msp430G2433: AD converter to get the voltage of capacity

Hi Yang,

What does "when working at 12MHZ, AD converter can not reach the target value. which means the voltage of capacity hasn't charge action" mean?

Are you using MSP430 to charge the capacitor?

Regards,

Ling

Hello Ling,

    Thank you for your reply and sorry for making you misunderstanding.

    I  am using the MSP430 AD convert to measure the voltage of capacity.

   It's a normal circuit like this:

 There is a voltage-time curve when we measure the voltage between point A and point B.

  I want use the AD convert of MSP430 to get the voltage so that I can get a voltage-time curve of the circuit.

  For test , I set the target voltage for A point, that means I programmed  MSP430 AD convert measure the A point , if the voltage of A reach the preset voltage ( what ever, for example 0x200), then success and return.

 That's the condition.

 I got the result is, when I use 1MHZ and 8MHZ(DCO), the MCU's AD convert can reach the preset voltage and return. But, once I set the MCU  working above 12MHZ(DCO) , It never reach the preset voltage. Just like there is no current charge action on the capacity(What I mentioned above).

After some research, I get the equivalent circuit of it:

 When start the MSP430 AD convert at the point A , we can get the equivalent circuit above.

Which C1 >> C2.

 If the frequency is slow, C1 will have the opportunity to get the enough time to be charged , so it can reach the preset voltage.

But if the frequency is high, The current mostly be consumed on the C2 but not C1, so, we can not make the C1 to reach the target voltage.

Am I right?

If it is true, How can I realize my target? To get the voltage-time curve of the circuit.

Thanks!

Hi Ling,

    That's a very import information, thanks a lot

Yang,

You should be able to verify the voltage at C1//C2 with a voltmeter. The ADC internal capacitance is extremely small. Even at high sampling frequency, I doubt it is the source of your problem.
How are you readying the ADC? are you using a sample-complete interrupt? or are you introducing software delay?
To me, it looks like your problem is related to the ADC not being allowed to complete its conversion.

Hi Yang,

Has your problem been solved?

Regards,
Ling