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F1232 ADC input impedance

Started by brer_spam July 26, 2005
I've been unable to locate the input impedance of the ADC analog input
pins for either the F12x2 or F161x devices.

The datasheet has the note : "The leakage current is defined in the
leakage current table with P6.x/Ax parameter." but I can never find
the specified table!

Any ideas?  

Could any one estimate the input impedance?

My application is to compare the (resistor voltage divided) MSP430
supply voltage with the internal 1.5V reference.  Since I'm battery
powered I'd like to use a 3Mohm divider.

Thanks,
Mark

PS : I've found the mux has 2K ohms of series impedance....







Beginning Microcontrollers with the MSP430

Hello Mark,

> I've been unable to locate the input impedance of
the ADC analog input
> pins for either the F12x2 or F161x devices.

Locating detailed hardware info on the MSP430 is not easy,
unfortunately. Wish TI was a bit more forthcoming in that area.

> The datasheet has the note : "The leakage
current is defined in the
> leakage current table with P6.x/Ax parameter." but I can never find
> the specified table!
> 
> Any ideas?  
> 
> Could any one estimate the input impedance?

You'll have to calculate leakage against your error budget. This app
note is from the competition but explains nicely on page 14 how it is
done:

http://www.freescale.com/files/microcontrollers/doc/app_note/AN1058.pdf

> My application is to compare the (resistor voltage
divided) MSP430
> supply voltage with the internal 1.5V reference.  Since I'm battery
> powered I'd like to use a 3Mohm divider.

3M seems quite a bit on the high side. Why so high? Connect the top of
the divider to a free port pin. Configure it as output and turn it
high only for the brief period when you want to check the supply
voltage, then back down. Allow for settling. IIRC the ports present a
resistance of about 30 Ohms to VCC when high.

Also, remember that resistors above 1M are less popular in a 1%
category. This might have cost and availability impacts.

> PS : I've found the mux has 2K ohms of series
impedance....

That won't matter much in the calculation.

Regards, Joerg

http://www.analogconsultants.com/