About the MC56F82748 of ANB1 ADC pin impedance

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About the MC56F82748 of ANB1 ADC pin impedance

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kriske
Contributor II

Hi

I measure the MC56F82748 demo board.

 

Measuring the impedance other impedance of ADC pins are normal about Mohm.

 

but measuring the GPIOB1 / ANB1 impedance just only about 3kohm.

 

I wanted to clarify whether the question on the register settings, the trouble you help.

 

1. want you to use your company's example code to measure ANB1 impedance.

 

2. If the measured impedance and 3kohm ANB1 many difference, please provide example code.

 

I have try the ADC function. I try use same resistor divider, but get difference result.

 

I use two 4.6kohm divider, but the measure is very difference.

 

If I use ANB1 get the ADC result is 10032 and use ANB0 get result is 16208.

 

According to calculations, the 16208 is right, why the ANB1 result is wrong.

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kriske
Contributor II

Dear Xiangjun

Thanks for your quick actions,

I know the ADC is a dynamic concept, I have check the DS of 9.5.1.1 Equivalent circuit for ADC input.

I have calculate the ADC impedance get the result. But It was still have some issue.

I use JUMP on the same resistor to get the ADC result on ANB0 and ANB1, but I get difference result.

DSC_0672.JPG

ANB1_issue00.png

ANB1_issue01.png

Kris

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xiangjun_rong
NXP TechSupport
NXP TechSupport

Hi, Kris,

I do not know how you test the impedance of the ADC analog channel pin, but the ADC impedance is a dynamic concept, the impedance is different for the ADC converting state and ADC idle state.I attach an1947, which addresses the ADC analog channel impedance. Hope it can help you.Generally, the ADC channel impedance is about 200K ohm, which is dependent on the ADC clock frequency.

I attach the an1947.pdf here.

BR

Xiangjun Rong

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mohammad_kamil
NXP Employee
NXP Employee

Hi Kris,

May i know device mask number (0N33G/1N33G/2N33G) used in TWR card.

Kamil

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kriske
Contributor II

Hi Mohammad Kamil

I already know the answer, my TWR card is use 1N33G.

So if I use 2N33G, the ANB1 can working properly?

Kris

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