Voltage burden is what is important here. Model 6517B is as close to an ideal ammeter as its voltage burden is extremely low. However, you can calculate the "effective" input resistance of each range. 20pA range is 20uV / 20pA = 1M ohm 200pA range is 100uV / 200pA = 500k ohm 2nA range is 20uV / 2nA = 10k ohm 20nA range is 20uV / 20nA = 1k ohm 200nA range is 100uV / 200nA = 500 ohm 2uA range is 20uV / 2uA = 10 ohm 20uA range is 20uV / 20uA = 1 ohm 200uA range is 100uV / 200uA = 0.5 ohm 2mA range is 2mV / 2mA = 1 ohm 20mA range is 5mV / 20mA = 0.2 ohms This is effective input resistance. The feedback - is the closest you can get to an ideal ammeter. This means the voltage burden is almost zero. Look in the specs for each range. So the effective input resistance is just a number. It usually does not matter.