shield means we can power analog front-end devices to
take full advantage of the bipolar range of the ADC and
I measured the bipolar fixed 5V supply as 4.V and -
4.84V when powered by the USB source, and 4.95V and -
4.93V when plugged into external power. Since these
voltages are regenerated by an onboard regulator, they
have a different voltage level and source resistance
depending on their power source.
In addition to the voltage level, the other important
metric of any power source is its output resistance. This
sets the maximum current draw before the voltage drop
on the rail is large, like 10% (see Resource 4). Table 1
summarizes the source resistances I measured and their
maximum current draw.
It’s important to note that when using an op-amp
powered with the - 7.5V supply, the maximum current draw
from the devices should be kept below 40 mA. This is not
unreasonable. When I use this rail to power other analog
front-end components, I always check their current draw
and so far, it has not been a limitation (see Resource 5).
The analog shield was created as a joint project
between TI and Stanford University. Dr. Gregory Kovacs, a
professor at Stanford University, worked with his graduate
student, Bill Esposito to design and build this shield as a
tool to teach analog design and data acquisition. I hope
Bill got an A for this project. He deserves it.
Installation of the DAS
The original library released by Digilent for the DAS
and posted on their website is not compatible with the
Due. In fact, this library is not very stable even with an
Uno. For use with the Due, you should install the library
available from Wespro’s GitHub site, last updated on
March 24, 2015 at https://github.com/wespo/
analogShield (see Resource 6). This library works with the
Uno and most other Arduinos.
Initially, I tried the library that Digilent recommended.
It never worked very well and gave me hours of
frustration. I read through dozens of Arduino forums on
related topics looking for a fix, but couldn’t find a solution.
Just before I was going to give up and accept that the Due
was not compatible with the DAS, I ran across a mention
of a library provided by Wespro. I tried it and it worked!
In fact, of all the posted libraries, this was the ONLY
library that worked and was stable for the Uno and Due
connected with the DAS.
When you go to Wespro’s GitHub landing
page, you will see a button in the upper right to
“Download the ZIP file.” Click this button and the
file “analogShield-master.zip” is downloaded.
Installing this library still in its .zip form is simple.
Open the Arduino IDE (integrated development
environment) and a new sketch. Under
Sketch/include library, you’ll see “add .zip library”
(Figure 4). Click this and point to where you
downloaded the analogShield-master.zip file. That’s
it. The DAS library is now in your library and you
can use it in any sketch. Be sure to close the IDE
window and then reopen it. This will ensure the
libraries are available.
Every sketch will need at least two libraries with
include statements at the top, the SPI library, and
the analogShield library. The SPI library is included
in the Arduino IDE in every distribution.
June 2017 17
Voltage rail Output resistance, Ω Maximum current in mA
Adjustable - 7.5V 18 43
Adjustable 7.5V 0.16 4,700
-5V 5 100
+5V 2. 7 190
5V Due 1.9 260
3.3V Due 0.55 590
■ FIGURE 4. Screenshot showing where to
find the “add new .zip library” menu item to
install the analog shield library.
1. Purchase Due at Sparkfun:
2. Purchase Due on eBay:
3. The Digilent Analog Shield:
4. See Bogatin, Eric, “Power
Sources: The Good, The Bad,
and The Ugly,” Nuts & Volts,
5. See Bogatin, Eric, “Why You
Need an Analog Front-End
and How to Set it Up,” Nuts &
Volts, February 2016.
6. Download the Arduino
7. See Bogatin, Eric, “Arduino
Based Data Acquisition,” Nuts
& Volts, June 2015.
8. Link to the PLX-DAQ v 2.9: