Seeking more information on NRF52833 ADC, reference, and buffers

[jdub]

Hi,

We are developing a product which is reading RTDs and thermistors from the NRF52833's internal ADC. We're working on compensating for the intrinsic errors of the ADC and gain buffers and have some questions not covered in the product specifications.

Do the ADC gain buffers have offsets associated with them? The datasheet says the ADC has an intrinsic offset of ±2LSB @ 10bit res but doesn't mention whether there are voltage offsets associated with the buffers.

Is there any procedure to internally determine the ADC buffer gains? For example, connecting the buffer inputs directly to the reference voltage?

Can you provide any more detail on what the ADC's internal calibration routine does or what it accounts for?

Are there any details on how the ADC input buffer gain is implemented?

Are there any specs on channel isolation in sequential measurement mode?

Is there a spec for temperature drift of the VDDH/5 divider?

Thank you!

[jdub]

Sorry, one other direct question came up, and then a high level request.

Direct ADC question:
The INL is listed as 4.7 bits, which is quite significant. Do you have any sense of whether the error at a given part of the transfer function is stable, or does it shift with time/temperature/supply voltage? Ie. Can we characterize the non-linearity for a given part and rely on that profile to calibrate future readings?

High level question:
Do you have any application notes related to reading a RTD or other high precision, low sensitivity resistance sensor?

In our application we are very space constrained and must do so with a simple voltage divider setup, with a fixed resistor on top and temperature sensor on bottom. By taking differential measurements with the same gain settings across the different legs of the voltage divider, we can cancel out any tolerance errors on the references or gains of the ADC and buffers. But our final temperature reading still has more error than we expect, so we are trying to find ways to improve it.

Thank you again

[hmolesworth]

Could you clarify the specific resistance range (at min to max temperature) of the RTD you are measuring?

[jdub]

Approx. 1kOhm (min) to 2.8kOhm (max)

[ketiljo]

jdub said:

Direct ADC question:
The INL is listed as 4.7 bits, which is quite significant. Do you have any sense of whether the error at a given part of the transfer function is stable, or does it shift with time/temperature/supply voltage? Ie. Can we characterize the non-linearity for a given part and rely on that profile to calibrate future readings?

In 12bit mode the INL is 4.7 LSBs, not bits. In 10-bit mode its around 1 LSB. The errors will likely be systematic. It is possible to compensate for some of the effects, however, it does require per device calibration. For example https://ieeexplore.ieee.org/document/7993659

jdub said:

Do you have any application notes related to reading a RTD or other high precision, low sensitivity resistance sensor?

No, sorry

jdub said:

In our application we are very space constrained and must do so with a simple voltage divider setup, with a fixed resistor on top and temperature sensor on bottom. By taking differential measurements with the same gain settings across the different legs of the voltage divider, we can cancel out any tolerance errors on the references or gains of the ADC and buffers. But our final temperature reading still has more error than we expect, so we are trying to find ways to improve it.

If you're using resistive input it's important that you select a TACQ that is long enough compared to the source impedance, see the product specification. 

[jdub]

> In 12bit mode the INL is 4.7 LSBs, not bits.

You're right, that was just a typo on my part, I wasn't making my calculations with 2^4.7 = 26 LSB of noise

> If you're using resistive input it's important that you select a TACQ that is long enough compared to the source impedance, see the product specification. 

Yes our input impedance is <1kOhm which is very low, so we can use a short acquisition time. But we have found that extending it out to the maximum makes no difference on this measurement.

Thank you!