February 2016 43
Rg = 100 kΩ/(G − 1)
In this application, I wanted to use a gain of 10.
The DC voltage was provided by a simple 10K ohm
10-turn pot connected between the 3.3V reference
and ground. The center tap signal — which I could
adjust — was connected to the V- input of the
AD623. The final circuit with the TMP36, pre-amp,
and instrumentation amplifier is shown in Figure 7.
This analog front end circuit was implemented in
a breadboard adjacent to the Arduino Redboard.
The amplifiers were powered off of the 5V rail of the
Redboard. The two sensitive voltages used the 3.3V
reference pin on the Redboard.
I added a few capacitors to the voltage rails to
keep the noise down. Figure 8 shows this
In this circuit, the original 100°C/V sensitivity
was changed to 100°C/V /3.245/10 = 3.1°C/V. With
a resolution of the least significant bit still 0.00323V,
the temperature resolution is 3.1 x 0.00323 = 0.01°C.
Now we’re talking.
We have three different sensitivity levels of
temperature measured in this circuit: the direct readings;
the direct readings increased in sensitivity by 3x; and the
direct reading increased in sensitivity by 30x. All the
conversion from ADUs into voltage and then into
temperature is done in the sketch.
It’s the final temperature values which were printed to
the serial port and available for plotting by my favorite
plotting tool, MakerPlot.
With these three different temperature readings
available, I did the same experiment: touching the sensor
very briefly and watching the temperature rise quickly and
fall slowly. In the slow decline in temperature, the
resolution limits of 0.3°C and 0.1°C are clearly seen.
The smoothly varying temperature on this plotted
scale of 0.1°C per division is an indication of the 0.01°C
resolution of the signal after the instrumentation amplifier.
Figure 9 shows these comparisons.
FIGURE 7. Complete circuit with sensor, pre-amp, and
FIGURE 8. The complete system configured on a breadboard connected
to an Arduino.
T5403 barometric pressure sensor from SparkFun:
TMP36 temperature sensor:
Makerplot for reading the serial port data and plotting:
A great op-amp applications handbook
you can download for free:
L358 from Jameco:
AD623 from Jameco: