Gain measurement of the OPA module on PIC18F16Q41

This code example measures the gain of the Operational Amplifier (OPA) module on the PIC18F16Q41. This project uses the on-board Analog-to-Digital Converter w/ Computation (ADCC) to sample both the input, which is derived from a DAC, and the OPA output, then computes the gain using either fixed point or floating point numbers.

Related Documentation

TB3280: Using Operational Amplifiers in PIC16 and PIC18

Code Example: Gain and Offset Calibration of the Analog Signal Conditioning (OPAMP) Peripheral (MPLAB)
Code Example: Gain and Offset Calibration of the Analog Signal Conditioning (OPAMP) Peripheral (Studio)

Software Used

• MPLAB® IDE 6.20 or newer
• Microchip XC8 Compiler 3.00 or newer
• MPLAB® Code Configurator (MCC)
• Serial Terminal

Hardware Used

• PIC18F16Q41 Product Information
  
• Microchip Curiosity Development Board (DM164137), Rev 4

Setup

Wiring

Figure 1 - Wiring Image (pin tables included below)

Wire	Name	Description
Orange	UART Transmit	UART Transmit Input for the Curiosity Board
Green	OPA1OUT	Output of the operational amplifier

Pin Table

Pin	Name	Description
RC2	OPA1OUT	Output of the operational amplifier
RB7	UART TX	UART TX (9600 BAUD)
RB5	OPA1IN0+	Input to the OPA module1
RA2	LED D6	Measuring indicator LED
RA5	LED D4	Timing Indicator2
TX	UART TX Input	Curiosity board UART transmit pin. Connect to RB7

Note:

1. RB5 is used so that the output is equal to gain x input. Input should be < 300mV to properly calculate the 16x range.
2. This pin is used as a tool to show that floating points are much slower than fixed point operations. LED is on when division is occurring. Integrate the active time of the waveform to find the difference in execution time.

Operation

Out of the box, this program does automatic gain switching to demonstrate the measurements. Every time the program prints the current gain, it changes the gain. The order of the gains is 1 (Unity Gain), 16/15, 8/7, 4/3 , 2, 8/3, 4, 8, and then 16. During the gain calculation routines, LED D6 is on. It should appear as a brief blink every 10 seconds, and can be used to measure the execution time. The gain calculated is printed through the UART at 9600 baud. The program can be run in fixed point or floating point mode - in fixed point, resolution is limited is limited to +- 0.0625, but the code is much faster and much smaller. In floating point mode, resolution is much better, but the program is much bigger and slower. The default configuration is floating point mode.

Figure 2 - Floating Point Measurement

Figure 3 - Fixed Point Measurement

Program Defines

There are 2 define statements in this example which can be used to change the behavior of the code example. Both define statements are present in the file "OPAconfig.h".

• ENABLE_CHANGING_GAIN

  • This option disables the gain switching demo on the code example. For correct operation, this option must be disabled when using external resistors⁽¹⁾. Gain measurement limit is 999.

• ENABLE_FLOATING_PT

  • This option selects the use of floating points for gain calculation, rather than fixed point numbers. Enabling this option provides an increase in the accuracy of the gain measurement, however it significantly increases code size and computation time.

Note:

1. To use external gain resistors, a reconfiguration of the OPA module in MCC is required.

Summary

This example shows the programmable gain functionality of the OPA module, and then measures the gain of the amplifier.