Experience excellence in resistance measurement with MAX4208 and PIC32MZ2048EFM100
Mastering resistance with Wheatstone's magic
Published Nov 08, 2023
Click board™
Wheatstone Click
Dev. board
Curiosity PIC32 MZ EF
Compiler
NECTO Studio
MCU
PIC32MZ2048EFM100
Discover the ultimate tool for precision resistance measurement – our Wheatstone bridge circuit solution is here to empower your journey towards accurate and reliable data.
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Hardware Overview
How does it work?
Wheatstone Click is based on the MAX4208, an ultra-low offset/drift, precision instrumentation amplifier, from Analog Devices. A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two branches of a bridge circuit, one branch of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements, in contrast with a simple voltage divider. The R2, R3, and R4 are resistors of known resistance (1k ohm), while the resistance R1 is brought to the terminal block and therefore it is changeable. With no resistance connected to the terminal block, the bridge on this Click board™ is in balance. At this point, the voltage between the two midpoints (IN- and IN+) will be zero. Therefore the ratio
of the two resistances in the known branch (R1 and R3) is equal to the ratio of the two resistances in the unknown leg (R2 and R4). If the external resistance is connected to the terminal block, bridge is unbalanced, and the voltage on the midpoints is proportional to the external resistor value. Wheatstone click is based around the MAX4208 IC, which is connected to an onboard Wheatstone bridge circuit, in order to precisely measure the resistance of an external element. The mentioned IC uses a spreadspectrum, autozeroing technique that constantly measures and corrects the input offset, eliminating drift over time and temperature and the effect of 1/f noise. This technique achieves less than 20μV offset voltage, allows ground-sensing capability, provides ultra-low CMOS input bias current and increased
common-mode rejection performance. It also provides high-impedance inputs, optimized for small-signal differential voltages (±100mV), which makes it ideal for an application such as wheatstone bridge disbalance measurement. This Click board™ also has TPL0501 onboard - 256-Taps, Single-Channel, Digital Potentiometer With SPI Interface, from texas instruments. It is connected to the MAX4208 in a way that it serves an gain adjust instead of with two external resistors The power supply voltage selection for the logic section is done by moving the SMD jumper labeled as VCC SEL to a desired position: left position to select 3.3V, right position to select 5V. This will allow both 3.3V and 5V MCUs to be interfaced with the Click board™ directly.
Features overview
Development board
Curiosity PIC32 MZ EF development board is a fully integrated 32-bit development platform featuring the high-performance PIC32MZ EF Series (PIC32MZ2048EFM) that has a 2MB Flash, 512KB RAM, integrated FPU, Crypto accelerator, and excellent connectivity options. It includes an integrated programmer and debugger, requiring no additional hardware. Users can expand
functionality through MIKROE mikroBUS™ Click™ adapter boards, add Ethernet connectivity with the Microchip PHY daughter board, add WiFi connectivity capability using the Microchip expansions boards, and add audio input and output capability with Microchip audio daughter boards. These boards are fully integrated into PIC32’s powerful software framework, MPLAB Harmony,
which provides a flexible and modular interface to application development a rich set of inter-operable software stacks (TCP-IP, USB), and easy-to-use features. The Curiosity PIC32 MZ EF development board offers expansion capabilities making it an excellent choice for a rapid prototyping board in Connectivity, IOT, and general-purpose applications.
Microcontroller Overview
MCU Card / MCU
Architecture
PIC32
MCU Memory (KB)
2048
Silicon Vendor
Microchip
Pin count
100
RAM (Bytes)
524288
Used MCU Pins
mikroBUS™ mapper
Take a closer look
Click board™ Schematic
Step by step
Project assembly
Start by selecting your development board and Click board™. Begin with the Curiosity PIC32 MZ EF as your development board.
Software Support
Library Description
This library contains API for Wheatstone Click driver.
