Research On Key Technology Of Source Measure Unit Based On FPGA

The source-measure unit(SMU)is a new test and measurement instrument with fourquadrant power output capability and it can be used as a programmable constant current source or a constant voltage source,electronic load,and digital multimeter.Due to these features of the SMU,it has been widely used in electronic product production testing,integrated circuit packaging testing,electronic measurement and other fields.This paper studies the implementation principle of SMU and proposes an SMU-based FPGA design scheme.By introducing a bipolar output circuit at the output of the DAC and using a switching module to switch the feedback loop of the high-power operational amplifier,the SMU can output two kinds of power,which are constant voltage and constant current.Thus this SMU have four-quadrant power output capability.When it was used as a measurement unit,it can be effective to access two ADC modules into the output of SMU to measure the voltage and current of the load.The feedback loop controls the output to constant voltage mode or constant current mode.In the constant voltage mode,the feedback loop of the power amplifier circuit is the conventional voltage series negative feedback circuit;in the constant current mode,the power amplifier is based on a kind of improved Howland current source circuit to realize the transform from the input voltage signal to output current signal in the same direction.When the SMU was used as a measuring unit to measure the voltage,one ADC of the hardware system was used to measure the voltage between the output pin and the ground pin.When it was used to measure the current,another ADC was used to measure the voltage at the sampling resistor.Then the current through the sampling resistor will be calculated according to Ohm’s law.Due to the connection between sampling resistance and the load is in series,so the current through the sampling resistor is equal to the current through the load.The program adopts the instruction set method to achieve remote control through the serial communication protocol.For the filtering of the ADC measurement data,the method is to adopt average filter algorithm to reduce the ripple effect and improve the measurement accuracy.In the software design,the PID automatic control system is used as the software main frame.Under the constant voltage or constant current mode,the output voltage or current is monitored in real time.Even if the load impedance changes,the system can respond to the adjustment output in time to ensure the stability of the output.The test results show that the scheme of this paper is feasible.The system output is stable.The fluctuation range of output is about 1‰.The dynamic output range of voltage is ±12V.The dynamic output range of current is ±5A.The output error and measurement error of voltage is less than 0.5m V.The output error and measurement error of current is less than 0.3m A.it can meet the design claim.