Hi G,
This TI PDF is more helpful.
The input impedance requirements for an MCU ADC input are is that it less than 5k for some models, 10k for others.

I would suggest you consider on using an OPA input filter and conditioner for the ADC input, in order to get the maximum resolution from the ADC conversion.

Limiting it to 200 to 300 is IMO under utilizing the ADC resolution.

E

Added: An ADC link.. @Goxeman
https://deepbluembedded.com/stm32-adc-tutorial-complete-guide-with-examples/

 

But be careful you don’t spoil it with a poor choice of op-amp.
The LSB of a 12 bit A/D on 3.3V is 800uV. Some op-amps will introduce an offset error that is greater than that, and look out for errors caused by op-amp input bias currents.

 

I think that I will be fine without the op amp that would introduce other variables.

Going back to the topic of the Figure 17 from ST. I am curious, what is C1 going doing? Blocking a frequency?

 

I think that I will be fine without the op amp that would introduce other variables.

Going back to the topic of the Figure 17 from ST. I am curious, what is C1 going doing? Blocking a frequency?

It’s an AC coupled input. I assume it is for audio.

 

Hi G,
Could you give us a clue to possible applications you are planning for, using the ADC input?
E

 

Hi G,
Could you give us a clue to possible applications you are planning for, using the ADC input?
E

Hi E,

We have already talked about it previously

 

Two contradictory points.
1. Increasing the resistances in the potential divider reduces the possible fault current that would go through the processor protection diode.
2. The ADC needs a low source impedance if it is to sample quickly.
A capacitor across the lower resistor would ensure the low source impedance, but would limit the frequency you can measure.
If the voltage you are measuring doesn't change much, then that isn't a problem, and a bit of filtering would help remove all sorts of interference and noise.

Hi Ian,

Would you use the capactitor close to the node point of the resistor divider or close to the ADC pin of the MCU?

 

Hi Ian,

Would you use the capactitor close to the node point of the resistor divider or close to the ADC pin of the MCU?

If you‘re thinking of pcb layout, then close to the device will have a small advantage. You have to charge the ADC’s sample and hold capacitor before it can take an accurate reading. It’s <10pF but it still takes time and current to do it.

 

I was thinking about 100nF at least

 

hi.
This is a typical ADC input response and timing, using the divider and 100nF

Do you follow it.?
E

 

hi.
This is a typical ADC input response and timing, using the divider and 100nF

Do you follow it.?
E
View attachment 299064

I am sorry, my mistake. I meant 100pF

100nF is not suited for acquisition of measures through the voltage divider ADC

 

Is your signal frequency >50Hz? If not, 100nF would be fine.

 

hi G,
For reference only, a typical MCU ADC

With and without the 100p, note the delayed sampling time.
As I asked earlier, exactly what voltages will you be sampling in order to determine what parameters?
E

 

hi G,
For reference only, a typical MCU ADC

With and without the 100p, note the delayed sampling time.
As I asked earlier, exactly what voltages will you be sampling in order to determine what parameters?
E

View attachment 299073

The output of the resistor divider will be between 0,5 and 3,3
I don’t mind having a bit of delay in sampling as this is only for readings

 

Hi guys,

I am back with my project and reading about different things I though that output impedance fo the voltage divider would be very high for the ADC dont you think? I read a bit about it and I have found in a couple of places that impedance to the ADC should be no more than 10K

 

The input impedance requirements for an MCU ADC input are is that it less than 5k for some models, 10k for others.

Hi,
I posted this information in post #41

E

 

Hi,
I posted this information in post #41

E

You are totally right. I will have to learn how to apply an OPAMP and select the appropriate part number for what I’m trying to achieve. I know the theory but I haven’t used any

Thanks E

 

Hi G,
Check out this option for an OPA.
E
Sorry to say, I did a full working simulation a week ago, but you did not want to use an OPA, so I deleted it.
Please post your OPA circuit and will will run it in simulation.

 

The impedance restriction is to ensure that the sample and hold capacitance is charged quickly enough. The capacitance is about 10pF. 100nF capacitor across the input holds 10,000 times the charge required by the sample and hold capacitance, so that will suffice and there is no need for an op-amp.

 

Those are contradictory opinions…