Regardless of inherent protection due to increased shunt resistance when switching ranges, and other factors, I'd still like to know what the maximum continuously sustainable safe current would be for each range [...]
Since nobody has replied about this and the BOM is now available, so we can get information for all the components, I'll take a stab at this myself. This will be a very conservative estimate based on the information available in the datasheets.
I'll assume that the worst-case scenario is with the power off. The shunts will still be connected across the current input terminals J1 and J2, and feed into the inputs of U1. None of the datasheets for the active components give internal schematics, so I'll assume that:
- +V and -V will be held to the same voltage level through a reasonably low impedance (at least via the 200K of R6 and R7).
- The inputs of U1 will have a very high impedance even with no power on the chip, at least within our safe operating range.
- There will be a reasonably low impedance path between VGND and either +V or -V. Therefore, the - input of U1 will couple through R5 to VGND (and thus +V and -V), so will not be a factor. I can't be sure of this without internal schematics but it's the safest assumption.
Current through the shunts will impress a voltage upon the + input of U1 via SW1. The + input is high impedance, so the voltage on it will be equal to the voltage across the shunt resistor(s) selected by SW1. The only absolute maximum rating given for U1's + input is "(-V - 0.3V) to (+V + 0.3V)". I'll assume this is even with no power on the chip. No information is given on what amount of current or power is safe if we exceed these voltages, so we can't exceed 0.3V across the shunt resistors(s), with current flowing in either direction. To be safe we should probably lower this to 0.28V.
The other limits we have are the power ratings of the shunts and the current ratings of SW1's contacts.
For the mA range:The shunt is 0.01ohm R1. SW1's contacts are rated 4A at 28V DC and no other higher currents are specified for other voltages, so we have to make 4A our maximum. R1 is rated 1W at ambient temperatures up to 70C. At 4A, R1 will draw 0.16W, so it's safe. At 4A, R1 will be at 0.04V, so U1 is safe.
For the uA range:The shunt is formed by a series-parallel arrangement of R9, R1 and R2 but R1 and R2 are insignificant for these calculations, so we can just consider 10ohm R9 to be alone. The safe voltage of 0.28V across R9 will require 0.028A. R9 is rated 0.1W at ambient temperatures up to 70C. At 0.28V, R9 will draw 0.00784W, so it's safe. SW1 obviously is safe.
For the nA range: The shunt for this range is 10Kohm R2. The safe voltage of 0.28V across R2 will require 0.000028A. R2 is rated 0.063W at ambient temperatures up to 85C. At 0.28V, R2 will draw 0.00000784W, so it's really safe (as SW1 still is).
Summary:For the mA range, the maximum safe continuous current is 4A, which is 3.2 times the 1250mA full scale value.
For the uA range, the maximum safe continuous current is 28mA, which is 22.4 times the 1250uA full scale value.
For the uA range, the maximum safe continuous current is 28uA, which is 22.4 times the 1250pA full scale value.
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