How can I achieve maximum resolution with Source Measure Unit(SMU)?

Solution

The more you choose narrow range, the more precise resolution you can get.
For example, if you are using PXIe-4136 for current measurement, you can get maximum 1pA resolution when you set the minimum range, 1uA.

To change the measurement range, you can use 

NI power supplies and SMUs use one or more ranges for voltage and current output, as well as one or more ranges for voltage and current measurement. Use the highest resolution (smallest) range possible for a particular application to get maximum output and measurement accuracy. Refer to the specifications document or the device book in this help file for your device for more information about what ranges are available for a particular channel on your device.

The measurement range is implicitly selected based on the configured output range. Thus, you cannot change the measurement range independently of the output range. The selected measurement range is large enough to measure any voltage or current within the configured output range.

When the output function is set to DC Voltage using the niDCPower Configure Output Function VI or NIDCPOWER_VAL_DC_VOLTAGE using the niDCPower_ConfigureOutputFunction function, the voltage level range and current limit range are in use. When the output function is set to NIDCPOWER_VAL_DC_CURRENT using the niDCPower_ConfigureOutputFunction function or DC Current using the niDCPower Configure Output Function VI, the current level range and voltage limit range are in use. The same relationships hold true during pulsing between pulse outputs, pulse level ranges, and pulse limit ranges.