voltage divider issue

[The_Babatian]

Hi All,

I am designing a digitally controlled boost converter. I have trouble with the voltage divider used to scale down the output voltage to fit the input range of the on-chip ADC of the Microcontroller (TMS320F28335) from TI. I noticed that the voltage divider ratio was at desired value when the output of the voltage divider was not connected to the ADC input node, but the voltage divider ratio increased a lot when when the output of the voltage divider was connected to the ADC input node.
I was wondering if this is caused by the input impedance of the ADC, so I tried to put a 0.1uF ceramic capacitor in parallel with the input of the ADC, but it did not make any difference.
Can anyone help me out on this issue? thanks in advance.

 

[pradhan.rachit]

I'm a bit confused...
Whether you are applying pulses or DC to the voltage divider?
If you're applying DC, then the capacitor will be blocking the DC voltage resulting in no voltage at the input of ADC...right?

 

[The_Babatian]

Thanks for your reply.

Sorry for the confusion. Originally, what I did was implementing a simple voltage divider with two resistors without the ceramic cap. I added a cap in parallel (not in sereies) with the input of the ADC just to see if this helps. But it did not. Any idea?

 

[ZASto]

You did not mention the resistor values of your divider.
You must take in consideration the input impedance of ADC input of your uC. The "low side" resistor should, at least, be 10 times lower than ADC input impedance.

 

[The_Babatian]

Thanks.

I used 1.8M Ohms for the low side resistor, and 10M Ohms for the high side resistor to achieve the desired voltage divider ratio. I thought the larger the resistor value the better (less power consumption of the voltage divider).

 

[mtwieg]

Yeah that's way too high, try decreasing them by a couple orders of magnitude at least. Buffering the voltage with an op amp would help too.

 

[pradhan.rachit]

Why don't you get started with a combination of 10K,1.8K...And then increasing the value of the resistors in the order of 10...

 

[ZASto]

Yaw, with 1M8 you are influencing a lot on the ADC input impedance. 1k8 would be more suitable.