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Topic: Minimum input current for a reliable reading? (Read 780 times) previous topic - next topic

bobthebanana

I'm going to be monitoring battery voltage and other voltages which are higher than the MCU's Vcc for my next project. Therefore I'm using a simple voltage divider to get the voltage down to an acceptable level. Of course, I'd like to waste as little power as possible in the divider so I need to know what the minimum input current to the ADC is so that I can adjust my resistors accordingly. The conversions will take place once a second if it makes a difference. I've already tested and found that less than 30uA is sufficient for digital inputs but I'm not sure about analog inputs.

HugoPT

The ADC input is a high impedance input so in theory the current is not important at all.It will drain a very very low current.
Be careful about the input voltage levels.You must insure the input level does not exceed the max input which is about the mcu power supply
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bobthebanana

That's what I figured. But how low is past the point of "not important at all"? Would you say 50uA is good enough?

MarkT

You need to put a 10 to 100nF capacitor to ground on the relevant analog pin - then the divider can be almost any resistance.

If you are only ever using one analog input this probably doesn't matter, but if you are reading more than one its important the impedance is 10k or less for full accuracy - a capacitor will drop the (dynamic) impedance to a very low value without compromising on resistor values.

Since you are measuring battery voltage the bandwidth can be very low without issues. (the cap and the resistor network form a low-pass filter).

[ When you read different analog input pins an internal multiplexer circuit will switch just before the ADC conversion - the voltage source to the pin then has to charge the ADC's sample/hold capacitor in a few microseconds, hence the 10k or less requirement.  Intrinsically the ADC's input impedance is extremely high at DC, you can treat it as infinite if not switching between pins ]
[ I won't respond to messages, use the forum please ]

bobthebanana

Alright then, I'll have a capacitor to ground as well. Seeing as the voltage will change less than 1 volt per hour there shouldn't be any problem with a 1uF cap or even higher, correct? I seem to be running out .1uF caps!

CrossRoads

"I seem to be running out .1uF caps!"
Time to order some more - they are quite inexpensive.
Order a bag of 100
http://www.dipmicro.com/store/index.php?act=viewProd&productCode=C5K10-50
cost you $2.80 and even less for shipping, have them in 2-3 days.
I buy all kinds of stuff from these guys. Got to have some parts on hands to experiment with.
16 MHz xtals, 22pf caps, 100nF caps, 10K resistors, LEDs, smaller current limit LEDs, shift registers, some of the chips just to have to experiment with.

I just did the same with Tayda Electronics to try them out.
Ordered $38 of switches, resistors & caps, pots, knobs. Was surprised how quick they showed up too.

