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App Notes

Software & Driver

 

2.7 - DAC

DAC Overview


There are two DACs (digital-to-analog converters or analog outputs) on the U6. Each DAC can be set to a voltage between about 0.02 and 5 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.

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)*65536

For a proper calculation, though, use the calibration values (Slope and Offset) stored in the internal flash on the Control processor (Section 5.4):

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 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 U6 to add 20 analog outputs. The LJTick-DAC has various differences compared to the built-in DACs on the U6:

  • Range of +10.0 to -10.0 volts. (~0-5 volts for the U6 DACs)
  • Range includes 0.0 volts. (Typical minimum of 0.04 volts for the U6 DACs)
  • Resolution of 14-bits. (Spread across 20V span, so voltage resolution is the same)

2 comments

I'd like to use the analog output to drive a buffer amplifier (high current op amp).  Can I attach one pin of the output to the non-inverting input of the op amp and the other pin to ground?

 

The DACs are single-ended, so DAC0 only has one output terminal and it is referred to GND.  Section 2.7.1.1 is about high current output.  It suggests the TLV246x as a possible amp and points you to Figure 2.6-1.  Vin in Figure 2.6-1 would be DACx.  Make sure the op-amp power supply (+V/-V) is referred to GND, which is obviously true if you use VS/GND to power the op-amp.