Internal Temperature Sensor - LabJack UE9 User Manual

In a situation where it is desired that a floating channel read a particular voltage, say to detect a broken wire, a resistor can be
placed from the AINx screw terminal to the desired voltage (GND, VS, DACx, ...). A 10 kΩ resistor will pull the analog input
readings to within 1 binary count of any desired voltage, but obviously degrades the input impedance to 10 kΩ. For the specific
case of pulling a floating channel to 0 volts, a 100 kΩ resistor to GND can typically be used to provide analog input readings within
100 mV of ground.

2.7.4 - Internal Temperature Sensor

The UE9 has an internal temperature sensor. Although this sensor measures the temperature inside the UE9, it has been
calibrated to read ambient temperature. For accurate measurements the temperature of the entire UE9 must stabilize relative to
the ambient temperature, which can take on the order of 1 hour. Best results will be obtained in still air in an environment with
slowly changing ambient temperatures.
The internal temperature sensor is also affected by the operating speed of the UE9. With Control firmware V1.08 or higher, the
UE9 is in high power mode by default, which is assumed by the LabJack UD driver.
With the UD driver, the internal temperature sensor is read by acquiring analog input channel 133 or 141, and returns degrees K.
2.8 - DAC
There are two DACs (digital-to-analog converters or analog outputs) on the UE9. Each DAC can be set to a voltage between
about 0.02 and 4.86 volts with 12-bits of resolution.
Although the DAC values are based on an absolute reference voltage, and not the supply voltage, the DAC output buffers are
powered internally by Vs and thus the maximum output is limited to slightly less than Vs. Another implication of this is that high
frequency power supply noise might couple to the analog outputs.
The analog output commands are sent as raw binary values (low level functions). For a desired output voltage, the binary value can
be approximated as:
Bits(uncalibrated) = (Volts/4.86)*4096
For a proper calculation, though, use the calibration values (Slope and Offset) stored in the internal flash on the Control processor
(Table 2-4):
Bits = (Slope * Volts) + Offset
The DACs appear both on the screw terminals and on the DB37 connector. These connections are electrically the same, and the
user must exercise caution only to use one connection or the other, and not create a short circuit.
The DACS on the UE9 can be disabled. Prior to control firmware 1.98 when disabled they are placed in a high-impedance state,
firmware 1.98 and later always leaves the DACs enabled. Both DACs are enabled or disabled at the same time, so if a command
causes one DAC to be enabled the other is also enabled.
The power-up condition of the DACs can be configured by the user. From the factory, the DACS default to enabled at minimum
voltage (~0 volts). Note that even if the power-up default for a line is changed to a different voltage or disabled, there is a delay of
about 100 ms at power-up where the DACs are in the factory default condition.
The analog outputs can withstand a continuous short-circuit to ground, even when set at maximum output.
Voltage should never be applied to the analog outputs, as they are voltage sources themselves. In the event that a voltage is
accidentally applied to either analog output, they do have protection against transient events such as ESD (electrostatic
discharge) and continuous overvoltage (or undervoltage) of a few volts.
There is an accessory available from LabJack called the LJTick-DAC that provides a pair of 14-bit analog outputs with a range of
±10 volts. The LJTick-DAC plugs into any digital I/O block, and thus up to 10 of these can be used per UE9 to add 20 analog
outputs.
2.8.1 - Typical Analog Output Connections
2.8.1 - Typical Analog Output Connections
The DACs on the UE9 can output quite a bit of current, but have 50 Ω of source impedance that will cause voltage drop. To avoid
this voltage drop, an op-amp can be used to buffer the output, such as the non-inverting configuration shown in Figure 2-3. A
simple RC filter can be added between the DAC output and the amp input for further noise reduction. Note that the ability of the
amp to source/sink current near the power rails must still be considered. A possible op-amp choice would be the TLV246x family
(ti.com).
2.8.1.2 - Different Output Ranges
There is an accessory available from LabJack called the LJTick-DAC that provides a pair of 14-bit analog outputs with a range of
±10 volts. The LJTick-DAC plugs into any digital I/O block, and thus up to 10 of these can be used per UE9 to add 20 analog
outputs.
The typical output range of the DACs is about 0.02 to 4.86 volts. For other unipolar ranges, an op-amp in the non-inverting
configuration (Figure 2-3) can be used to provide the desired gain. For example, to increase the maximum output from 4.86 volts
to 10.0 volts, a gain of 2.06 is required. If R2 (in Figure 2-3) is chosen as 100 kΩ, then an R1 of 93.1 kΩ is the closest 1% resistor
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