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  • Wheatstone Bridge Reference Design

  • Created: May 17, 2016

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This design is a circuit board that showcases the MCP6N11 and MCP6V2x devices from Microchip. The MCP6N11 is a single instrumentation amplifier with CMOS, Schmitt-triggered input to control modes of operation and several minimum gain options. While the MCP6V2Xs are auto-zeroed op amps that provide input offset voltage correction for very low offset and offset drift. They are used to process the output of a Wheatstone bridge and determine the temperature and resistance reading of an RTD temperature sensor used as the input to the bridge circuit. Each of the MCP6N11 and MCP6V2x devices will have its own signal processing circuit, which will give separate outputs. The circuit demonstrates to the customers the application of these devices and helps them to evaluate its performance.

The circuit is basically comprised of the bridge circuit, signal processing circuit, power supply and the PIC microcontroller (MCU). The bridge circuit is configured as a Wheatstone bridge. This circuit is used to measure an unknown resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component, which in this case is the RTD. The R2 (RTD), R3, R4, and R5 form the bridge. The resistors R1 and R6 set the current, which is ratiometric with the 5.0V supply. The bridge’s output is a small Differential Mode (DM) signal with a large Common Mode (CM) interference signal superimposed. The CM signal is a PWM waveform generated by the microcontroller, which simulates real world interference. The signal processing circuit is composed of the VREF, INA#1, and INA#3 circuits. The VREF block, which produces a buffered ratiometric (proportional to VDD) voltage (Vref). The Vref keeps the INAs within their range, even when VDD changes. Errors in Vref are subtracted out later in firmware. The INA#1 and INA#2 circuits process the bridge’s output, rejecting the CM interference and providing large gains for the DM signal. The INA#1 uses the MCP6N11, which is set at a high DM gain (200 V/V) and has good CMRR at 25kHz (92dB, at the PWM frequency). The INA#2 is a traditional three-op amp instrumentation amplifier. It uses Microchip’s auto-zeroed MCP6V27 (dual) and MCP6V26 (single) opamps. It is also set at a high DM gain (200 V/V) and has good CMRR at 25kHz (e.g., 96dB). The INA #2 provides a means of comparing the MCP6N11 to a traditional INA. Both of the INA circuits have an input filter that provides a low-pass function for both CM and DM signals. They are fast enough to follow supply variations and to let the INA reject CM mains noise. They also have an output filter that provides a low-pass function for both CM and DM signals out of the INA. It is slow to minimize noise and interference. The INA#1 outputs are labeled as Vad1 and Vad2, while the INA#2 outputs are Vad3 and Vad4. These signals from the INA circuits are then inputted to the 12-bit ADC of the PIC MCU. The firmware of the MCU directly converts the INA voltages to temperature readings. Calculation of RTD resistance is then processed based on the temperature measured. A GUI sends configuration information to the firmware, via the USB and collects the following (averaged) data from the firmware: VAD1, VAD2, VAD3, VAD4, TRTD1, TRTD2, RRTD1 and RRTD2. These data are then displayed in the GUI.

It is possible for the user to modify configurations in this circuit board and modify also the firmware. The PIC MCU can be re-programmed via the ICSP connector. This modification may include averaging scheme for speed performance and output data conversion flow.