8 channel bridge input with a 24 bit sigma-delta ADC

• 8 full bridge channels

• optimised for low power consumption (15 mW)

• 240 SPS max (all channels)

• 22 ENOB maximum at lowest sample rate, 16 ENOB at 240 SPS

• nwr-2u form factor (55 x 70 mm)

Warning

Documentation is WIP

ain-bridge-8ch-24-nwr2u board is a 8 channel multiplexed digitiser for full bridge sensors, such as strain gauges. Sensors are excited with a 1.8 V voltage source and measured by a AD7124-8 analog-to-digital converter with a complete analog frontend comprising of a multiplexer, input buffers, programmable gain amplifier, sigma-delta modulator and configurable digital filters.

Conversion results are read by a microcontroller connected to the NW-R backplane using CAN bus.

Front panel includes a 36 pin MDR connector for sensors and two LEDs with configurable blinking pattern.

Project log

Date	Description
2021-05-29	1.0.0 prototype boards were submitted for manufacturing (JLCPCB)
2021-06-22	Prototype boards have arrived
2021-06-24	Two prototype boards populated
2021-06-25	Testing firmware, ADC communication, configuration, conversion result reading

Contents

• 8 channel bridge input with a 24 bit sigma-delta ADC

  • Specifications

    • Operating conditions

    • Power consumption

  • Downloads (design files, firmware)

  • Pictures

  • Hardware design description

  • Notes

Specifications

Operating conditions

Parameter (abbr.)	Conditions	Min	Typ	Max	Unit
Temperature		-40	25	+85	°C
Humidity	non-condensing	0		95	% RH
Input voltage		4.5	5.0	5.5	V

Power consumption

The following figures are based on first measurements running testing firmware without all features enabled. The consumption may change a bit as a result of firmware optimisations and featureset changes.

Unless otherwise stated, measurements are done in the following conditions:

• input voltage: 5 V

• LED pattern: both LEDs disabled

• no backplane communication

Parameter (abbr.)	Conditions	Min	Typ	Max	Unit
Input current	continuous conversion of all 8 channels, 1 SPS		2.0		mA
.	continuous conversion of all 8 channels, 30 SPS		2.0		mA
.	continuous conversion of a single channel, 1 SPS		2.0		mA
Input current	White LED fully on		+0.6		mA
.	Blue LED fully on		+0.6		mA

Downloads (design files, firmware)

Description	Download links
Schematic (PDF export)
Board layers (PDF export)
Board manufacturing files (Gerbers)
KiCad project files

Pictures

KiCad 3D renders

Hardware design description

Notes

The board was designed and drawn in about 15 hours. Many compromises have to be made in order to finish the board withing the specified time. Hence, some wanted features were not implemented. When the board is validated in the field and gives reasonable measurement results, these features will be implemented in next revisions:

• programmable excitation voltage from about 1 V to 10 V, probably using a SMPS buck-boost with a linear post regulator