Created: Apr 25, 2017
No description available.
The MCP6S21 and MCP6S26 devices featured in this reference design belong to the MCP6S2X family of Programmable Gain Amplifiers (PGA). The MCP6S21 has one input channel while the MCP6S26 has six input channels. The MCP6S2X family has eight gain selections (+1, +2, +4, +5, +8, +10, +16 or +32 V/V). For devices with two to eight inputs, an internal multiplexer selects the input channel based on the configuration it receives from its SPI port. The SPI interface can also be used to put the device into shutdown mode to conserve power. The MCP6S2X family operates from 2.5V up to 5.5V with a low current supply, typically 1mA.
In this reference design, the MCP6S21 and MCP6S26 devices are used to amplify two signal sources connected to channel zero and channel one of the MCP6S26 device. The MCP6S26 device accepts and amplifies the signals with a gain of 32V/V. As shown in the circuit, the output of the MCP6S26 serves as an input to the MCP6S61 device. If both devices are configured for a gain of 32V/V, the MCP6S61 can amplify the signals more with a total gain of 1024V/V. In order for the MCP6S21 and MCP6S26 devices to operate, they need to be programmed through their SPI ports. The PIC16F676 microcontroller in the circuit is used to program the devices according to the user inputs. User inputs are selected using the DIP switch. The DIP switch allows the user to program the channel, gain, or even shutdown the devices. When the momentary READ push button switch is pressed, the microcontroller reads the DIP switch configuration through the I/O ports. The DIP switch settings are specified on Microchip's MCP6S2X evaluation board user manual. The PIC16F676 microcontroller determines the proper command and data bytes to be transmitted to the PGAs and then transmits a 16 or 32-bit word through the SPI port. The digital lines of the PGAs are daisy chained. The MCP6S26 has a Serial-Out (SO) line that can be used to serially program another device. So in the circuit, the SO line of MCP6S26 is connected to the Serial-In (SI) line of the MCP6S21 to program it. If the MCP6S26 is removed from the circuit, the MCP6S21 cannot be programmed.
In the circuit, there are three reference voltage options that can be used to offset the output signal of the PGA devices. There is a fixed 2.5V from the MCP1525 device, a variable reference voltage from 0 to 5V that can be adjusted by tweaking RV1, and a ground. These options can be selected using header J2. Since the reference voltage input of the PGA requires a low-impedance source, these reference voltages are buffered using the MCP6022 operational amplifiers. The ICSP header J3 provides the capability of programming the PIC16F676 from a PC. This requires additional hardware, such as MPLAB ICD 2. The SMA connectors J4 and J5 can be used to input the two signal sources while J6 and J7 can be used to get the outputs. The header J1 selects the power source for the circuit. In stand-alone mode, connect a 5V power supply to the screw terminal ST1 and short the pin 1 and pin 2 of J1. When in program mode, short pin 2 and pin 3 of J1 since the debugger provides the supply for the circuit.