PN7150 AGC question in understanding system

[daniel13]

Hi, we are currently bringing up a PN7150 chip on a custom NFC coil and are following the AN11755, Antenna Design and Matching Guide

I have a question on the Rx tuning path in understanding the AGC in tuning in reader mode page 3.6.3
Rx resistance selection process.

I understand that the higher the AGC, the more attenuation is done on the Rx signal.

What don't understand is why does the AGC need to be between 500 and 800 with no tag on top for tuning. Perhaps we don't fully understand the role of the AGC, but it seems as if we would want a lower AGC value with no Tag on top so then there is more headroom for attenuation when a card comes in.

Can someone help explain why the AGC needs to be between these two values, or else point to some documentation we can look at aside from the AN11755?

In addition, what are the effects of having a lower AGC with no Tag?

Thanks! 

[Kan_Li]

Hi,

The range between 500 and 800 are helped to reach the target Vrx_pk, please kindly refer to the following for details.

Have a great day,
Kan

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[daniel13]

Hi Kan Li, what would be the downside to having a lower AGC which I understand to be a high Vrx_pk?

Our testing shows a larger range on our coil, when tuned with a lower AGC.

[estephania_mart]

Hello,

Sorry for the late response.

The Attenuation Gain Control (AGC) will help you attenuate the signal to the receiver can reach the optimum input voltage.

Yes, indeed the ACG attenuates the RX signal from the antenna by using a voltage divider, the recommended values are set to that to achieve the optimum input values.

I believe everything it's explained with more detail in the chapter 3.6.1 Receiver Block functionality in figure 22 as referenced from Kan Li as well as the 3.6.2 Rx connection.

If you checked the paragraph under the figure 22 all the recommendations and some reasons why those values are recommended for that device.

If you have any other question, please let us know.

Regards, 

Estephania