about the S32k144 Vref internal block diagram

[qiaoqin_wang]

Hi

i have read the S32K144 datasheet ,but i can't understand about the Vref volateg for ADC,

can you help me to understand it better;

Does the ADC reference voltage VREF come directly from an external power supply, will the MCU chip do the conversion? For example, blood pressure, or other forms?

thanks for you 

looking forward your reply

[lukaszadrapa]

Hi,

yes, you are supposed to connect VREFH to external supply voltage. Notice that the difference between VREFH and VDDA should not be higher than 0.1V, so it means the best way is to use the same supply voltage for both.

And just in case if you are not aware of this app note:

https://www.nxp.com/webapp/Download?colCode=AN5426

Regards,

Lukas

[qiaoqin_wang]

Dear lukaszadrapa

thanks for your replying

but i want to konw the internal bolack diagram for detail about the vref voltage，can you provide it，thanks， for example：when the external supply voltage for the vref，how the MCU internal to change the voltage for the ADC to use，thanks.

[qiaoqin_wang]

Dear lukaszadrapa

could you help me to understand this confusion,thanks for your help and looking forward your reply.

[lukaszadrapa]

I'm afraid we do not provide internal block diagram.And I'm not sure if I can understand your question. Could you please provide better explanation?

Regards,

Lukas

[qiaoqin_wang]

  Dear  lukaszadrapa

sorry， Maybe what I described is not very clear，i will try to descirbed it； thanks for your patience;

because we use the MCU is S32K144,and we use the LDO to supply 5V for the Vref;

we want to konw if we supply 5V  voltage for the Vref,if the ADC convert the voltage that use the Vref voltage  form the LDO ; (exp: down the voltage for 3.3V ,or high accuracy compoenent to keep the Vref voltage don't change when the external LDO change),so i want to know the internal Vref voltage internal detail block digaram;

formula

1 LSB = (VREFH - VREFL)/2N:

S32K144 block diagram  ：

as the below is the Vref，i want to konw the internal detail for the Vref. 

[qiaoqin_wang]

Dear  lukaszadrapa

thanks for your so quick reply,and i have undestand ,but i have anthoer question and i need your help 

we use the MCU is s32K144,and we want to use the bandgap  to monitor the Vref voltgae(Vref)

 as the below picture  show:

so we want to check :

1.    The bandgap channel delivers a fixed 1 V that the  voltage doen‘t change,Even if the MCU's power supply changes, its voltage does not change, it is always a constant 1V？

2. whether the  bandgap channel voltage will change when the MCU external power supply voltage is lower than a certain value （exp:1.5V )?

[lukaszadrapa]

Hi,

1. yes, that's the nature of bandgap voltage.

2. operating voltage of this device is 2.7V up to 5.5V. If the supply voltage falls below 2.7V, low voltage reset circuit will trigger reset and the device will stay in reset until the voltage is high enough again. So, down to 2.7V it will still work.

Regards,

Lukas

[qiaoqin_wang]

Dear lukaszedrapa

1. yes, that's the nature of bandgap voltage.

WQQ ---this explataion i have understand ,thanks!

2. operating voltage of this device is 2.7V up to 5.5V. If the supply voltage falls below 2.7V, low voltage reset circuit will trigger reset and the device will stay in reset until the voltage is high enough again. So, down to 2.7V it will still work.

WQQ ---I’m still very confused about this explanation. I understand what you mean, when the MCU’s power supply voltage is lower than 2.7V，the MCU will reset，but i want to know that the operating voltage of this device is lower than 2.7V,if the bandgap voltage is still 1.0V ,if  the voltage is lower, as low as 0.5V, is the bandgap voltage is still 1.0V ?

thanks for you so patienece explanation,thanks

Regards,

[lukaszadrapa]

Hi,

if the voltage is lower than 2.7V, the device is held in reset. The application is not running and you can't read the bandgap voltage, of course. So, this question doesn't make sense.

If the reset is released (i.e. supply voltage is above 2.7V), the bandgap voltage will meet the specification. 

Regards,

Lukas

[qiaoqin_wang]

dear lukaszadrapa

Thank you for replying to me so quickly and patiently, sincerely thank you, hope you have a happy day every day.

[lukaszadrapa]

Hi,

the reference is used directly. If the Vref is changed, the "1 LSB = (VREFH - VREFL)/2N:" is changed accordingly. If there's a noise on the Vref, the noise will affect also the accuracy. It's responsibility of user to ensure that the voltage is kept constant as much as possible.

Regards,
Lukas

[qiaoqin_wang]

Hi 

can you help me to understand this confusion, i need to konw this informtaion, so that i can design my schematic better,thanks for you help ,and looking forward your reply.