Simulation Capacitive divider - Signal Interfacing with Micro-controller ADC

Thread Starter

mishra87

Joined Jan 17, 2016
687
Hello All,

I am facing issues to interface a signal from capacitive divider to MCU ADC.

Why does signal goes negative once I connect with 1 Meg... and if i remove 1 Meg signal looks good.
My intention is if i connect this signal to MCU ADC it should not distort.
So could anybody guide me how to interface with MCU

MCU to be used for application is STM32F205

Please help me out .

Attached schematic for reference .

upload_2019-5-13_21-13-26.png


upload_2019-5-13_21-15-3.png

Thanks in Advance !!!
 

Wolframore

Joined Jan 21, 2019
1,742
figure one you're biasing the signal so that 0V at output is biased at the crossing...

figure two bias is the ground with all signal referenced to it from the bottom.

a third option is to bias on top which will make all the pulses negative...

what are you trying to achieve?
 

Thread Starter

mishra87

Joined Jan 17, 2016
687
figure one you're biasing the signal so that 0V at output is biased at the crossing...

figure two bias is the ground with all signal referenced to it from the bottom.

a third option is to bias on top which will make all the pulses negative...

what are you trying to achieve?
Thank for your reply.

I am trying to give below signal to ADC of MCU.

upload_2019-5-13_22-7-42.png

Regards,
Due to loading of 1 Meg signal goes negative. So how to overcome this.
 

Thread Starter

mishra87

Joined Jan 17, 2016
687
My problem statement is that, how to interface capacitive divider to MCU ADC pin.
Same phase input signal to be interfaced.

Thanks !!!
 

Thread Starter

mishra87

Joined Jan 17, 2016
687
Hello,

Still struggling with the same problem.
Why voltage at Vadc coming negative in simulation.

upload_2019-5-14_22-37-45.png

Could anybody give any clue.
Pulse switching frequency is 100KHz
Any suggestion would really appreciated.
I want interface this signal to STM32F205 ADC so how to do this.
???
 

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Thread Starter

mishra87

Joined Jan 17, 2016
687
If I remove 1 Meg load the signal becomes negative.
How to simulate this because i want to interface this signal to MCU.

upload_2019-5-14_22-46-23.png
 

Jony130

Joined Feb 17, 2009
5,155
What have you expected?
Your circuit with a 1M Ohms load resistance behavior just like a simple RC circuit with R = 1MΩ and Cz ≈ 20pF and the signal amplitude is equal to 3.3V/2 = 1.65V.

And you are getting the negative voltage because the capacitor is charging and discharging via a 1MΩ resistor. And during the discharge phase the current flow in the opposite direction via a 1M resistor hence the negative voltage.

But can you explain to us why do you want to use a Capacitive divider in the first place?
 

Attachments

Thread Starter

mishra87

Joined Jan 17, 2016
687
My Application is to measure sensor signal using capacitor divider.
Now there is three capacitor C1 , C2 & C3.
C1 & (C2+C3) makes a capacitive divider.
Since C1 & C2 are in pF and C3 is in nF hence Series of (C2+C3) doest have much impact on divider circuit.
Between C2 & C3 there is a sensor now there is two cases -2

1. When C2 is connected with C3 with medium sensor.
2. When C2 is not connected with C3.

These signal i want to monitor with MCU ADC.
So could anybody advice me how to do this ???

upload_2019-5-14_23-21-28.png

upload_2019-5-14_23-23-23.png

How to interface this signal to ADC.

Regards,
 

Alec_t

Joined Sep 17, 2013
11,133
we are using capacitive divider
Why? What is wrong with simply connecting the sensor directly to the ADC input, or using a resistive divider if necessary?
I suspect, with only a 10p coupling capacitor, your signal source impedance (some 300KOhms at the frequency you are using) will be too high to give a good ADC reading.
 
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