PB-11D02 Push Button Panel Mount SPST-NO PB-11D02-TH1-00 A-5050 $0.25 Ordered: 10 Shipped: 10 $2.50
PB-11D02 Push Button Panel Mount SPST-NO PB-11D02-TH1R00 A-5051 $0.22 Ordered: 10 Shipped: 10 $2.20
22pF 50V Ceramic Disc Capacitor A-523 $0.009 Ordered: 100 Shipped: 100 $0.90
0.1uF 50V Ceramic Disc Capacitor A-553 $0.009 Ordered: 100 Shipped: 100 $0.90
100 Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3041 $0.009 Ordered: 100 Shipped: 100 $0.90
150 Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3045 $0.009 Ordered: 100 Shipped: 100 $0.90
1.0K Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3065 $0.009  Ordered: 100 Shipped: 100 $0.90
1.5K Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3069 $0.009 Ordered: 100 Shipped: 100 $0.90
220 Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3049 $0.009 Ordered: 100 Shipped: 100 $0.90
270 Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3051 $0.009  Ordered: 100 Shipped: 100 $0.90
390 Ohm 1/8W 1% 0805 SMD Chip Resistors  A-3055 $0.009 Ordered: 100 Shipped: 100  $0.90
270 OHM 1/4W 1% Metal Film Resistor  A-2283 $0.012  Ordered: 100 Shipped: 100 $1.20
1N5818 Diode Schottky 1A 30V A-252 $0.05 Ordered: 20 Shipped: 20  $1.00
DC Power Jack 2.1mm Enclosed Frame With Switch A-2237 $0.16  Ordered: 1 Shipped: 1  $0.16
10uF 100V 105C Radial Electrolytic Capacitor 6x11mm A-4535 $0.03 Ordered: 40 Shipped: 40 $1.20
3 x AA Battery Holder A-766 $0.14 Ordered: 5 Shipped: 5 $0.70
Black Plastic Knob with Red Pointer A-327 $0.19  Ordered: 4 Shipped: 4 $0.76
Black Plastic Knob with White Pointer A-322 $0.19  Ordered: 4 Shipped: 4 $0.76
Black Plastic Knob with Green Pointer A-5098 $0.19  Ordered: 4 Shipped: 4 $0.76
Black Plastic Knob with Yellow Pointer A-326 $0.19  Ordered: 4 Shipped: 4 $0.76
Black Plastic Knob with Blue Pointer A-324 $0.19  Ordered: 4 Shipped: 4 $0.76
10K OHM Logarithmic Taper Potentiometer with Solder Lugs A-1957 $0.50  Ordered: 10 Shipped: 10 $5.00
10K OHM Linear Taper Potentiometer with Solder Lugs A-1961 $0.50  Ordered: 10 Shipped: 10 $5.00
2x40 Pin 2.54 mm Double Row Pin Header Strip A-198 $0.28  Ordered: 10 Shipped: 10 $2.80
2x40 Pin 2.54 mm Right Angle Double Row Pin Header A-638 $0.25  Ordered: 10 Shipped: 10 $2.50
40 Pin 2.54 mm Single Row Pin Header Strip A-197 $0.15  Ordered: 10 Shipped: 10 $1.50
40 Pin 2.54 mm Angle Single Row Pin Header A-199 $0.11  Ordered: 10 Shipped: 10 $1.10

Some of the stuff I don't have an immediate need for, but too many times I've needed something and Radio Shack didn't have it or even carry it, nearest 'local' place is 45 minutes away, and I wanted to get something done and not wait a week for cheap delivery. So I stock up a little.
The DC jack I ordered just to try one out, not sure if it will mate with 5.5/2.1mm walwart cords, but if it does it will be a great panel mount alternative to $1.49 panel mount connectors.
The battery holder I figure will go great with a mini 1284 board, hot glue one on and run wires off to LEDs and buttons or something.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

bobthebanana

Well seeing as my patience is exceedingly low, I can rarely wait 2 and a half weeks for a Tayda delivery (even though it's cheaper overall) when digikey can ship overnight for $8. Never tried dipmicro though, I'll have to check them out.

doughboy


Well seeing as my patience is exceedingly low, I can rarely wait 2 and a half weeks for a Tayda delivery (even though it's cheaper overall) when digikey can ship overnight for $8. Never tried dipmicro though, I'll have to check them out.


it's always a tradeoff between time and money.
it can get quite expensive though if you order 1 or 2 parts at a time and have it shipped overnight.
if possible to plan ahead like crossroads, you can save money and at the same time use a lot less packaging material for shipping. (digikey use the same box packed with the shredded brown paper even if I order just 1 smd part that is as small as a dot)

CrossRoads

"digikey can ship overnight for $8"
Really?  Has cost me $20 for overnight from them.

Dipmicro ships USPS first class mail, flatrate kind of box, very inexpensive.
The material from Tayda showed up really quick, wasn't anything like 2.5 weeks.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

doughboy

good catch. I just tried getting a digikey shipping estimate and don't see anything for $8 overnight lol.

bobthebanana

The shipping rate baffles me for sure. According to the digikey box, it's being shipped from over 2000 miles away!

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