I'm trying to find the minimum current which I can measure with the 1T feedback resistor...
Time to build a greater voltage divider.
Besides the prior 1/10 and 1/100 voltage division, I added 1/1,000 and 1/10,000 options. That allowed measurements of 1.6 fA and 160 aA via my ADA4530-1 current to voltage converter using its largest 1T feedback resistor. The ADA4530-1_cal picture shows my converter with my grounded battery powered calibration box attached. There's not much unshielded space for power line noise to influence the reading. While the banana connectors aren't anything special, I'm using Teflon insulated wires.
The 1.6 fA measurement didn't look too bad after the reading stabilized--see the 1.6fA attachment. Due to the large value resistors used with the small current, the stray capacitance in my circuit produced a significant time constant and delay before the reading became steady. While the reading did stabilize close to the expected 1.6 mV output level for 1.6 fA, it wasn't steady enough for me to rate it as a success. With the accuracy at 16 fA adequate to satisfy me, about 10 fA may be the limit for my circuit to have reasonable accuracy. Based on my results in using increased value resistance, feedback accuracy and my recent results using the ADA4530-1 IC, it's likely if I paid $68.96 for a 10T resistor I would be able to have accurate readings down to 1 fA.
With the mentioned results at 1.6 fA, I wasn't expecting much luck in my final measurement at 160 aA. I'm not used to dealing with attoamps. As the 160aA attachment shows, it wasn't a complete failure. The reading did center around 0 to the desired -0.16 mV to indicate 160 aA. But, it only points to a very vague low current range. The big jumps on both sets of today's readings could be due to power supply spikes.
I also wanted to take some readings using my ADA4530-1 circuit with my 1T ohm current limiting resistor along with up to 1,000 volts [with 2 mA current limiting] to take a comparative set of readings using my circuit's different feedback resistors. The difference in the indirectly measured resistance of my external 1T resistor would partially depend on the voltage coefficient of the external resistor and also on the relative accuracy of my at least 1% accuracy feedback resistors (except for both 1T resistors being 10%). I don't have any way of confirming which was more influential on my results other than what the 1% accuracy resistors suggests. The results were:
| FB R | V to R | Voltage Out | Measured R|
1T 1.0000V -1.0013V 0.9987T
100G 10.000V -1.0451V 0.9568T
10G 100.00V -1.0561V 0.9469T
1G 1000.0V -1.0915V 0.9162T
This has been an interesting project.