AD consume when making a convert?

[hmmpic]

PIC18F26K22.

AD converter consume in "ma" when making a convert.

In the data sheet I can see it consume about 230ua when the AD module is turned on. I can't find out how much it consume when making a convert, I think it will be a lot more because it may switch the "hold" cap. on.

Where to find that information?

[Ttelmah]

The hold capacitor doesn't draw current from the supply rail. There is more current drawn from the Vref (which may also be the supply rail), and from the switching in the ADC itself.

Assuming Vref = supply, you need D013 (supply current to the ADC when switched on), plus D017 (differential ADC current - why use the same name as a digital to analog converter - designed to confuse), plus the Vref current, for the total ADC current.
The Vref current, is typically 5uA when the ADC is idle, rising to up to 150uA during the conversion cycle. This is 'Iref' and for some (unknown) reason is missing from some data sheets (it is for your chip). However it is present in several others with the same voltage range, and converter, and when asked, Microchip said to use the value from one of these....

The sequence is when you enable the channel multiplexer to select the ADC channel, the hold capacitor is connected to the analog input (this is where it draws current from), taking a few uSec to charge close to the voltage on this pin. This is Tacq. Then during the conversion, it is _disconnected_ from the input, but discharges slowly because of internal leakage. The ADC is a successive approximation type, so you have the current to switch the FET's selecting the internal comparator and latches, and the load on the Vref to generate the comparison voltages.

The current is not a 'lot more'. On most chips it is under 0.5mA total during the conversion.

Best Wishes

[hmmpic]

Thanks.
Reason I ask is the system is powered form LI-PO cell when the supply is not switch on.

I use the internal VRef (about 30ua from datasheet) and the then AD converter.
Both are on all the time, but can be switch off in software.
Before making a lot of change I want to know how much power I will save.
Every hour the AD make 10 full conversion. So maybe I can save about 7-10 ma/h over 24 hour, or 300ua/h every hour, it will not change anything, the cell is 200ma/h, I only need power for 3 days.

Thanks.

[Ttelmah]

A total conversion only involves Tacq (depends on the impedance of the source feeding the ADC, but say 20uSec 'worst case), and then the cycles to do the conversion (typically about 6uSec). So the ADC only needs to be powered for under 30uSec/conversion, with under 1mA drawn at this time. So 0.00003mA/h per conversion.

I have logging systems using a single LiPo cell, that run in excess of 5 years doing 60 conversions/hour, on a smaller cell than yours (140mA/h).

What are you using the internal Vref for?. You do realise this cannot power the ADC?. To use the internal Vref with the ADC, you have to feed the ADC Vref from the supply, and then perform a second conversion 'reading' the Vref, which allows you to calculate the battery voltage from this, but doubles the time the ADC has to be on....

Best Wishes

[hmmpic]

About the VREF, I explain it wrong, it is the "FVR BUF2" I use. It is assigned to the AD converter...

About my system, it is a embedded GSM system. The GSM module make some connect every days, and it drain! When sending data it drain about 2,5 Amp in burst. The 3 days is maybe 3-6 GSM connect.

[temtronic]

query:
Is there any reason why you can not use larger batteries? The 'mantra' of 'smaller is better' does NOT apply to batteries.
comments:
I typically use 4 AA where 2 coin cells would work. I can get years of trouble free performance. Modules like GSM can take a LOT of power,'dip' the voltage down to brownout level, reset, oops...what's wrong now..it worked on the bench just fine (powered by wallwart.).
Also consider working temperature! The colder it gets the less energy batteries can supply. Often overlooked, again, 'worked fine in the (nice warm) lab' scenario.

hth
jay

[Ttelmah]

Very true.
Downgrade battery 'expectation' to perhaps 50% of their rating in really cold climates. Also battery ratings are typically at '1S'. a current like 2.5A, even in a short pulse will kill a 200mAh battery very quickly indeed.
One single GSM transaction is going to take more out of the battery than having the ADC converting thousands of times per hour.
You can improve things here by reducing the pulse seen by the battery. A super-capacitor. is ideal for delivering a pulse like this.

Best Wishes

[temtronic]

comment:
Yes, forgot about supercaps ! Nowadays a high Farad unit might be cheaper that a bigger battery. And they do smooth a power supply nicely.
Depending on the project status, might be easier to add than big battery too.

jay

[gpsmikey]

You definitely need a good sized cap across the supply battery - batteries tend to increase in internal resistance as they discharge and trying to pull that kind of current from a small battery will quickly take you to a point where the battery resistance limits the current. Many years ago with the old transistor radios that used the 9V batteries, we found that putting an electrolytic cap across the battery could double the life of the battery simply because the cap kept a low impedance path across the battery as it discharged so the speaker drive could still work well.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[hmmpic]

Hi

So many nice reply and information on this:-)

Just to inform, the li-po cell is custom made, long-life, with PCM. (+25.000 now). The spec is 200ma/h - 25C discharge. When I looked for the cell in the first time, i have a lot cell from different company for testing. Some have the same spec as this one, but on live test they failed! My cell drop maximum 100mv with 3 amp PULS load (on off load) this is real nice, I think. I charge the cell with only 0,5C, and there no defect cell's over 3 year now. There are so many company there made these cell's, but not all of them is well made!

The GSM module is only consuming the drain for short time, 200ms in pulse of us. I agree in the beta test for the design, there was some power supply problem, because it is a real tiny and compact design...