It's like 40 millionths of an amp (as in 0.00004 A). If you order from them, MAKE SURE you order the 'EM1' optical head and not the 'HEDS' one. They skin you for an extra $3.15 but at least it provides a +/-8mA sink/source capability. It can drive a short cable (<6') while I wouldn't trust the HEDS spec to drive a 1" cable.
It used to be their encoders were rock-solid. Now we have been dealing with stuff like "The G320 passed 'encoder test' but goes nuts when I connect the motor!".
Begin rant:
Something must have changed at USdigital. 40uA is not something in good conscience I would ever use to drive a cable! I would even think twice to even drive a 0.1" long pcb trace with that current. 8mA is barely acceptable but not 40uA.
Figure on 50pF per foot of cable; a 6' foot (2 meter) cable has 300pF of capacitance. How long does it take to charge 300pF to 5V at 40uA? 37.5uS is how long and it's also the rise time. It also means if CH_A goes from 5V to 0V it injects a 38uS 'notch' in the CH_B wire if it is misfortunate enough to be at 5V at the time (50% chance). That kind of 'notch' guarantees malfunction of the encoder signals. Malfunction on the encoder lines means malfunction of the drive.
They offer "Line Drivers" at $15 and change. The 'line driver' is a $0.12 IC that should have been included inside of their encoder. They ship a cable with their encoder, the expectation is the encoder can drive that cable so why isn't the miserable $0.12 driver IC inside the encoder where it belongs? Why are they charging more than 100 times what it costs when it should have been for free inside the encoder?
I can no longer recommend USdigital encoders until they get their act together, stop the cost-cutting and include a proper driver for their cables they supply.
If you want a really superior encoder for just $29, go to
www.digikey.com and tap in 'AMT102-V'. It has a serviceable line driver, has nearly perfect 90-degrees quadrature and get this, it is 16 encoders in one. It has a DIP switch that sets the encoder line-count resolution from 48-lines to 2,048-lines with 14 settings in between. It's a capacitive-type encoder so there is no dirt sensitive optical code-wheel. Finally, the pinout and cable connector exactly matches what's used on the USdigital encoders.
Why the rant? It is because USdigital is costing us time and money dealing with application support calls that devolve into the short-comings of their encoder cable drivers. These calls wouldn't come up if their encoders could actually drive the supplied cables.
End of rant.
Mariss