RMS to DC click
PID: MIKROE‐3311
Weight: 25 g
RMS to DC click is a Click board™ that is used to convert the RMS of the input
signal into a DC voltage, with a value directly readable over the I2C
interface. The Click board™ is equipped with the LTC1968, an RMS-to-DC
converter IC, which outputs an analog voltage depending on the RMS value
of the input signal. The key feature of this IC is its very good linearity of the
output voltage with respect to the RMS of the input signal. This is due to the
innovative delta-sigma computational technique, used in this IC. Besides the
high linearity, this IC also features high accuracy, bandwidth independent of
signal amplitude and thermal stability. It can be used to accurately measure
the RMS value of various alternating signals.
The Click board™ is supported by the mikroSDK compliant library, which
includes functions that simplify software development. The Click board™
comes as a fully tested product, ready to be used on a system equipped with
the mikroBUS™ slot.
Featuring a very high linearity, a rail-to-rail common mode voltage, a true
RMS-to-DC conversion with a minimum number of external components
required, an excellent linearity that allows direct application with no
compensation elements required, a very wide signal bandwidth, and more,
RMS to DC click is an ideal solution for development of various true RMS
digital multimeter applications, panel meters and gauges, AC + DC
measurement applications, a true RMS measurement of an audio signal and
other similar applications that require accurate RMS value readings.
How does it work?
RMS to DC click is based around the LTC1968, a precise RMS-to-DC converter
with the wide input signal bandwidth, from the Linear Technology division of
Analog Devices. This IC uses the proprietary delta-sigma computational
techniques to achieve a highly linear DC voltage output at its output in
respect with the RMS of the input signal. The RMS is typically associated
with the alternating signals. This Click board™ is capable of measuring the
RMS of both bipolar and unipolar periodically alternating signals, over a wide
range of frequencies.
The RMS or Root Mean Square is used to describe the power of the input
signal: the RMS value of current is equal to a DC current value that would
produce the same heat dissipation on the resistive load. Therefore, it is often
important to know the RMS value of the signal. RMS to DC click allows
measuring of the RMS value of a periodically repetitive signal.
As mentioned before, the LTC1968 provides a highly accurate and linear
conversion of the RMS value at its input, to a constant voltage at its output.
The constant voltage directly depends on the RMS value of the input signal,
thanks to the innovative sigma-delta conversion technique of the LTC1968,
which is described in details within the LTC1968 datasheet. Due to a high
output voltage linearity, no compensation elements are required, except a
single filtering capacitor. The output voltage of the LTC1968 is then fed to an
analog-to-digital converter (ADC). For the voltage-to-digital conversion
purposes, RMS to DC click utilizes the MCP3221, a 12-bit ADC with I2C
interface, from Microchip. This ADC uses the voltage at its power supply pin
as a conversion reference. This simplifies the Click board™ schematics,
allowing the reference voltage to be changed along with the power supply
voltage of the ADC.
The communication logic voltage level, as well as the ADC power supply
voltage, can be changed by switching the SMD jumper labeled as VCC SEL to
either 3V3 position or 5V position. Note, however, that this will cause the
reference ADC voltage to change accordingly. This should be accounted
for when calculating the output value.
The input signal can be connected to the two-pole input signal connector.
The LTC1968 IC accepts both bipolar and unipolar signals at its input, thanks
to the differential input. The negative differential input is used as the
reference input on this Click board™, therefore it is offset at 2.5V in respect
to GND, while the positive differential input is decoupled by a 100nF
capacitor, allowing only the AC component of the input signal to be
processed. This allows signal input within the ±2.5 range to be applied.
RMS to DC click also features an #ENABLE pin, used to enable or disable the
LTC1968 when used in power sensitive applications. This pin is pulled to a
LOW logic level by a resistor, so the IC is enabled by default. The user can
disable the IC by pulling the #ENABLE pin to a HIGH logic level. This pin is
routed to the CS pin, and it is labeled as EN on this Click board™.
Due to an overall circuit simplicity allowed by the LTC1968 IC, the ADC
directly outputs the RMS value of the input signal. However, the Click
board™ is supported by a mikroSDK compatible library, which contains
functions that simplify the development even further, allowing the RMS data
to be read in almost a single line of code.
