Weigh Scale Applications for the MCP3551

AN1030 Weigh Scale Applications for the MCP3551 This application note will focus specifically on load Author: Jerry Horn, Gordon Gleason cells, a type of strain gauge that is typically used for Lynium, L.L.C. measuring weight. Even more specifically, the focus will be on fully active, temperature compensated load cells whose change in differential output voltage with a INTRODUCTION rated load is 2 mV to 4 mV per volt of excitation (the There are many different types of sensors whose excitation voltage being the difference between the underlying realization is based on a Wheatstone +Input and the -Input terminals of the load cell). bridge. Strain gauges are one such sensor. As a The goal is to develop a variety of circuits that can material is strained, there is a corresponding change in quantify this change via an analog-to-digital converter resistance. In many cases, each side of the Wheat- (ADC), which will be a MCP3551, 22-bit Delta-Sigma stone bridge may respond to the strain by lowering or ADC. The analysis for each circuit should be applicable increasing in resistance (see Figure 1). to other resistive bridge sensors. The different circuits will allow cost versus performance trade-offs. +Input The circuits presented in this application note have been realized in the MCP355X Sensor Application Developer's Board whose block diagram is shown inCompression Tension Figure 2. This board includes two microcontrollers. The PIC16F877 performs the basic weigh scale function -Output +Output while the PIC18F4550 sends data to a personal com- puter (PC) for analysis and debugging. The board Tension Compression includes a display as well as input switches that are used for calibrating the zero point and full-scale point of the load cell and for setting various processing options. -Input Conversion results from the currently selected ADC are communicated to the PC over the USB bus. This data FIGURE 1: Wheatstone Bridge of a can be viewed on a PC using the DataView software Typical Strain Gauge. that comes with the reference design. All of the testing In the case of Figure 1, the bridge is said to be fully and results shown in this application note were done active. In some cases, only half of the bridge may be with an MCP355X Sensor Application Developer's active (half active). For some sensors, only a single Board, the DataView software, and various load cells element of the bridge may change in response to the and/or load cell simulators that are either described in stimulus. this document or that can be easily purchased. USB to PC running DataView PIC18F4550 LCD Display C 2 8 I Push Button Control Switches 3 s on MCP3551 ?? i t GAIN