M5213EVB ADC voltage input limit

[jrowland]

Hi everyone,
    I'm still new this field and i'm learning as I go with this microcontroller. My question is, Is there a limit on how much voltage that the analog input pins can take? I've read that by default the voltage reference high is 3.3V. However, I did read that you can change this voltage reference. I'm kind of hesitant to change the voltage reference due to the fact that i might "fry" the board. Any info that you can provide will be much apprieciated, thanks.

[jrowland]

Thank you for the response, it was informative. I would like to ask one last question. I currently have an analog input from a source that ranges from 0V - 5V . Even if the VREFH is set to 3.3V will it still be safe to accept a max input signal of 5V? Or is there a chance that i might damage the board?

[mjbcswitzerland]

Hi

5V is out of the specification limits so you will need to divide it with a potentiometer. The question is whether you need to buffer it with an op-amp too. The op-amp will introduce its own (small) offset error, but will ensure that the impedance is low so that the sample and hold circuit at the ADC input can be changed quickly.

Regards

Mark

[JimDon]

It appears that the spec sheet rates the sample cap at 1,4pf.
pg 38 of the Spec Sheet:

"Sampling capacitor at the sample and hold circuit. Capacitor C1 is normally disconnected from the input and is only connected to it at sampling time; 1.4pF"

So perhaps this is not too big of an issue. I have used voltage dividers, but lower is better.

[mjbcswitzerland]

Hi Jim

You are right that it is usually not an issue but it is worth knowning about possible limitations. I remember using the 5213 in a project which was measuring NTCs, which can have resistances of about 100k and we did use op-amps in that case. However there was also some active filtering too, although I think that a large capacitor would probably have been as good (slow signals - and the large capacitor [large in this case would only need to be a few nF] also acts as a good current source for short periods).

I am no longer sure of the details but I think that the sampling period (the time that the input is actually connected to the sample-hold) is about 200ns at 5MHz (maximum ADC clock). A resistive source of 30k would take about 200ns to charge the 1.4pF input capacitor to the input signal level (using 5xRC as approximately fully charged). Any stray capacitance or real capacitors in parallel help charge, although they can have a filtering effect on the input signal too.

Therefore I suspect that any source with less than 10k should not be an issue at all.

Probably using a slower ADC clock also increases the connection time.

Regards

Mark

www.uTasker.com

[mjbcswitzerland]

Hi

If you are using the internal 3.3V reference, the limits on the analogue inputs are 0..3.3V.

It is possible to use other reference values between 0V and 3.3V (eg. VREFL = 1V and VREFH = 2V). In this case the analogue inputs must also remain within this range (in the example 1V..2V). These can be multiplexed with AN6 and AN2 respectively (set in CAL register). The references should no be connected to any voltages below 0V or higher than 3.3V.

To measure larger signal ranges you will need to scale the input (generally using op-amp which can supply the short current surge to charge the internal sample-hold circuit).

Also take a look at the uTasker forum for a discussion of the use (more the programming...) of the ADC:
http://www.utasker.com/forum/index.php?topic=280.0
The ADC in the 5213 is the same as the one used in the M5223x so you may like to look at the uTasker project (it does support the 5213 and FlexCAN - although used primarily for the Ethernet enabled parts) which has a useful ADC interface and also allows the ADC to be simulated!

Regards

Mark

www.uTasker.com