nRF52840 SAADC Input Stage

[ThePinkPanther]

Hi,

We are using the nRF52840 SoC.

1. The following image is taken from the datasheet:

It shows a Gain block before the SAR core

a. Does the Gain block serve also as a buffer?

b. What is the Gain BW?

 

1. The following image describes a simplified SAADC sample network:

- Does the Rsource external to the nRF52840 SoC?

 

Thank you

[Jørgen Holmefjord]

Hi,

Does the Gain block serve also as a buffer?

This is a simplified block diagram. There are input buffers between the MUX and the SAR core.

What is the Gain BW?

I'm not sure we have any parameters for this. Can you elaborate more on what you are looking for?

Does the Rsource external to the nRF52840 SoC?

Yes, this is the external source resistance.

Best regards,
Jørgen

[ThePinkPanther]

Thanks,

We’re trying to understand the bandwidth requirement of an external amplifier, which is connected to the nRF52840 SoC and driving the ADC. 

This external amplifier should have the ability to load the sample and hold capacitor in the SAADC fast enough, in order to withstand a certain sample rate/acquisition time - while maintaining the gain it supposes to provide. This ability is referred, per my question, as Gain BW. If the internal Gain buffers, between the MUX and the SAADC, have adequate Gain BW, we can relax the requirement in this aspect from the external amplifier in our application.  

Are there are any other recommendations on how to drive the SAADC input, besides the acquisition time configuration per Rsource? 

[Jørgen Holmefjord]

Sorry for the slow reply. I will check your questions with the SAADC developers. I will get back to you as soon as I receive their response.

[Jørgen Holmefjord]

Forwarding the response from our developers:

"The Gain block is a switch capacitor amplifier. As long as the input capacitors settles sufficiently, then the gain block has sufficient BW to drive the internal ADC. The Rsource criterias is determined by settling, as such it depends on TACQ. 

In general the SAADC does not need any external amplifier, but of course, it depends on the source of the signal. If the output impedance of sensor, or input network your using exceeds 800 kOhm (TACQ = 20us) or you need a faster TACQ, then you do need a buffer. 

What type of buffer would depend on your recquirements for gain accuracy, single ended or differential, signal source etc, so we would need to understand the usecase better to come with recommendations."