MAX Voltage Input on ADC - 13193EVB

[ramanoar]

Hi there,

Does anybody know what is the max voltage input of the ADC on a 13193EVB Board?
I have a 5V input to digitalize, and don't know if the board supports it.
Thanks in advance.

Rafael.

[rocco]

Hi, Rafael:

The 13193EVB uses the MC9S08GT60 microcontroller, which is designed for a maximum voltage of 3.3 volts. However, the voltage regulator supplies the GT60 with 3.0 volts. The Vrefh input is 3 volts as well, and that sets the maximum input voltage for the ADC on this board.

If your voltage source allows, you could use a pair of precision resistors to divide down the 5 volts to 3 volts.

hope that helps. . .

Message Edited by rocco on 05-29-200602:31 PM

[ramanoar]

Hey there rocco,

I wasn't clear enough, sorry.
My problem is, my input signal on the ADC is up to 5V, and I wanted to know if the board supports this level of signal to digitalize.
I didn't mean the input voltage, as to energy source, sorry about this.

Rafael.

[peg]

Hi Rafael,

You were clear enough the first time, I think you have misinterpreted rocco's response.

The S08GT can be powered by MAX 3.3volts on this board it is powered by 3.0volts. You CANNOT feed more than the supply voltage into the ADC!

If you do you risk damaging the MCU. There is no benefit in trying as when the ADC input equals the supply rail to the MCU you will get max value and it won't go any higher. You can probably use a resistive voltage divider into the ADC input to reduce your input voltage. But be careful not to make your input impedance to high or you will get inaccurate readings using this type of ADC.

Regards David

[ramanoar]

OK, I got it.
But... which valeu range should I consider to be of high impedance?

[bigmac]

Hello ramanoar,

For 10-bit accuracy, the equivalent source resistance at the ATD input pin should not exceed 10k ohm.  This is the value of the parallel combination of the two resistors of the voltage divider.  For example, if you chose resistor values of 10k (series) and 15k (shunt), this would give a source resistance of 6k ohm.  A capacitor, typically 100nF, should also be connected between the input pin and ground.

However, this also assumes that your source resistance for the 5 volt signal is low.  With the above voltage divider values, and for an error less than 1 LSB at 10-bit resolution, the source resistance should not exceed 240 ohms.

Regards,
Mac

Message Edited by bigmac on 2006-06-07 10:37 AM