How to interface NTC thermistor to Kinetis ADC input

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How to interface NTC thermistor to Kinetis ADC input

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2,557 Views
dkh
NXP Employee
NXP Employee

Hello,

I have a circuit with 100K thermistor in series with 25k resistor to ground. I connect my adc input to the point where

the thermistor and resistor connect and also a capacitor. This approach works on some adc channels but not others.

reading through the datasheet I notice there is an internal adc resistance of 5k Ras. So it appears having a high impedance input circuit may be an issue.

Is there a reliable way to make this approach work ?

On some of the eval boards like the FRDM-KL03z I notice they use lower value 10k NTC with 5k above and 5k below in series with 2 adc channels. Why do they do this as compared to approach above.

Regards,

1 Solution
1,823 Views
Hui_Ma
NXP TechSupport
NXP TechSupport

Hi

The TWR-KM34Z75M board provides below temp sensor circuit:

pastedImage_1.png

Customer doesn't need to use the serial resistor R, for the ADC input equivalency diagram shows there with serial Radin (input resistance), which typical value is 2Kohm(Max. value is 5Kohm). Or customer can use a very small serial resistor R, such as 100ohm.

pastedImage_5.png

The FRDM-BC3770-EVB provides below NTC thermistor interface circuit design:

pastedImage_2.png


Wish it helps.

Have a great day,
Ma Hui
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3 Replies
1,823 Views
Hui_Ma
NXP TechSupport
NXP TechSupport

Hi

The Ras (analog source resistance) Max. value is 5Kohm, while there with below note:

This resistance is external to MCU. To achieve the best results, the analog source resistance must be kept as low as possible. The results in this data sheet were derived from a system that had < 8 Ω analog source resistance. The RAS/CAS time constant should be kept to < 1 ns.

The FRDM-KL03Z board using below thermistor circuit to enhance the system robust.

pastedImage_3.png

Customer also could consider to use below circuit:

pastedImage_4.png


Wish it helps.

Have a great day,
Ma Hui
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------

0 Kudos
1,823 Views
dkh
NXP Employee
NXP Employee

Thanks for reply. On second circuit are there any guidelines for Rs, R, and NTC resistance ?

How about the capacitors ?

Since this is a slow changing signal measuring temperature can high values for the Resistors and NTC be used ?

100k + ?

The reason I ask is this is common approach, but power consumption is very important and must be minimized.

If you can use high resistance devices what software configurations of the ADC are optimal ?

0 Kudos
1,824 Views
Hui_Ma
NXP TechSupport
NXP TechSupport

Hi

The TWR-KM34Z75M board provides below temp sensor circuit:

pastedImage_1.png

Customer doesn't need to use the serial resistor R, for the ADC input equivalency diagram shows there with serial Radin (input resistance), which typical value is 2Kohm(Max. value is 5Kohm). Or customer can use a very small serial resistor R, such as 100ohm.

pastedImage_5.png

The FRDM-BC3770-EVB provides below NTC thermistor interface circuit design:

pastedImage_2.png


Wish it helps.

Have a great day,
Ma Hui
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------