Other Parts Discussed in Thread: OPA189, OPA2189, OPA350, REF70, REF6025, REF7025
I am designing a calibration board for use with the ADS1256 and a 12-bit MCU SAR and I would appreciate some advice:
If I understand correctly, the ADS1256 SELFCAL can be used for any PGA setting, but is it based on the same 2.5V reference for all PGAs? In other words, does the ADS1256 have internal division of the reference voltage?
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Is it reasonable to have e.g. a 1-year calibration interval, meaning that we can have a separate post-production/calibration device that is used once a year? Or, should the product itself incorporate a buffered resistor divider for each internal reference so that it can self-calibrate every time the product powers up? (This would of course still rely on the built-in 2.5V reference.)
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Due to tolerances, if the precision reference base for the calibration board were an exact 2.5V, there would be both a difference compared to the less exact 2.5V reference inside the product and inaccuracy for each of the resistor divider-generated voltages. In some 50% of the cases this would result in overranging the ADS1256. Is such a slight overranging (probably below some 0.2%) a practical problem in terms of accuracy? The 'ideal FSC' has some margin before 'binary saturation' (or whatever it might be called).
Or, should we make sure that the voltage levels used for calibration are guaranteed to be below the less exact product-internal 2.5V reference? (This is perhaps more of a pedagogical task for the staff measuring and adjusting the calibration board: "Don't ask why, just adjust the calibration board to be 0.5% less than nominal values."...)
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Is it a good idea to design such a calibration board with a 2.5V precision reference that feeds a number of resistor dividers with OPAMP unity gain buffer so that full-scale differential and single-ended measurements can be made of known full-scale input voltages for each PGA setting? This should enable not SYSGCAL but measurement data for adjustment of SELFGCAL so that the practical result would be equivalent to SYSGCAL?
(And another with a 5.0V or 4.096V precision reference for unbuffered calibration.)
Or would connecting the calibration board precision 2.5V reference to each product connector/interface in turn enable SYSGCAL without a need for all these gradually lower voltages?
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Could you recommend a suitable OPAMP unity-gain buffer for the resistor dividers? If I understand correctly, such OPAMPs must be designed to work with high input impedance, which rules out some zero-drift, low-noise OPAMPs?
I am currently looking at OPA189/OPA2189, but are there other ones better suited for this?
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Should you always incorporate a feedback resistor for the unity-gain buffer? Would 100 to 330 ohm be good as a rule of thumb or should it match the input impedance from the resistor divider?
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Should there be an RC network at the output of each unity-gain buffer?
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For the product's 2.5V reference, it is sometimes a good idea to use an OPAMP buffer. The OPA350 is commonly used as an example, e.g. in the ADS1256 EVM, but are there newer, higher-performance OPAMPs, that are currently recommended?
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An unrelated question:
In this recent post Bryan Lizon86 says "you should not be measuring ground-referenced signals with the buffer enabled.".
e2e.ti.com/.../ads1256-differential-inputs-and-single-ended-inputs
Is this because the ADS1256 buffer is not precise near GND?
"With the buffer enabled, the voltage on the analog inputs with respect to ground [...] must remain between AGND and AVDD − 2.0V." (datasheet p15)
Best regards
Niclas