Load Cell 5 Click Board™

How Does the Load Cell 5 Click Board™ Work?

The Load Cell 5 Click Board™ as its foundation uses the AD7780, a pin programmable, low power, low drift 24-bit ΣΔ ADC from Analog Devices, that includes a PGA and uses an internal clock. The AD7780 typically consumes only 330μA and simplifies this weigh scale design since most of the system building blocks are already on the chip. The AD7780 has two filter options selectable via FIL pin(low state - the 16.7Hz, high state - 10Hz) and a Power-Down Mode allowing the user to switch off the power to the bridge sensor and power down the AD7780 when not converting, increasing the battery life.

Since the AD7780 provides an integrated solution for weighing scales, it interfaces directly to the load cell. The only required external components, which are also on the Click board™, are filters on the analogue inputs and capacitors on the reference pins for EMC purposes. The low-level signal from the load cell is amplified by the AD7780's internal PGA programmed via the PWM pin of the mikroBUS™ socket, labelled as GN, to operate with a gain of 128 or 1. The conversions from the AD7780 are then sent to the MCU through SPI serial interface, where the digital information is converted to weight.

The Load Cell 5 Click Board™ uses the 6-wire load cell configuration, which has two sense pins, ground, power supply, and two output connections. The load cell differential SENSE lines connected to the AD7780 reference inputs create a ratiometric configuration immune to low-frequency changes in the power supply excitation voltage. Those sense pins are connected to the high and low sides of the Wheatstone bridge, where voltage can be accurately measured, regardless of the voltage drop due to the wiring resistance.

The AD7780 has separate analogue and digital power supply pins. The analogue and digital power supplies are independent of each other to be different, or the same, potentials achieved with the AVDD SEL jumper. This feature allows selecting the AD7780 power supply between an external power supply (2.7 - 5.25V) and logic voltage levels supplied via mikroBUS™ rails.

Load Cell 5 Click communicates with MCU using a standard SPI interface with a dual-purpose DOUT/RDY line. This line can function as a regular data output pin for the SPI interface or as a data ready pin (interrupt) labelled as RDY and routed on the INT pin of the mikroBUS socket. Also, it uses the RST pin on the mikroBUS™ socket, which performs the Hardware Reset function by putting this pin in a logic low state, and a blue diode labeled as ACTIVE is used to indicate the device's Active Operational Status.

The Load Cell 5 Click Board™ can operate with both 3.3V and 5V logic voltage levels selected via the VCC SEL jumper. This way, it is allowed for both 3.3V and 5V capable MCUs to properly use the I2C communication lines. However, the Click board™ comes equipped with a library containing easy-to-use functions and an example code that can be used, as a reference, for further development.

SPECIFICATIONS

PINOUT DIAGRAM

This table shows how the pinout of the Load Cell 5 Click Board™ corresponds to the pinout on the mikroBUS™ socket (the latter shown in the two middle columns).

Notes	Pin					Pin	Notes
NC	1	AN	PWM	16	GN	Gain Selection
Reset	RST	2	RST	INT	15	RDY	Interrupt / Data Ready
Filter Select	FIL	3	CS	RX	14	NC
SPI Clock	SCK	4	SCK	TX	13	NC
SPI Data OUT	SDO	5	MISO	SCL	12	NC
NC	6	MOSI	SDA	11	NC
Power Supply	3.3V	7	3.3V	5V	10	5V	Power Supply
Ground	GND	8	GND	GND	9	GND	Ground

ONBOARD SETTINGS AND INDICATORS

LOAD CELL 5 CLICK ELECTRICAL SPECIFICATIONS