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Electronic Modules
Using Electronic Modules with Jim Rowe
16-bit precision
4-input ADC
This month we’re looking at the tiny ADS1115 that can add up to four
high-speed 16-bit analog-to-digital conversion (ADC) channels to almost
any microcontroller. It has a built-in I2C serial interface, so it can be easily
connected to popular microcontrollers like an Arduino Uno or Nano.
L
et’s say you want to make precise
measurements of analog voltages
or currents with one of the common
microcontroller units (MCUs). The
ADC in most MCUs provides a resolution of only ten bits over a range of
either 3.3V or 5V, which corresponds
to a precision of ±1.6mV (3.3V ÷ 1024
÷ 2) or ±2.5mV (5V ÷ 1024 ÷ 2), where
1024 is equal to 210.
That is acceptable for many applications, but not good enough if you
want to make precision measurements,
especially of small voltages.
That’s where this module is worth
considering because it allows you to
add precision 16-bit ADC capability to
any of the popular MCUs. As a result,
you will be able to make much more
precise measurements, even on quite
small signals. It offers a precision improvement of 64 times compared to a
10-bit ADC or 16 times compared to
a 12-bit ADC.
It is not just useful for a singleended full-scale range of 3.3V or 5V
either, because the module gives you a
choice of six different full-scale ranges:
±6.144V, ±4.096V, ±2.048V, ±1.024V,
±512mV or ±256mV. This means that
the smallest step size on the highest
range is 187.5µV, while on the lowest
range, it’s 7.8125µV.
Note though that the inputs must go
no more than 0.3V beyond the supply
rails and those measurement ranges
can be between two inputs or from an
input to ground. So you can’t actually measure voltages very far below
ground or above the (typically 3.3V
or 5V) supply range.
A further feature of the module is
that it has four analog inputs, which
can be used to measure either four different voltages with respect to ground,
or to provide two differential inputs.
Another feature of the module which
adds to its appeal is the ability to
make measurements at eight different
rates, from eight per second to 860 per
second. It connects to the MCU via
a standard two-wire I2C serial interface, with the ability to set the module’s I2C port to one of four addresses:
48h, 49h, 4Ah or 4Bh (h = hexadecimal). That means you can connect up
to four modules to a single I2C port
on an MCU, each set for a different
I2C address.
Even if you don’t need the improved
precision, if you’ve run out of ADC
channels on your micro, it might be
worth considering this module for
the extra analog inputs it provides. If
you’re already using an I2C serial bus
in your project, it won’t even take up
any more pins on the micro to add as
many as 16 more analog inputs. Otherwise, you can dedicate two digital
pins – not a bad swap.
In short, it’s a very flexible and impressive precision ADC module. All
of these capabilities are due to the IC
that forms the ‘heart’ of the module:
an ADS1115 made by Texas Instruments. So let’s look at the innards of
this impressive chip.
Inside the ADS1115
Fig.1: a block diagram of the ADS1115 IC. This shows the ADS1115 can be
configured as four single-ended channels or as two differential channels.
42
Fig.1 shows the basic block diagram
of the ADS1115. The 16-bit delta-
sigma ADC is in the centre, with the
chip’s built-in voltage reference just
above it and the internal clock oscillator just below.
To the left of the ADC (on its input
side) is a programmable-gain differential amplifier (PGA), providing the
chip’s six full-scale ranges.
Left of the PGA is the input multiplexer (MUX), which selects which of
Practical Electronics | October | 2024
Four-channel 16-bit precision ADC
the four single-ended inputs (AIN0AIN3) are connected to the input of
the PGA, or which two inputs are
connected as a differential input.
When one of the single-ended
inputs is selected, the lower input
of the PGA is connected internally to
ground. In contrast, when two inputs
are selected for differential measurements, each is connected to one of
the PGA inputs.
You can see the chip’s I2C interface
to the right of the ADC. This provides
two-way communication between
the ADS1115 and the external MCU,
with programming and control data
inwards, and the measurement data
stream outwards.
This is done using the SCL and SDA
pins; the ADDR pin sets the chip’s
I2C address by linking it to one of the
Vdd, GND, SCL or SDA pins, as will
be explained shortly.
