The current measurement seemed very easy task for me in the beginning. I thought that measuring voltage over low resistance shunt is piece of cake. Later, I realized that this is not true : it is really a challenge and the market prove it. I found only 4 DMM, which have less than 500 ppm relative accuracy per year in the A/mA/uA range (with one exception : 2A range for Keithley 2002).
These are: Fluke 8508A, HP/Agilent/Keysight 3458A, Transmille 8081 and Tek/Keithley 2002. In the table below, the accuracy specification for one year is show:
According the datasheets, the most accurate current measurement can be done with the Fluke 8508A and Transmille 8081 DMM, following by the Keysight 3458A and Keithley 2002.
The rest bench-top DMM on the market have accuracy starting from 500 ppm and can finish to 1500+ ppm accuracy for the reading.
The accuracy of the current measurement depends on the:
- Value/stability/TCR of the shunt resistor
- The CMRR/Vos/Vos∆Time/Vos∆Temp/OpenLoopGain/GainError∆Temp of the used operational/instrumental/differential amplifiers in the current measurement schematics.
- Matching resistors tolerance/TCR for the differential operational amplifier.
So a lot of factors contribute to the current measurement accuracy and trade-off has to be made. May be the biggest trade-off is the value of the shunt resistor. For bigger current ranges, the value should be in the sub-ohms range for decreasing the power dispersion. But there are not too much resistors in the sub-ohms range which have let say less than 100ppm per year stability. For example the 0.5 Ohms resistor from Vishay series Y1690 have typical 50ppm load life for 2K hours. However the trade-off is the availability and the price: there is minimum order quantity of 500 pcs, which cost ~7K+ USD.
So a lot of factors contribute to the current measurement accuracy and trade-off has to be made. May be the biggest trade-off is the value of the shunt resistor. For bigger current ranges, the value should be in the sub-ohms range for decreasing the power dispersion. But there are not too much resistors in the sub-ohms range which have let say less than 100ppm per year stability. For example the 0.5 Ohms resistor from Vishay series Y1690 have typical 50ppm load life for 2K hours. However the trade-off is the availability and the price: there is minimum order quantity of 500 pcs, which cost ~7K+ USD.
If a bigger value of the shunt resistor is selected, the stability and the availability are better, but the trade-off is the parameter which can be critical for some type of circuits under measurement. This parameter is called "Burden Voltage" and it is the voltage drop caused by the DMM in the measurement circuit. It should be kept as low as possible. The voltage drop of the DMM includes :
- the voltage drop in the cables from the circuit under test to the main DMM PCB.
- the voltage drop in the shunt resistor.
- the voltage drop in the fuse, which vary with the current amount.
I will give an example, how the burden voltage can be a blocker issue. In the example I will use the internal idle power consumption for the Xilinx Spartan 6 FPGA. The power supply voltage Vccint of the FPGA core have nominal value of 1.23V, but if the value drop below 1.2V, the FPGA can stop to work. That means that no more than 30 mV should be the burden voltage of the DMM used for measurement. The quiescent Vccint supply current (idle current) of the Spartan 6 LX150 is 51mA and lets check if we can measure it using these high end DMM:
- The Fluke 8508A have 1.2 Ohms input impedance for 200mA range, which will generate 61.2mV voltage drop. But using 2A range with 0.3 input impedance this will generate only 15.3mV voltage drop.
- The Transmille 8081 have 10 Ohms input impedance for 200mA range, which will generate 510mV voltage drop. But using 1A range with 0.5 input impedance this will generate only 25.5mV voltage drop.
- The Keysight 3458A have 10 Ohms shunt resistor for 100mA range, which will generate minimum 510mV drop voltage (without voltage drop over the fuse). For the 1A range, the shunt resistor is 0.1 Ohms, which will generate 5.1mV voltage drop.
- The Keysight 3458A have 10 Ohms shunt resistor for 100mA range, which will generate minimum 510mV drop voltage (without voltage drop over the fuse). For the 1A range, the shunt resistor is 0.1 Ohms, which will generate 5.1mV voltage drop.
- The Keithley 2002 have maximum burden voltage 0.35V for the 200mA range. The burden voltage for 51mA is : 0.35V*(0.051A/0.2A) = 0.08925 V which exceed the maximum allowed voltage drop with ~40mV. But if the 2A current range is used, the maximum burden voltage is 1.1V, which will generate 28mV voltage drop which is in the 30mV range limit (1.1V*(0.051A/2A) = 0.02805V).
Given examples above shows, that in mA ranges the voltage drop can be blocker issue, but using the A range, user can measure the idle current for the Xilinx Spartan 6 FPGA. These examples are not only valid for the mention high end DMM, but also for the low and middle class DMM.
One workaround for the burden voltage issue is the uCurrent™ product made by Dave Jones from eevblog.com. It is an adapter, which use lower shunt values and amplifies the voltage drop. Here the trade-off is the missing current protection.
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