Post by thread:[PIC] ADC voltage divider

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 eMail: spam_OUTbrent.brownTakeThisOuTclear.net.nz

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 divider. Regards, Nigel Brent Brown wrote: {Quote hidden}> > 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. > >

{Quote hidden}>-----Original Message----- >From: .....piclist-bouncesKILLspam@spam@mit.edu [piclist-bouncesKILLspammit.edu] >On Behalf Of Jake Anderson >Sent: 11 February 2007 08:23 >To: Microcontroller discussion list - Public. >Subject: Re: [PIC] ADC voltage divider - reducing power >consumption in sleep > > > >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 >> divider. >> >> Regards, >> >> Nigel >> >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. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================