Above the I2C interface is a comparator with its output connected to
the chip’s ALERT/READY (or ALRT/
RDY) pin. The comparator can be
programmed to perform one of two
functions: either an alert ‘flag’ whenever the ADC output reaches the top
or bottom threshold of its measurement range, or as an indication that
a measurement has been made and
the result is ready for ‘collection’ by
the MCU.
The final section of the ADS1115
is below the I2C interface. This comprises four 16-bit registers:
1. The Conversion register, which
holds the last conversion data.
2. The Configuration register, which
holds the programming bits for the
chip’s input multiplexer, the gain/
range settings for the PGA, the sampling rate setting and whether the
device is to operate in single-shot
or continuous conversion mode –
see Fig.2.
3. Lo-thresh, the lower threshold
value for the Alert Comparator.
4. Hi-thresh, the upper threshold
value for the Alert Comparator.
Data in the Conversion, Lo-thresh
and Hi-thresh registers is stored in
signed two’s complement format:
from 8000h (-32768) to 7FFFh
(+32767). Because this ADC has differential inputs, it can produce negative results, meaning that a signed
number is needed. This also means
that there are effectively 15 bits of
resolution when single-ended samples are taken.
Practical Electronics | October | 2024
Fig.2: how the configuration register is arranged. This 16-bit register
handles the chip’s input multiplex (bits 12-14), the PGA (bits 9-11), whether
the device is operated in single-shot or continuous conversion mode (bit 8)
and the sampling rate (bits 5-7).
Fig.3: the ADS1115-based module is extremely simple, as can also be seen
in the lead photo. The values of the L1 & L2 inductors are unknown, they
could be ferrite beads.
Fig.4: it’s easy to connect the ADS1115-based module to an Arduino Uno or
similar, as all you need to do is connect its I2C interface (SCL & SDA) and
power rails to the Arduino.
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Electronic Modules
As you can see, there’s quite a
lot inside the ADS1115’s tiny (3.0
× 3.0mm) 10-pin VSSOP (very-thin
shrink small-outline) package.
It can be powered from supply
voltages (Vdd) between 2.0V and 5.5V.
However, the analog input voltages must be kept within the range of
(GND − 0.3V) to (Vdd + 0.3V) to prevent the ESD diodes inside the input
MUX from conducting, which would
degrade the accuracy of the ADS1115
as well as possibly damaging it.
When the ADS1115 is initially
powered up, it is set to its reset/default mode: the input MUX selects
AIN0 and AIN1 in differential mode,
the PGA is set for a full-scale range
of ±2.048V, the sampling rate set to
128 samples per second and the conversion mode set to one-shot mode.
If any of those need to be changed,
it can be done by sending the appropriate instructions from the MCU.
The I2C address of the chip is not
determined by anything in the Configuration register, but by the connection to the ADDR pin.
The module circuit
As you can see from the circuit in
Fig.3, there’s very little in the module
apart from the ADS1115 chip itself.
There are three pull-up resistors connected between its SDA, SCL and
ALRT/RDY pins and the Vdd line, a
pull-down resistor between the ADDR
line and GND, two small inductors (L1
and L2) of unknown value (they might
even be ferrite beads), plus two 100nF
capacitors which provide filtering for
the module’s input power.
The only other item is 10-pin SIL
header CON1, which makes all the
connections to the module.
Connecting to an Arduino
Fig.5: this wiring diagram shows how
to connect the ADS1115 module to an
Arduino Nano.
Fig.6: by default the ADS1115 has
an I2C address of 48h. If you plan to
connect multiple ADS1115 modules to
communicate with a microcontroller,
then you will need to link the ADDR
pin to one of the other pins as shown.
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As mentioned earlier, one of this
module’s features is how its I2C interface makes it easy to connect to
one of the popular MCUs.
This is illustrated in Fig.4, which
shows how easily it can be connected
to an Arduino Uno. The module’s Vdd
pin connects to the Arduino’s +5V pin,
its GND pin to one of the Arduino’s
GND pins, its SDA pin to the Arduino’s A4/SDA pin and its SCL pin to
the Arduino’s A5/SCL pin.
With R3 and later versions of the
Uno, the last two pins can be connected to the SDA and SCL pins at
upper left on the Arduino, just to the
left of the AREF pin. These locations
have the advantage of always being in
the same position on Uno-compatible
boards, regardless of which pins the
micro actually uses for I2C.
