# ADC resolution after voltage divider [duplicate]

I am working on a project where I have to monitor Vehicle fuel levels using an ADC of a micro controller. My fuel signal is a 0-12V signal & my 10 bit ADC is 0-3.6V range. I have 2 questions :-

1. Is only putting resistor divider for 0-12V to 0-3.6V sufficient? Do I need to take care of anything else here?
2. How do I calculate resolution of 10 bit ADC after I drop down the voltage as mentioned in point 1 using a divider circuit?

For an ADC with a high impedance input then a resistive voltage divider is the best approach in terms of simplicity. I'd put a 100nF cap across the lower resistor just to give the imperfect (and sometimes glitchy) input impedance of the ADC a better chance of an accurate reading especially if sampling is multiplexed among a few other analogue inputs.

You might need to make the range greater than 12 volt because on charge the voltage might look closer to 14 volts.

If your ADC is 10 bit and you choose a 0 to 15 volt range then resolution is 15/1024 = 14.6 mV per bit.

• thank you for your reply. First, do I need to use a voltage follower??? second , 14.6mV is 15V/1024. But I am feeding a dropped voltage to my ADC like in this case 0-3.6V. So, I will calculate as 3.6/1024??? – Oshi Mar 21 '16 at 12:48
• You don't need a voltage follower in most situations but if there is a risk of significant over-voltage then using one might be a good idea. Use a R2R type of op-amp. 14.6 mV is the resolution of the 15 volt range. If your ADC reference voltage is 3.6 volts then resolution at the ADC input is 3.6 volts / 1024 = 3.516 mV. Smallprint - check ADC zero offset and gain slope - this will usually reduce the actual range by about 5%. – Andy aka Mar 21 '16 at 12:51
• The I/P impedance of my ADC is 36K. Will that affect anything? – Oshi Mar 21 '16 at 13:17
• Effectively that is in parallel with the potential divider so it's probably a good idea to use a buffer. Are you sure this isn't just an AC impedance quoted in the data sheet. Usually (whatever that means), the DC resistance is Mohms but the AC input impedance is low and glitchy. – Andy aka Mar 21 '16 at 14:47

1) Yes. I'd use... simulate this circuit – Schematic created using CircuitLab

You may want to tweak to utilize the full range of your ADC.

2) (Vref/2^ADCbits) > 3.52 mV per step