ADC performance

There are several ways.

1) select 1.5V reference. This pumps the resolution from 2.44mv to 1.46mV
2) apply an external reference voltage which is even lower. The minimum is, however, 1.4V (1.37mV). I don't recommend it.
3) boost your input signal. You can use the internal OpAmp. With its open-loop voltage gain of 100dp, you can theoretically reach a resolution of 21nV. Of course this is only theory. But for an input frequency of <1kHz you can reach a gain of up to 2200, pumping your input voltage into the full ADC input range. At the cost of some added noise and nonlinearity. Of course you can use an external OpAmp instead. And some added trouble, as the input offset voltage of the internal or external OpAmp needs to be compensated (internal is up to +-10mV). I'd say an external chopper OpAmp may help you here better than the internal one. After all, it is quite a small voltage you're measuring.

Jens thanks for reply,

Let me explain what I'm doing, I got a sensor that for each measure it 

takes, sends me 5.5 nA. If i put a resistor between his terminals, I can 

read the volt . There is a problem with the choise for the resistor, 

because if I increased the value of the resistor leads to decrease the responds of the 

sensor. The perfect choise is near 200 K, that leads to 1mV. But for 10 bits of resolution and 1.5 V of reference, leads to 1.5 mV of sensibility of the ADC. It cross my mind  use the oamp of the msp430. I will read about this.

Regards

Gastón

If the sensor outputs a current, then you should use a current-driven OpAmp to amplify it. It's much better than to convert the current into a voltage which increases the power output of the sensor up to a level where it won't be linear anymore.

Basically, any OpAmp is current-driven, but the input circuitry is optimized for a response to input voltages rather than input currents. The output settles if the input currents on both inputs are identical. Since usually an OpAmp is driven by input voltages, the absolute input current does not count and the input stages are just calibrated to have the same current on the same voltage.

In your case, what you need is to directly amplify the input current as provided by the sensor. There are some OpAmps where the inputs are optimized for input current rahter than voltage (the input of the OpAmp plays the role of the load resistor). I don't know how these types are called. TransImpedance Aplifiers or TransConductance Amplifiers? Anyway, I think you're much better served with one of these.

A good source for some op amp designs isOp Amps for Everyone

http://www.ti.com/litv/pdf/slod006b

It goes through all the basics and has several examples for interfacing sensors to ADCs using Op Amps. It even covers Single Supply op Amps which is usually not covered

in most engineering courses and might come useful to you if you're powering from a battery.

GL

HI, Gustavo

Thanks for the link. 

Regards

Gastón