible to replace the OPA333 devices with OPA388 devices, but the design needs to be carefully optimized where the ADC RC kickback filter and all passive components need to be tuned using simulation.

As mention on section 7, it is possible to use a monolithic INA for the front end such as the INA333 (or INA828). The compromise to using this INA is that it converts differential input signals to single-ended, meaning that the output could be more easily subject to external noise coupling which could be more easily rejected in a differential configuration. Also, the INA requires a reference buffer to offset the single-ended output to mid scale. In addition, your MSP432 ADC will need to be configured as single-ended input when using an INA.  If you use a relative slow sampling rate (i.e. 10-kSPS), the INA may be able to drive the ADC directly without a buffer.

Please refer to the slau516 design for simulation guidelines. Also, for information to create a SAR ADC model and tuning the RC filter you can refer to TI's Precision Labs series below on SAR ADC topics. Section 5 of the tutorial covers the selection and optimization of the R and C values used in the RC kickback, as well as to how to model the sample-and-hold of the ADC using TINA SPICE simulation. You can find the TI Precision Lab for ADCs in the link below:

https://training.ti.com/ti-precision-labs-adcs

Please let me know if you have questions,

Best Regards,

Luis