μSMU

μSMU is a small source-measure unit designed for the very low-cost electrical characterisation of photovoltaic cells.

Background

SMUs are "4-quadrant" devices, meaning they can both source and sink current at both positive and negative voltages. This makes them very useful for semiconductor device characterisation - including LEDs, transistors and solar cells.

In photovoltaic research laboratories, a SMU is typically used to vary the voltage applied to an illuminated solar cell, whilst simultaneously measuring the current. This voltage sweep allows us to plot the solar cell's I-V characteristics, and calculate its light-to-power conversion efficiency.

SMUs are generalised pieces of test equipment, designed to be highly sensitive over vast current & voltage ranges. For example, the workhorse Keithley 2400 has a voltage range between 100 nV and 200 V, and a current range between 1 pA to 10 A. This is likely overkill for most research and education applications concerning solar cells, which tend to operate between 0-5 V and μA to mA. The μSMU doesn't intend to replace precision SMUs, rather to supplement them in cost-sensitive areas where such precision is not required.

The μSMU is a USB-powered SMU with a +/- 5 V voltage range and +/- 50 mA source/sink capability. The PCB is only 70mm x 43mm

Function

The μSMU was originally inspired by Linear Technology's DC2591A evaluation board, which demonstrates an I2C address translator IC to interface up to 8 modules containing several I2C devices with an Arduino-style board. Somewhat consequentially, these boards also contain fantastic SMU circuits!

The voltage applied to the device-under-test (DUT) is supplied by a LT1970 opamp driven by a 16-bit DAC on the non-inverting input and a 2.048V reference on the inverting input. The current flowing through the DUT is measured by amplifying the voltage drop through a high-side 50 Ohm shunt resistor using a precision programmable gain amplifier. Both the DUT voltage and shunt resistor voltage drop are measured using a 16-bit ADC. The whole system is controlled using a STM32F072 microcontroller, which presents a USB virtual communications port for interfacing.

Capabilities

Errata

Version 10 (release 1.0)

• The board layout is missing grounding on the MCU for some reason. Make sure to place a couple of vias in the MCU's exposed pad to ensure proper grounding. Sorry!

Changelog

Version 10 (release 1.0)

• Voltage DAC changed from a 12-bit Microchip MCP4725 to a 16-bit TI DAC8571
  • Can now achieve sub-mV voltage steps
• Current sense amplifier changed from an Analog LT1991 to a TI PGA281
  • PGA281's gain can be programmed using 5 GPIOs ranging from 0.125 to 176. This allows low currents to be gained more than high currents, improving current resolution.
• Current shunt resistor increased from 10 to 50 Ohms.
  • The programmable current sense amplifier (PGA) means we can use a high value shunt resistor and decrease the gain when measuring high currents.
  • Voltage drop across a 10 Ohm shunt is sensed by the power amp (U10) to impart programmable current limiting. Measuring this across the main 50 Ohm shunt would limit the current output to ~10 mA due to limitations in the LT1970
  • The increase in current shunt value along with the new PGA means we can now sense currents on the order of ~10 nA
• Buffer amplifier changed from a quad Maxim MAX44252 to 4x Gainsill GS8331
  • Less expensive and lower V_OS
• Electrostatic discharge protection added to USB port (U5)
• Isolated DC-DC converter replaced with bipolar switching regulator TI TPS65131 supplying ±9.7V
  • Extra headroom for power amplifier to push higher currents through 50 Ohm current shunt
• 4.5V LDO added for DAC and ADC
• USB-C port replaced with lower cost 2.0-pinned version

License

Hardware

CERN Open Hardware License Version 2 - Permissive (CERN-OHL-P-2.0).

Software

GNU General Public Licence v3.0.

Documentation:

This work is licensed under a Creative Commons Attribution 4.0 International License.