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) + Offset
Since 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.
2.6.2.1 - Analog Inputs With DAC1 Enabled (Hardware
Revisions 1.20 & 1.21 only)
This Section only applies to the older hardware revisions 1.20 and 1.21. Starting with hardware revision 1.30, DAC1 is always
enabled and does not affect the analog inputs.
The previous information assumed that DAC1 is disabled. If DAC1 is enabled, then the internal reference (Vref = 2.44 volts) is not
available for the ADC, and instead the internal regulator voltage (Vreg = 3.3 volts) is used as the reference for the ADC. Vreg is
not as stable as Vref, but more stable than Vs (5 volt power supply). Following are the nominal input voltage ranges for the analog
inputs, assuming that DAC1 is enabled.
Max V
Min V
Single-Ended
3.3
0
Differential
3.3
-3.3
Special -10/+20
N/A
N/A
Table 2.6.2.1-1. Nominal Analog Input Voltage Ranges (DAC1 Enabled)
Note that the minimum differential input voltage of -3.3 volts means that the positive channel can be as much as 3.3 volts less than
the negative channel, not that a channel can measure 3.3 volts less than ground. The voltage of any analog input pin, compared to
ground, must be in the range -0.3 to +3.6 volts, for specified performance. See Appendix A for voltage limits to avoid damage.
Negative channel numbers 30 and 32 are not valid with DAC1 enabled.
When DAC1 is enabled, the slope/offset calibration constants are not used to convert raw readings to voltages. Rather, the Vreg
value is retrieved from the Mem area, and used with the approximate single-ended or differential conversion equations above,
where Span is Vreg (single-ended) or 2Vreg (differential).
2.6.3 - Typical Analog Input Connections
A common question is “can this sensor/signal be measured with the U3”. Unless the signal has a voltage (referred to U3 ground)
beyond the limits in Appendix A, it can be connected without damaging the U3, but more thought is required to determine what is
necessary to make useful measurements with the U3 or any measurement device.
Voltage (versus ground): The single-ended analog inputs on the U3 measure a voltage with respect to U3 ground. The differential
inputs measure the voltage difference between two channels, but the voltage on each channel with respect to ground must still be
within the common mode limits specified in Appendix A. When measuring parameters other than voltage, or voltages too big or
too small for the U3, some sort of sensor or transducer is required to produce the proper voltage signal. Examples are a
temperature sensor, amplifier, resistive voltage divider, or perhaps a combination of such things.
Impedance: When connecting the U3, or any measuring device, to a signal source, it must be considered what impact the
measuring device will have on the signal. The main consideration is whether the currents going into or out of the U3 analog input
will cause noticeable voltage errors due to the impedance of the source. To maintain consistent 12-bit results, it is recommended
to keep the source impedance within the limits specified in Appendix A.
Resolution (and Accuracy): Based on the measurement type and resolution of the U3, the resolution can be determined in terms of
voltage or engineering units. For example, assume some temperature sensor provides a 0-10 mV signal, corresponding to 0-100
degrees C. Samples are then acquired with the U3 using the 0-2.44 volt single-ended input range, resulting in a voltage resolution
of about 2.44/4096 = 596 µV. That means there will be about 17 discrete steps across the 10 mV span of the signal, and the
temperature resolution is about 6 degrees C. If this experiment required a resolution of 1 degrees C, this configuration would not
be sufficient. Accuracy will also need to be considered. Appendix A places some boundaries on expected accuracy, but an in-
system calibration can generally be done to provide absolute accuracy down to the linearity (INL) limits of the U3.
Speed: How fast does the signal need to be sampled? For instance, if the signal is a waveform, what information is needed: peak,
average, RMS, shape, frequency, … ? Answers to these questions will help decide how many points are needed per waveform
cycle, and thus what sampling rate is required. In the case of multiple channels, the scan rate is also considered. See Sections 3.1
and 3.2.
2.6.3.1 - Signal from the LabJack
One example of measuring a signal from the U3 itself, is with an analog output. All I/O on the U3 share a common ground, so the
voltage on an analog output (DAC) can be measured by simply connecting a single wire from that terminal to an AIN terminal
(FIO/EIO). The analog output must be set to a voltage within the range of the analog input.
2.6.3.2 - Unpowered Isolated Signal
An example of an unpowered isolated signal would be a photocell where the sensor leads are not shorted to any external voltages.
Such a sensor typically has two leads, where the positive lead connects to an AIN terminal and the negative lead connects to a
GND terminal.
2.6.3.3 - Signal Powered By the LabJack
A typical example of this type of signal is a 3-wire temperature sensor. The sensor has a power and ground wire that connect to Vs
and GND on the LabJack, and then has a signal wire that simply connects to an AIN terminal.
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