More detailed specification of SAADC input impedance?

[snygaard]

I’m searching for some more detailed information of the SAADC input impedance.

Please see attached file for details and questions.

Nordic Semiconductor question 20190315.pdf

[Carsten Wulff]

a) The R_INPUT modells leakage path's, for example, leakage of ESD clamps, thus this is a hard question to answer, there will be device to device variation, and large temperature variation (lower resistance when hot). What we've measured is in excess of 50 M Ohm. In general, we're here into leakage currents of nA, 

b) R_INPUT does not change with gain setting. For the other settings I think it depends on what you mean by "change". During sampling, the network of transistors connected to the pad will be different from the transistors during idle, so there is a theoretical change in the amount of leakage (pA). R_INPUT is not a physical resistor, but rather a model of all the leakage paths from the pad, to VDD, and ground. 

c) The value for R_ladder is given for typical process, however, this could vary up to +-20 % for a typical silicon process technology

d) C_sample will depend on process variation (device to device), which could be up to +-20 % in a typical silicon process technology, in addition the C_sample changes with gain setting.

Maybe you could elaborate on your source resistance? It would be helpful to understand your application. 

[snygaard]

Thank you for the input.

I'm using an ADC as battery monitor using a simple resistor divider and want to predict the obtainable precision of the voltage measurement. The nRF52810 is running at regulated 1.8V and during battery monitoring single ended ADC input and internal reference is used. I was looking at this case for inspiration:

https://devzone.nordicsemi.com/b/blog/posts/measuring-lithium-battery-voltage-with-nrf51

and was wondering how Mohm in resistor divider is compatible with ADC datasheet values of Rinput in the same Mohm range.
For nRF52810, the datasheet says "Rinput >1Mohm (typically)".
If using an equivalent source resistanse of e.g. 400k, it makes a significant difference of the ADC value sampled, if the equivalent Rinput is 1Mohm or 10Mohm.
I expect you to have a specification stating a typical and a guarenteed minimum value of Rinput over the temperature range. Else the precision of the ADC measures cannot be predicted.

[Carsten Wulff]

Hi,

Have you taken a look at this one?

https://devzone.nordicsemi.com/b/blog/posts/measuring-lithium-battery-voltage-with-nrf52 

The ADC on nRF51 is a different architecture and the concerns from that do not translate to nRF52. 

On nRF51 there was DC current flowing into the ADC, while on nRF52 there is an AC current given by the sampling frequency, internal cap, and voltage, which is why we recommend  max 800 kOhm source resistance in the PS for TACQ = 40 us

[snygaard]

Thank you for pointing out the difference. It would have been very valuable to have a short description of this in the datasheet...

So I read it like the nRF52 SAADC input impedance consist of a DC leakage path (nA range) and a AC load introducing an equivalent resistance calculated by the formula:

Rinput = 1 / (fsample * Csample)

Right understod?

A few more questions:

1) In your first answer above (subject b), the R_INPUT you talk about is the DC leakage (independent of gain) and not the equivalent resistance determined by 1 / (fsample*Csample) because Csample is stated to be dependent on gain setting. Right?

2) What is the relation between the stated typical value "Rinput>1Mohm" in the datasheet and the formula "Rinput=1/(fsample*Csample)"?
Using maximum sampling rate (200kHz) and maximum value of Csample (2.5pF), the calculated Rinput makes 2Mohm.

3) Regarding electrical specifications in the datasheet:
What is the temperature conditions for the typical values stated? 25degC?
What is the temperature conditions for the max/min values stated? -40degC to 85degC?

[Carsten Wulff]

1) Correct R_INPUT != 1/(fsample*Csample), The R_INPUT is purely to model the DC leakage. It does not include the effects of the sample cap

2) No relation between the two, they are independent factors. They both contribute to the apparent current that goes into the SAADC though. 

3) This can be seen in the Recommended Operation conditions of the product specifcation of the product that you're using