ADC leakage on LPC2148....?

Started by myfriendmarkis February 8, 2006
I'm using the LPC2148 and have a serious problem. I'm using one of the 
ADC channels to measure battery voltage (across 2AA cells). When the 
microcontroller is off there is a current drain path from the 
batteries through the ADC which flattens the batteries in a few days 
with the board off.... 

The input impedance of the ADC is shown as 20k ohms so it is not 
possible to put a large resistor in series... Please can you help?
	

An Engineer's Guide to the LPC2100 Series

Perhaps buffering with a one-gain op-amp could help? (e.g., LP339).

The problem, as I see it, is that you shouldn't present any voltage to 
the A/D inputs while the chip is out of power. In fact, you can't tell 
if those pins are connected to the A/D block when VDD is 0!

Anyway, by the same manual, you can use a 40K resistor which will give 
you *some* protection (~82A).

Guille

--- In lpc2000@lpc2..., "myfriendmarkis" <myfriendmarkis@...> 
wrote:
>
> I'm using the LPC2148 and have a serious problem. I'm using one of 
the 
> ADC channels to measure battery voltage (across
2AA cells). When the 
> microcontroller is off there is a current drain path from the 
> batteries through the ADC which flattens the batteries in a few days 
> with the board off.... 
> 
> The input impedance of the ADC is shown as 20k ohms so it is not 
> possible to put a large resistor in series... Please can you help?
>
	
Could you use a P-channel MOSFET to switch that connection on or off?
Something like a FDV302P would do the job nicely - is is about 13 ohms
when turned hard on (i.e. with gate 2.7V less than source/drain).
Ensure that the gate floats up to battery voltage when the LPC2148 is
off.

Is the ADC input pin you're using to read the voltage where the
leakage is? There is an input multiplexer to choose which pin each ADC
is connected to. I suspect that when you tell the ADC to use a
different pin, that leakage path does not apply. Unless you allow VddA
or VRef to go below the analogue input value.

I would have expected more current to flow into VddA and VRef then any
analog input. Do you turn these off when powering down the LPC2148?

Hope these comments help,
Danish
--- In lpc2000@lpc2..., "myfriendmarkis" <myfriendmarkis@...>
wrote:
>
> I'm using the LPC2148 and have a serious problem. I'm using one of the 
> ADC channels to measure battery voltage (across 2AA cells). When the 
> microcontroller is off there is a current drain path from the 
> batteries through the ADC which flattens the batteries in a few days 
> with the board off.... 
> 
> The input impedance of the ADC is shown as 20k ohms so it is not 
> possible to put a large resistor in series... Please can you help?
>