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Datasheets and User Guides

Software & Driver

 

15.0 DAC [T-Series Datasheet]

Range: ~0V to ~5V

Resolution: T4 = 10-bit
                  T7 = 12-bit

Source Impedance: 50 ohms

Max Output Current: 20mA*

*See Appendix A-4 for details on voltage drop related to current draw.

Overview

There are two DACs (digital-to-analog converters, also known as analog outputs) on T-series devices. Each DAC can be set to a voltage between about 0 and 5 volts with 10 bits of resolution (T4) or 12 bits of resolution (T7).

For electrical specifications, See Appendix A-4.

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 voltage is limited to slightly less than VS.

The T7 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. 

Register Listing

To set DAC output voltage, write to the following registers:

DAC Registers
Name Start Address Type Access
DAC#(0:1) 1000 FLOAT32 R/W    

Output Range

The gain of the DAC output amplifiers is designed so the nominal max output is 5.0 volts.  The actual max voltage can vary and will always be limited to the supply voltage.  To determine the actual max of each DAC, write an impossibly high value such as 6.0 to the DAC, and then read back the value.  This is the expected full-scale output with no load and with a supply voltage greater than the returned value.

The output range is also limited by the ability of the output amps to drive near the power rails (0 and VS).  They can drive quite close, especially at light load, but will never be able to drive all the way to exactly 0.0 or VS.

The output range is further limited under load by the drive ability of the output amp and the 50 ohms of source resistance.  The latter is dominant at lower currents, so for example if you set a DAC to 4.000 volts and draw 2 mA from it, the output will be closer to 3.900 volts.

See Appendix A-4 for more information.

For voltages all the way to 0.0 and 5.0 volts (and beyond), the LJTick-DAC is a great solution.

Power-up Defaults

The power-up condition of the DACs can be configured by the user. From the factory, the DACs default to be 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.

Protection

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.

10 Hz Square Wave Output

DAC1 can be configured to output a 10 Hz, 3.3 V square wave. Writing a 1 to DAC1_FREQUENCY_OUT_ENABLE will enable the 10 Hz output:

Name Start Address Type Access
DAC1_FREQUENCY_OUT_ENABLE 61532 UINT32 W    

The square wave output will be disabled:

  • when 0 is written to DAC1_FREQUENCY_OUT_ENABLE,
  • when any value is written DAC1,
  • or when a stream-out channel targeting DAC1 is enabled.

T7 requires firmware version 1.0234 or later. 

Increasing Output to ±10V

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 many can be added to a T-series device. 

Testing

A good way to test the DACs is to see if they output the expected voltage when loaded.  The source impedance of the DACs is about 50 ohms, and that source impedance interacts with load impedance to form a voltage divider.  Remove all user connections and connect a 470 ohm resistor from DAC# to GND.  A voltage divider is formed such that:

Vout = Vset * 470 / (470 + 50) = Vset * 0.9

So whatever the DAC is set to, the output at the terminal should be about 10% less.  You will see the 10% drop with test voltages up to about 4 volts.  Above 4 volts, other effects come into play since this test is drawing substantial current from the DAC line.