2.6.2 - Converting Binary Readings to Voltages
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Export allFollowing are the nominal input voltage ranges for the low-voltage analog inputs. This is all analog inputs on the U3-LV, and AIN4-AIN15 on the U3-HV.
Note that the minimum differential input voltage of -2.44 volts means that the positive channel can be as much as 2.44 volts less than the negative channel, not that a channel can measure 2.44 volts less than ground. The voltage of any low-voltage analog input pin, compared to ground, must be in the range -0.3 to +3.6 volts.
The “special” range (0-3.6 on low-voltage channels and -10/+20 volts on high-voltage channels) is obtained by doing a differential measurement where the negative channel is set to the internal Vref (2.44 volts). For low-voltage channels, simply do the low-voltage differential conversion as described below, then add the stored Vref value. For high-voltage channels, do the same thing, then multiply by the proper high-voltage slope, divide by the single-ended low-voltage slope, and add the proper high-voltage offset. The UD driver handles these conversions automatically.
Although the binary readings have 12-bit resolution, they are returned justified as 16-bit values, so the approximate nominal conversion from binary to voltage is:
Volts(uncalibrated) = (Bits/65536)*Span (Single-Ended)
Volts(uncalibrated) = (Bits/65536)*Span – Span/2 (Differential)Binary readings are always unsigned integers.
Where span is the maximum voltage minus the minimum voltage from the tables above. The actual nominal conversions are provided in the tables below, and should be used if the actual calibration constants are not read for some reason. Most applications will use the actual calibrations constants (Slope and Offset) stored in the internal flash.
Volts = (Slope * Bits) + OffsetSince the U3 uses multiplexed channels connected to a single analog-to-digital converter (ADC), all low-voltage channels have the same calibration for a given configuration. High-voltage channels have individual scaling circuitry out front, and thus the calibration is unique for each channel.
See Section 5.4 for detail about the location of the U3 calibration constants.
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Comments
#1
Hello,
I have recently purchased a U3-HV to act as a datalogger for a number of small "shed" projects. I am currently trying to log a simple 1-5V signal outputed from a thermal gas flow meter. Not being an electrical guru (chemical engineer) the amount of circuitry knowledge in your explanations is very difficult to follow. However, i am having a lot of trouble simply getting a correct signal to appear on the LJLogUD.
My first issue is that there appears to be a large amount of interference in the measured signals, particularly a random cyclic interference which is different on each single ended wire. I find that not being able to directly use a differential measurment (due to it converting it directly into binary) is enormously aggravating, particularly when the converted binary (ie by dividing by 65536) still gives me a resultant cyclic interference. Please help, i had very high hopes for this little device, but have so far found other dataloggers far more user friendly and accurate.
#2
I think Section 2.6.3.4 would be better for this topic, although to continue in detail about your particular signal I would start a forum topic or email support@labjack.com.
Start with all connections removed except for USB and your signal. Connect your signal to AIN0 and GND, monitor it in the test panel in LJControlPanel or in LJLogUD, and let us know what you see.
We can't provide a single calibration for the differential high-voltage channels on the U3-HV, because the calibration depends on the common-mode voltage on both inputs, so we leave it to the user to calibrate in their actual system. However, a differential measurement seldom makes sense with high-level signals. The differential app note has more information.