I have a question regarding very basic op-amp and adc theory that has been bothering me because I haven't found a clear answer to anywhere:
On a 12-bit ADC, if my voltage reference is 1.24 V, my resolution will be ~0.6 mV. If I'm interested in having a resolution of 0.1 mV, will amplifying my sensor's analog voltage range (0 to 200 mV) to cover the entire range of the adc (1.24 V) effectively increase my resolution?
Can I calculate my new resolution by dividing 200 mV by 2048 (= ~0.1 mV) or is the resolution fixed no matter what percentage of the adc range i use? In other words, will using an opamp help me get a higher effective resolution or will it stay at ~0.6 mV no matter the amplification. Is the equation for calculating the new resolution different when adding an amplifier? Do I lose the simple linearity between the output codes and voltages.
As a follow-up question: If amplifying my sensor's output doesn't get me to 0.1 mV. Is there a way I can get down to 0.1 mV using a 12-bit ADC and 1.24 V reference? Are there any external circuit components I can integrate to achieve this? Am I forced to use a separate 16-bit adc and connect it to my microcontroller? Ideally, I want to minimize my circuit's complexity and use the adc in my microcontroller (BlueGiga BLE112) and use an op-amp externally, but I'm open to other ideas.
Thank you for taking the time to read this, and pardon my gaps in knowledge, trying to get past the learning curve as rigorously and best as possible.
Best Answer
The nominal resolution of any ADC is V(span) / 2^^n where V(span) is the range of voltage it can convert (1.24V in your case) and n is the number of bits.
For your 12 bit device that is 1.24 / 4096 = 303 microvolts per step. This is the base resolution regardless of what you feed the ADC. Note that the actual precision of the ADC is a bit more complex.
If you have a signal with a maximum size of 200mV, then amplifying it to take advantage of the full span of the ADC will yield more accurate results in the measurement, but you will need to take care in your choice of amplifier.
There are devices specifically designed as ADC drivers, and which one you use is dependent on the electrical characteristics of the ADC you are using.
There is an excellent online tutorial on the signal chain at Analog Devices