LAUNCHXL-F28379D: Questions about ADC's on Launchpad

Joel Holland said:

First of all, in the ADCD, there is an additional differential operational amplifier on board. What is the purpose of this? If you can set up the ADC to perform differential conversions, why include an additional differential amplifier prior to the MCU? 

The intent was to provide some assistance for conditioning external signals for differential sampling. On-board buffering is often better with respect to managing noise, which is a greater concern for 16-bit sampling vs 12-bit sampling.

Joel Holland said:

Second of all, looking through the documentation for this board, there doesn't seem to be any unity gain buffers/filter components before the input signals are passed to the ADC sensor inputs, even though TI have a document detailing how you need these to provide an ideal driving source to the ADC's. Why is this not included, and should it be designed externally to provide best ADC performance?

A persistent goal for the LaunchPad platform is to keep user costs low. When it comes to ADC buffering of the BoosterPack signals, the tradeoff decision was made to minimize cost by leaving out buffering circuitry while accepting some raw performance degradation that users can mitigate through external buffering.

For general evaluation and learning purposes, the non-buffered ADC path is sufficient. For cases where maximum precision is required, an external buffer is recommended.

Joel Holland said:

In a previous question I asked about providing a precision voltage reference external for the ADC's, and was told this is possible using the analog 14/15 input pins. That is fine. However, the differential amplifier in the ADCD has its own voltage reference. Does this mean you can't use an external precision reference, and does this only apply to the differential input for ADCD

I am not aware of a simple way to supply an external ADC reference voltage on the LaunchPad. The reference is generated on-board with no test point access.

Joel,

I think he misinterpreted your question.

His reply was: "...ADCIN14 and ADCIN15 can be used to calibrate all of the ADCs to an external reference"

This is not the same as supplying an external reference to the ADC VREFHI/VREFLO pins.

-Tommy

Joel,

The ADC voltage reference will determine the full-scale range of the converter, which directly influences its precision.

The shared 14/15 input pins allow the user to sample a single input voltage across all ADCs in order to infer the ADC-to-ADC error so that the results from different ADCs can be normalized (calibrated) using software post-processing.

-Tommy