Specifications
Type
Measurements
Applications
RMS to DC click is an ideal solution for development of
various true RMS digital multimeter applications, panel
meters and gauges, AC + DC measurement applications,
a true RMS measurement of an audio signal and other
similar applications that require accurate RMS value
readings.
On-board
modules
LTC1968, a precise RMS-to-DC converter with the wide
input signal bandwidth, from the Linear Technology
division of Analog Devices; MCP3221, a 12-bit ADC with
I2C interface, from Microchip.
Key Features
It offers an excellent linearity that allows direct
application with no compensation elements required, a
rail-to-rail common mode voltage, a true RMS-to-DC
conversion with a minimum number of external
components, a good thermal stability, a very wide signal
bandwidth, and more.
Interface
GPIO,I2C
Input
Voltage
3.3V or 5V
Click board
size
M (42.9 x 25.4 mm)
Pinout diagram
This table shows how the pinout on Thermo 8 click corresponds to the pinout
on the mikroBUS™ socket (the latter shown in the two middle columns).
Notes
Pin
Pin
Notes
NC
1
AN
PWM
16
NC
NC
2
RST
INT
15
NC
EN
3
CS
RX
14
NC
NC
4
SCK
TX
13
NC
NC
5
MISO
SCL
12
SCL
I2C Clock
NC
6
MOSI
SDA
11
SDA
I2C Data
Power Supply
3V3
7
3.3V
5V
10
5V
Power
Supply
Ground
GND
8
GND
GND
9
GND
Ground
LTC1968 Chip
Enable
Onboard settings and indicators
Label
Name
Default
PWR
PWR
-
JP1
VCC SEL
Left
VIN
IN+,IN-
-
Description
Power LED indicator
Power supply voltage selection: left position
3.3V, right position 5V
Input signal connector
Software support
We provide a library for the RMS to DC Click on our LibStock page, as well as a
demo application (example), developed using MikroElektronika compilers. The
demo can run on all the main MikroElektronika development boards.
Library Description
The library performs an RMS to DC signal converting with 12bit resolution.
It is also possible to get the averaged output DC signal (voltage), with a
minimalized noise. For more details check the documentation.
Key functions:
uint16_t rms2dc_readADC( void ) - The function returns a 12bit result of AD conversion.
T_RMS2DC_V rms2dc_voutADC( T_RMS2DC_V vccSel ) - The function returns the output
voltage value calculated to mV, depending on the power voltage selection.
T_RMS2DC_V rms2dc_avrgVoutADC( T_RMS2DC_V vccSelect, uint8_t nSamples ) - The
function returns the averaged output voltage value calculated to mV, depending on the power
voltage selection.
Examples description
The application is composed of the three sections :
System Initialization - Initializes peripherals and pins.
Application Initialization - Initializes I2C interface and turns ON the device.
Application Task - (code snippet) - Reads averaged DC output voltage calculated to mV, and
sends results to the serial plotter. Note: The input voltage frequency should be in the range from
50Hz to 250kHz. Als,o the input voltage amplitude must be lower than 5V. In these conditions,
the device can convert the RMS signal, in every form, to DC signal.
void applicationTask()
{
outVoltDC = rms2dc_avrgVoutADC( _RMS2DC_VCC_3V3, 25 );
plotData( outVoltDC );
Delay_ms( 5 );
}
Additional Functions :
void plotData( T_RMS2DC_V plotY ) - Sends data (DC voltage) to the serial plotter.
void logData() - Sends DC voltage value to the uart terminal.
The full application code, and ready to use projects can be found on
our LibStock page.
Other mikroE Libraries used in the example:
Conversions
I2C Library
UART Library
Additional notes and information
Depending on the development board you are using, you may need USB UART
click, USB UART 2 click or RS232 click to connect to your PC, for development
systems with no UART to USB interface available on the board. The terminal
available in all MikroElektronika compilers, or any other terminal application
of your choice, can be used to read the message.
https://www.mikroe.com/rms‐to‐dc‐click/1‐23‐19