MEP EP#287: Biased For Arduino

Bias Test System

• Got the first board built and installed on an arduino
• First tests were a success
• Arduino library - works “first” time
  • Adien Akhmad
• 2.5V reference voltage
  • MCP1501T-25E/CHY
  • 0.1% initial accuracy
  • 50ppm drift
  • I have measured a handful of these at work and have found the output to be within this range - 2.498
• Arduinos and USB are noisy
• Resistive measurements via constant current
  • Opamp driven constant current source
  • 1mA output current measured @ 1.001mA with a 1k resistor
  • Output current is somewhat load dependent - higher at higher resistance values
  • Possibly should have gone with a parallel opamp config
• Validation
  • Voltages seem to be reading well.
  • 0 to 300V was within 1% of meter
  • Small “offset” reading
    • This is due to input buffer amp offset.
      • Datasheet calls out 125 as typical and 750 as max
    • I have seen the 100 to 400uV range
    • Will calibrate out
  • When doing a simple offset correction I can get 0.00000X readings
  • For resistive measurements there is a strange gain offset error
  • Verified the current and voltage but the readings to not match up
  • About 1.7% low for all readings
• What is next
  • Finish validating the hardware with better standards and meters
  • Merge the scipi and ads1256 library
  • Make the gui
  • Profit

Gripes about arduino

• When do you bite the bullet and fix a problem?
  • Pin header
  • Mounting holes
• Solution….
  • Custom pin header
  • Custom plastic “sled”

Supply chain oddities

• Surprisingly Linear Tech devices are in stock but not Analog Devices?
• Working on that Linear Tech Automotive power supply design
• LT8672