Input Impedance of A0-A5

I could not find any information in the data sheets about what the input impedance(s) of the analog input pins (A0-A5) were. I want to know if they would work for a certain experiment I'm running that requires that the voltmeter has an input impedance in the order of megaohms. If they are not of the exact value I need, I can put a load resistor on it to make the current through it the value I want. Any information on the input impedance would be helpful.

The ATmega328P datasheet has this info way in the back: Table 28-16 (page 328). The analog input resistance is claimed to be 100 Mohms.

During an actual sample, the input resistance is temporarily a lot lower as the sampling capacitor is charged up so it is recommended that whatever you connect to the A/D have an output impedance of 10k or less for best accuracy.

-- The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

it is recommended that whatever you connect to the A/D have an output impedance of 10k or less for best accuracy.

This is to allow the A/D input capacitor on the sample and hold to charge up in the time allotted to it between switching the input multiplexer over and starting the conversion. If it is higher than this you might see what looks like cross talk between channels as you switch them over.

Big thanks Grumpy_Mike, I had this exact issue and now it makes perfect sense. I was using a voltage divider with a pair of 1M resistors to step down to half voltage so it was safe to measure voltage from a 2 cell lithium polymer battery (which goes as high as 8.4V). I couldn’t figure out why there was tons of cross talk until I read this, and of course with such a high impedance, plus the high sample rate I was using, it makes sense that the capacitor didn’t have time to charge or drain after the multiplexer switched.

The datasheet has a circuit schematic for the relevant parts of the analog circuitry, multiplexer & sample/hold cap - try looking in the section about the ADC!

There is full discussion of the impedance requirements of a source. The bit that's not immediately obvious is that the sample/hold cap has only 6us in which to charge and settle in normal operation, fortunately its very small (14pF)

You can use an op amp voltage follower between the voltage to be measured and the analog input. The op amp has high impedance and low output impedance in the voltage follower configuration so it functions as an impedance matching circuit.

1 Like

gerdes1723: it makes sense that the capacitor didn't have time to charge or drain after the multiplexer switched.

That's why you use a 100n capacitor from the tap of the voltage divider to ground. When the A/D muxer comes past to measure the divider, it sees a solid voltage (the 100ncap) to sample from. Now you can even use 10Megohm resistors.

http://jeelabs.org/2013/05/16/measuring-the-battery-without-draining-it/

Edit I seems the 100Megohm input impedance is just a number grabbed out of a hat. I have measured >>1000Megohm, at room temp, with nothing else connected to adjacent pins. Charge a low leakage large value film cap to e.g. 3.3volt, and measure it every second. With noting else connected than the cap, the A/D value will stay there for days. Leo..

1 Like

so in short using an LM324 as a buffer might be a good idea.

In the case of 1000 meg ohm input resistance for say use with PH electrodes and redox potential electrodes the ca3130 or ca3140 with guard ring round the + input and the op connected to the - input.

Warning there is a risk of static damage.

I want to know if they would work for a certain experiment I'm running that requires that the voltmeter has an input impedance in the order of megaohms. If they are not of the exact value I need, I can put a load resistor on it to make the current through it the value I want. Any information on the input impedance would be helpful.

All digital DMMs would have an input impedance of about 100 Mohms. Wawa's recommendation to add a 0.1 uF cap across any analog input that should be steady state (such as a battery) is a good suggestion you should follow.

There is full discussion of the impedance requirements of a source. The bit that's not immediately obvious is that the sample/hold cap has only 6us in which to charge and settle in normal operation, fortunately its very small (14pF)

SuusiMB: so in short using an LM324 as a buffer might be a good idea.

Depends. One drawback could be output voltage swing (V+ −1.5).

raschemmel: All digital DMMs would have an input impedance of about 100 Mohms.

AFAIK common DMMs have a 10Megohm impedance. Try measuring a 9volt battery normally and through a 10Meg resistor. Think of the DMM being part of a two-resistor voltage divider. Leo..

Wawa: Depends. One drawback could be output voltage swing (V+ −1.5).

I have seen lots of lm324 applications that use +- 5 volts and even more frequently powered by a pp3 battery. but you are right it is best to warn of the risks of +- 15 volts :)

SuusiMB: ...it is best to warn of the risks of +- 15 volts :)

Read it again. The dot might be hard to see. Opamps like the LM324, LM358 etc. have an output voltage swing that is ~1.5volt less than the positive supply. So if the opamp's supply is 5volt, the output of the opamp can only swing between (almost) 0volt and ~3.5volt. About 700-720 A/D values. Leo..

If the point is to buffer an analog input using an op amp voltage follower you wouldn't be running the op amp from 5V anyway. You would power it with the 8.4V battery ( obviously), and put yor voltage divider on the op amp output ( again obviously) because the ADC input impedance is way too high to load the op amp and also because you were talking about using large values for the voltage divider. I've used voltage followers powered by their input voltage ( Vcc connected to +V input) because the op amp I used was the LT1215 which doesn't draw much . I don't think the LM324 draws much either. If the op amp is not a "Rail to Rail" type there will be a slight difference between input and outout but it is consistent and can be measured so it's not a problem if you are aware of it.

The input common mode voltage range of the LM324 is 0 to (V+ -2volt). Leo..