I’ve been meaning for a while now to cycle back and work on the SSMU project again. The Hackaday Prize entry submission deadline was quickly approaching so I decided to put something together for motivation. Most of the details will be tracked over on their projects site: http://hackaday.io/project/2424-Simulation-Source-Measurement-Unit
The idea is still the same – integrate core functions from lab equipment over the audio frequency range and integrate with software running a circuit simulator. Instead of sticking to through hole components and an MSP430, I’m going to use a STM32L100RBT6 and a custom PCB. The current planned features are the following:
1-2* voltage sources (0-15V, 1MSps DAC)
1 current source (up to about 50mA)
2+ voltage measurements (12bits, 1MSPS ADC)
2 current measurements
2 Bode plot measurements (gain and phase up to 100kHz)
Fixed and adjustable power supply (3.3V, 5V, 0-9V)
Adjustable current limits for 0-9V rail and DAC output
Protocol analyzer (at least I2C, SPI, and UART)
Speaker output stage
USB and wireless (nRF24L01+) connectivity
*One voltage source is used for the current source but will be accessible when the current source is not in use
I’ve picked most of the components except for some current and voltage protection chips. The BOM is sitting somewhere around $20-25 so I’m on track to not break the bank.
Instead of measuring magnitude and phase by brute force sampling and software, I’m going to extract it with a few op amps. In the picture below, U1 attenuates the voltage at the DUT (can range up to 9-15V) and optionally provides amplification for lower amplitude signals. C2 compensates for some of the input capacitance the DUT might present. U2 and D1 form a precision rectifier and along with C1 constitute a peak detector. This relatively stable voltage can then be sampled by the microcontroller’s ADC as the magnitude of the waveform.
This value is then divided and compared with the input waveform again to produce a digital output representing when the waveform is above or below this fixed voltage. When compared with the digital output for the other Bode circuit (one at either side of the DUT) the microcontroller can determine the phase shift.