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'[PIC] ADC voltage divider - reducing power consump'
I've got a circuit where I want to measure the voltage of the battery
supplying power. Since this is much higher than the VCC (I'm using a
regulator to provide power), I need to use a voltage divider resistor
network to drop the voltage to the correct input range. However this
resistor network draws power even when in sleep mode - I'd like to reduce
this as much as possible, since I don't want to drain my battery when it is
switched off, as it may be for long periods (hours, days).
First of all I can use high value resistors - however according to
datasheets the max input impedance is 10k, which would suggest that is the
maximum value of the lower resistance. Given I only need to measure every
second or so though and the voltage isn't varying rapidly, can I use much
higher value resistors and rely on not needing to charge the input capacitor
up very much each time, and so ignore the datasheet?
Secondly, are there any clever techniques I can use with FETs or other input
pins to cut of the current flow through the divider? At present I have the
ground end connected to an input pin and set that to a low output when
running, but to a high impedance input when going in to sleep mode - that
helps a bit, but I still have the current flow through the upper resistor
into my analogue input pin due to the clamping diode which prevents the
voltage there rising much above VCC. Got a couple of ideas using FETs, but
interested in anything anybody has done.
p.s. my apologies, this should in a way be OT, as it's actually being
implemented on an AVR at present, but given I've done similar things on a
PIC (in fact my development circuit was originally designed around one), and
the input requirements for the ADC appear almost identical to that on a PIC,
it is at least pretty relevant to that, and PIC based solutions others have
used will be extremely relevant!
|Chris McSweeny wrote:
> First of all I can use high value resistors - however according to
> datasheets the max input impedance is 10k, which would suggest that is
> the maximum value of the lower resistance. Given I only need to
> measure every second or so though and the voltage isn't varying
> rapidly, can I use much higher value resistors and rely on not needing
> to charge the input capacitor up very much each time, and so ignore
> the datasheet?
You may be correct that this would reduce the error induced by
charging/discharging the internal sampling capacitance. Another question worth
asking is does this capacitance charge/discharge each time an A/D conversion
takes place or only each time the ADC input selection is changed?
In such situations I have no problems with using very much higher resistances
(100's of K or several M's) but also add external capacitance directly to the ADC
pin, say 100nF. Overall response time is slow due to the RC constant, but is not a
problem as you're just measuring battery voltage which doesn't change quickly. The
AC impedance to the ADC is now low, and reduces the effect of the sampling
capacitance on the reading. The high resistances will then mostly be a factor to be
considered in conjunction with input bias current and the offset error it may produce.
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: 027 433 4069
I've done this successfully a few times, 100nF adjacent to the PIC ADC
pin acts as a low-impedance source to charge the PICs internal sampling
cap, you can then use hundreds of K or even M Ohms for your potential
Brent Brown wrote:
Nigel Duckworth wrote:
> I've done this successfully a few times, 100nF adjacent to the PIC ADC
> pin acts as a low-impedance source to charge the PICs internal sampling
> cap, you can then use hundreds of K or even M Ohms for your potential
Just watch for leakage currents through the cap, it may skew your readings.
Leakage through an external cap is insignificant unless you are using ultra high value resistors and/or and electrolytic cap. Much more of a problem is the leakage into the PIC's sample hold circuit which is obviously temperature dependant, but which rules out very high value divider resistors if you want reasonable accuracy. The magnitude of the leakage current is described in the datasheet.
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