# Basic function principles

The KL3351 analog input terminal permits direct connection of a resistor bridge. The bridge voltage, UD, and the supply voltage, Uref, to the bridge are digitized with 16-bit resolution, and are transmitted, electrically isolated, to the higher-level automation device. The input channels are available in the form of two 16-bit values for further processing.

The measurement result can be calculated from the formula: measured value = UD / Uref.

Precise acquisition of the supply voltage along with the bridge voltage in a converter compensates for long-term and temperature drift.

## LED display

The green Run LEDs indicate the operating state of the associated terminal channel.

• On: Normal operation
• Off: Watchdog-timer overflow has occurred. If no process data are transmitted by the Bus Coupler for 100 ms, the green LEDs go out.

The red error LEDs indicate an overflow (range exceeded):

• Error LED1: outside the acceptable range for the signal voltage (UD), or broken wire
• Error LED2: outside the acceptable range for the reference voltage (Uref)

## Output of the measured value

The bridge voltage between +UD and -UD is output with a resolution of 500 nV/digit, so that 10 mV corresponds to an output value of 20000 in the data bytes D0 and D1 of channel 1.

The supply voltage for the measuring bridge is output with a resolution of 500 µV/digit, so that 10 V corresponds to the output value 20000 in the data bytes D0 and D1 of channel 2.

The resolution can be adjusted by means of gain register R38.

## Example

A load cell was connected to the KL3351 in an experimental test bed.

The load cell was fed with the terminal’s 5 V supply voltage (UV), while the reference voltage (Uref) and the bridge voltage were captured with the Uref and UD channels respectively.

Technical data for the load cell:

5 kg = 2 mV / V

The weight is calculated as follows:

Weight = UD / Uref * 5 / 0.002 [kg]

## Bridge supply

If the internal supply voltage is overloaded (I_load > 20 mA), the use of a KL9505 or a KL9510 it is recommended.