Burden voltage is because current is measured by voltage drop across a known low value resistor. The Keithly does this instead by an active measurement of using an opamp virtual earth to generate a voltage proportional to the current internally to keep the input voltage close to zero. TO do this for higher currents than the 1mA offered means you need an opamp capable of sinking or sourcing the current on it's output thus you will need one capable of doing 1A linearly for a 1A measurement, 10A for a 10A unit. This current has to come from a power supply, so will make it both a very big and very expensive unit, especially if you are looking for high accuracy, as high current power amplifiers normally do not have the best offset and drift characteristics at low currents, and definitely not at full power. not much good having a 10mV burden voltage if the opamp has 100mv of drift over a period of 1 minute, or 200mV of offset as it goes from 0 to 10A.