Connecting the module to an Arduino Nano is just as easy, as shown
in Fig.5. As you can see, the module’s Vdd pin connects to the Nano’s
+5V pin, the GND pin to one of the
Nano’s GND pins, its SCL pin to the
Nano’s A5 pin and its SDA pin to the
Nano’s A4 pin.
By default, the ADDR pin is connected to ground by a 10kW resistor,
so the module will have the address
48h. To change it, link the module’s
ADDR pin to one of the pins to its
left, as shown in Fig.6.
Linking this pin to the V dd pin
sets the I2C address to 49h; linking it
to the SDA sets the address to 4Ah,
while linking it to the SCL pin sets
the address to 4Bh.
It should be just as easy to connect
the ADS1115 module to just about
any other MCU, including one of the
‘Mite’ series (Maximite, Micromite,
PicoMite etc).
Whichever MCU you want to connect the module to, you will need
software to configure it and interpret its output data stream. Let’s
now consider how this can be done
with an Arduino. This will involve
finding a software library designed
to communicate with the ADS1115,
plus (hopefully) an example sketch
to show how it’s done.
Arduino software libraries
Searching the web for Arduino libraries for the ADS1115, I came up
with three choices:
1. A library called ADS1x1x, written
by someone named hideakitai, with
the documentation and the zippedup library code at https://github.com/
hideakitai/ADS1x1x
2. A library called ADS1x15, written
by Rob Tillaart, with the documentation
and the library code at https://github.
com/RobTillaart/ADS1x15
3. A library called ADS1115_WE,
written by Wolfgang Ewald, with the
documentation and the zipped-up
library code at https://github.com/
Wollewald/ADS1115_WE
All three come with at least one example sketch, while the third comes
with no fewer than 10 examples illustrating different ways of using the
ADS1115.
After trying each of these libraries
and their example sketches, I decided
that the ADS1115_WE library and its
examples were probably the easiest to
use. I also found that Mr Ewald had
a very informative piece on his blog
site (linked at the end of this article)
giving a lot of information regarding
the ADS1115, how it works and how
it is programmed.
Practical Electronics | October | 2024
Four-channel 16-bit precision ADC
I made a few minor changes to Mr
Ewald’s single-shot example and tried
using it to take measurements of different DC voltages.
You can see the results in the Serial
Monitor listing shown in Fig.7. I had
connected the test voltage to AIN1,
with the AIN0 and AIN2 inputs connected to ground and the AIN3 input
left floating.
The ADS1115 was programmed to
set its measurement range to ±2048mV
and to compare AIN1 to GND.
When I set the sketch at 7:09:36am,
the voltage fed to the AIN1 input
was +100mV and remained so
until 7:09:49am. Then I changed
it to +200mV before changing it
to +500mV at 7:05:58am. Then at
7:10:07am, I changed it to +50mV,
passing briefly through +100mV. Finally, at 7:10:16am, I changed the
voltage to +20mV.
The AIN0 column remains fixed at
readings of -0.00, as does the AIN2
column, reflecting the fact that both of
these inputs were grounded. But because the AIN3 input was left floating,
the readings in this column remained
fixed at 0.27V.
If you decide to try out my example
sketch, remember to change the address in the code to match what you
have configured your module for, as
shown in Fig.6.
Conclusion
Overall, this module is easy to use,
flexible, and far more accurate than
most microcontrollers for measuring
analog voltages. It doesn’t have as
much precision as a good DMM, but it
is nonetheless extremely handy.
If you want to learn more about the
ADS1115, you can view the datasheet
at: www.ti.com/product/ADS1115
Also see the technical write-up on
Wolles Elektronikkiste: https://pemag.
au/link/abph
Above: the ADS1115
module can run from a
2-5.5V supply, making
it easy to use both 3.3V
and 5V powered micros.
The available sampling
rates are: 8, 16, 32, 64,
128, 250, 475 and 860
samples per second
(SPS).
Fig.7 (right): we ran
Wolfgang Ewald’s singleshot example code, with
some minor changes,
and used it to measure
different DC voltages.
Where you can get it
This ADS1115 module is available
online from a few different suppliers.
You can find multiple offers on eBay
or AliExpress starting at just over £1
each (eg, https://pemag.au/link/abyy).
Search for “ADS1115” but check the
description and photos to make sure
they match the module shown here.
There are some other modules using
the ADS1115 that look a little different, but they are very similar in terms
of circuitry.
PE
Practical Electronics | October | 2024
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