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Datasheets and User Guides

App Notes

Software & Driver

 

2.2 - AO0 & AO1

The LabJack U12 has 2 screw terminals for analog output voltages. Each analog output can be set to a voltage between 0 and the supply voltage (+5 volts nominal) with 10-bits of resolution.

The output voltage is ratiometric with the +5 volt supply (+5V), which is generally accurate to ±5% (see Appendix A). If an output voltage of 5 volts is specified, the resulting output will be 100% of the supply voltage. Similarly, specifying 2.5 volts actually gives 50% of the supply voltage. The maximum output voltage is almost 100% of +5V at no-load, and decreases with load. See the specifications in Appendix A relating to maximum output voltage. Also note that loading either analog output will cause an IR drop through the source impedance of each.

If improved accuracy is needed, measure the +5 volt supply with an analog input channel, and the actual output voltage can be calculated. For instance, if an analog output of 2.5 volts is specified and a measurement of +5V returns 5.10 volts, the actual output voltage is 2.55 volts (at no-load). Alternatively (and preferably), the analog output can itself be measured with an analog input.

There is a 1st order low-pass filter on each analog output with a 3dB frequency around 22 Hz.

The analog outputs are initialized to 0.0 volts on power-up or reset.

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 overvoltages such as ESD (electrostatic discharge) and continuous overvoltage of a couple volts. An applied voltage that exceeds the capability of this protection will most likely damage the resistor R63 (AO0) or R62 (AO1) on the LabJack U12 PCB. The symptom of such a failure would be reduced voltage from the analog outputs, particularly at load, and could be verified by measuring the resistance of R62/R63 (should be less than 50 ohms but a damaged resistor will measure higher). A simple repair for such damage is to remove the damaged resistor and simply make a short with a blob of solder.

Software

The analog outputs are set using the function EAnalogOut (easy function) or AOUpdate, which take up to 20 ms to execute, providing a maximum update rate of about 50 Hz per channel. AOUpdate also controls/reads all 20 digital I/O and the counter.

2 comments

It would be helpful to state the drive capabilities of the AO ports.

What is the output impedance? Maximum allowed load? Maybe a diagram output voltage vs. load current...

The maximum allowed load, and other specifications can be found in Appendix A under the Analog Outputs (AO0 & AO1) section. Though adding a diagram to this section of the datasheet is a good idea. http://labjack.com/support/u12/users-guide/appendix-a