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
- 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?
- 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