Key functions:
wheatstone_set_potentiometer- Set potentiometer ( 0 - 100k )wheatstone_read_an_pin_voltage- This function reads results of AD conversion of the AN pin and converts them to proportional voltage level.
Open Source
Code example
The complete application code and a ready-to-use project are available through the NECTO Studio Package Manager for direct installation in the NECTO Studio. The application code can also be found on the MIKROE GitHub account.
/*!
* \file
* \brief Wheatstone Click example
*
* # Description
* This example demonstrates the use of Wheatstone Click board by measuring the input
* resistance.
*
* The demo application is composed of two sections :
*
* ## Application Init
* Initializes the driver and logger and sets the default potentiometer (gain) level.
*
* ## Application Task
* Reads the AN pin voltage and calculates the input resistance from it.
* All data are being displayed on the USB UART where you can track their changes.
*
* @note
* The following formulas you may find useful:
* AN_PIN(V) = ( ( 1kOhm + R_INPUT(kOhm) ) / ( 1kOhm + 2*R_INPUT(kOhm) ) - 1/2 ) * VCC(V) * GAIN
* VOUT(V) = AN_PIN(V) / GAIN
* R_INPUT(kOhm) = ( VCC(V) * GAIN - 2*AN_PIN(V) ) / ( 4*AN_PIN(V) )
* R_INPUT(kOhm) = ( VCC(V) - 2*VOUT(V) ) / ( 4*VOUT(V) )
*
* \author MikroE Team
*
*/
// ------------------------------------------------------------------- INCLUDES
#include "board.h"
#include "log.h"
#include "wheatstone.h"
// ------------------------------------------------------------------ VARIABLES
static wheatstone_t wheatstone;
static log_t logger;
// ------------------------------------------------------ APPLICATION FUNCTIONS
void application_init ( void )
{
log_cfg_t log_cfg;
wheatstone_cfg_t cfg;
/**
* Logger initialization.
* Default baud rate: 115200
* Default log level: LOG_LEVEL_DEBUG
* @note If USB_UART_RX and USB_UART_TX
* are defined as HAL_PIN_NC, you will
* need to define them manually for log to work.
* See @b LOG_MAP_USB_UART macro definition for detailed explanation.
*/
LOG_MAP_USB_UART( log_cfg );
log_init( &logger, &log_cfg );
log_info( &logger, " Application Init " );
// Click initialization.
wheatstone_cfg_setup( &cfg );
WHEATSTONE_MAP_MIKROBUS( cfg, MIKROBUS_1 );
wheatstone_init( &wheatstone, &cfg );
wheatstone_set_potentiometer ( &wheatstone, WHEATSTONE_POT_MAX );
log_info( &logger, " Application Task " );
}
void application_task ( void )
{
float an_pin_v = 0;
float vout = 0;
float r_kohm = 0;
if ( WHEATSTONE_OK == wheatstone_read_an_pin_voltage ( &wheatstone, &an_pin_v ) )
{
vout = an_pin_v / wheatstone.gain;
if ( 0 != vout )
{
r_kohm = ( WHEATSTONE_VCC_5V - 2 * vout ) / ( 4 * vout );
}
log_printf( &logger, " VCC : %.3f V\r\n", WHEATSTONE_VCC_5V );
log_printf( &logger, " GAIN : %.3f\r\n", wheatstone.gain );
log_printf( &logger, " AN_PIN : %.3f V\r\n", an_pin_v );
log_printf( &logger, " VOUT : %.3f V\r\n", vout );
log_printf( &logger, " R_INPUT : %.3f kOhm\r\n\n", r_kohm );
Delay_ms ( 1000 );
}
}
int main ( void )
{
/* Do not remove this line or clock might not be set correctly. */
#ifdef PREINIT_SUPPORTED
preinit();
#endif
application_init( );
for ( ; ; )
{
application_task( );
}
return 0;
}
// ------------------------------------------------------------------------ END
Additional Support
Resources
Category:Measurements
