MEP EP#287: Biased For Arduino

Podcast Notes

Bias Test System

  • Got the first board built and installed on an arduino
  • First tests were a success
  • Arduino library – works “first” time
  • 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

Special thanks to whixr over at Tymkrs for the intro and outro!