Best Tag to passively measure impedance?

[leo94]

I have a sensor that changes its impedance when exposed to radiation. I want to integrate it with an NFC/RFID tag to meausre the impedance passively. Tipical impedance values vary betweeen 2 and 20 MOhms. 

I am novice at RFID/NFC tech, so I dont quite know where to look.

I was thinking of using NTAG I2C plus. Is it possible to read the impedance (or resistance) value with only the tag and the sensor ? How would i connect it ? 

Is there any better suited tags? 

[xiangjun_rong]

Hi, Leo,

I suppose that the " passive operation mode" is low power modes, as you know that the LPC family has 4 low power modes: sleep, deep sleep, power down, deep power down modes, in all the cases, the core stops working. In other words, you can not sample external analog channels with ADC and save samples in low power modes. Yes, the Op-Amp to build a current source would require a battery, because the OP-AMP needs power supply.

Hope it can help you

BR

Xiangjun Rong

[leo94]

Very useful xiangjun.rong‌!

Just to  conferm:

Low power modes = Powered by NFC ? 

And more specifically: 
The moment NFC power becomes available, i cannot sample external analog channels with ADC ? 

Since my sensor's resistance changes permanently, I only need to sample resistance when Powering it through the RF filed - this mode doesn't allow to sample external analog channels with ADC? 

thanks for your help, 

Leo Torchia. 

[driesmoors]

Hello Leo,

To clear up some misunderstandings for all: 

• A passive tag is a tag that draws all its power from the NFC field. All three mentioned tags can do this.
• When a MCU is in play, it usually features low power modes to reduce the current consumption. Hence the sleep, deep sleep, deep power down options. Typically all LPC ICs have these, but these options are not available one all mentioned tags.
• All tags can also be powered via an external power supply - a battery. This is necessary to allow them, without the presence of an NFC field, to either communicate over the I2C bus (NTAG I2C and NTAG 5), or to fully operate independently (NTAG SmartSensor). When a battery is in play, a more suitable name could be semi-passive tag, as both active and passive (e.g. when the battery is depleted) uses are possible.

Your questions cannot be answered: we don't know what you want to achieve, and with what. We don't know what your use case is, which impedance sensor you plan to use, and what kind of radiation we're talking about.

Please have a look at the datasheets of the mentioned tags and check their pin functions. All of them allow to download SDKs, user manuals and application notes, which should help in determining their validity for you.

Kind regards,
Dries. 

[driesmoors]

Hi,

NTAG I2C | NXP  seems like a good choice. You may as well want to have a look at NTAG 5 link: NFC Forum-compliant I²C bridge for IoT on demand | NXP or possibly at NTAG SmartSensor_NXP 

Best,
Dries.

[leo94]

driesmoors‌ Thanks! will check it out. One more thing, do you know to what ports you would connect the 2 ends of my sensor to measure its Resistance/Impedance ?  

[xiangjun_rong]

Hi, Leo,

As you said "I have a sensor that changes its impedance when exposed to radiation", so you have to test the impedance which represents the  radiation. As far as I know that you can use OP amplifier to develop a current source, in other words, you can have a fixed value current flow the impedance of sensor, you can test the voltage of the sensor with ADC converter to get the impedance and the radiation.

Hope it can help you

BR

XiangJun Rong

[leo94]

Thanks for your answer, but to be more precise I would like to know if I could achieve a resistance measurement also in passive operation mode. The Op-Amp to build a current source would require a battery.

thanks, 

Leo