I'm working on a project that will necessitate bipolar current measurement across a wide current range. It will need to accurately measure current from 750 mA down to 1 nA, although greater. The amplifier I'm using is an ADA4254, and my ADC is a 24-bit LTC2449.
My preliminary design involved two sense resistors connected to a relay. One resistor was 15 O, and the other was 56 kO. The ADA4254 has programmable amplification up to 128 V/V. See the diagram below.
This should work handsomely for my purpose, except that this circuit will be replicated 48 times, and relays are big and comparatively expensive. So I was wondering if I could do away with the relay altogether.
A 15 O resistor is pretty well the largest resistor I could use (15 O * 750 mA = 11.25 V).
The voltage across the sense resistor at Imin is 15 O * 1 nA = 15 nV. Once amplified, that would be 1.92 uV.
The resolution of the ADC is (5/2)/2^24 = 149 nV. Note that I divided by 2 because the operation is bipolar.
Since 1.92 uV >> 149 nV, I should be able to achieve sufficient resolution, however I do not have a lot of experience with very small signals and I have some potential concerns, so my questions are as such:
- Is this practically achievable?
- Given that the sense resistor cannot be particularly close to the op-amp (~3.5 cm), would this pose a risk of amplifying noise at very low currents?
- Does anyone see another feasible solution?