Simple non-inverting op-amp amplifier circuit - how to boost mV to 0-2.5V DC.
For millivolt range input voltage to the data logger, you will need to amplify (multiply) your signal to utilize the data logger's 2.5V DC dynamic range. You'll need to make an op-amp circuit. An op-amp circuit is really just an amplifier with infinite gain controlled by a voltage divider. An amplifier's gain will multiply the voltage of the input signal to a useful level that the data logger can measure.
First determine the needed gain ratio: divide the needed output volts from your amplifier by the output volts of your sensor.
(There are two basic types of op-amp circuits: inverting and non-inverting. The non-inverting is used here for its simplicity. For more information on how to build such circuits, a good reference is "The Art of Electronics" by Horowitz and Hill.)
G = output volts / input volts
For HOBO external inputs this usually means: G = 2.5 / input volts
If you wish to use the HOBO's full-scale range of 2.5V DC and the maximum input signal is 250 mV, the gain is 10:1 or just 10. Once you have determined the gain, G, you can calculate for the Resistor R1. For now, pick a value for R2 (typically 10K) then calculate for R1.
R1 = R2 / (G-1)
From the above example G = 10 and if you picked a value of 10000 for R2.
R1 = 10000 / (10 - 1)
R1 = 1111.1 Ohms
The closest value resistor you could purchase for R1 is 1100 ohms, which still gives you a 1% nominal accuracy.
There are a few practical considerations. If the sum of R1 and R2 is too low (Rmin) you could overload your power supply. Rmin or 1000 ohms will draw 2.5 milliamps, if you are powering your OP amp from the logger's switched 2.5v output. If the sum of R1 and R2 is too high (Rmax), anything over 200000, the OP amp's internal offsets may create errors. Additionally, resistors do not come in every possible resistance value. You will have to plug in available resistance values, until you reach an OP AMP circuit that is both accurate enough, and keeps the sum of R1 and R2 within practical limits. Sensors with full-scale outputs in the 100mV or less range will need to be amplified in order to get good measurement resolution and accuracy. EME Systems offers amplifiers that have proved successful for some HOBO users. http://www.emesystems.com
Note: The UX120-006M includes a 16-bit A/D converter that greatly increases the logger's resolution and ability to log data from small voltage sources. UX120-006M