2.6 - AIN [U6 Datasheet]

AIN Overview

The LabJack U6 has 14 user accessible analog inputs built-in. All the analog inputs are available on the DB37 connector, and the first 4 are also available on the built-in screw terminals.

The analog inputs have variable resolution, where the time required per sample increases with increasing resolution. The value passed for ResolutionIndex is from 0-8, where 0 corresponds to default resolution, 1 is roughly 16-bit resolution (RMS or effective), and 8 is roughly 19-bit resolution. The U6-Pro has additional ResolutionIndex settings 9-12 that use the alternate high-resolution converter (24-bit sigma-delta) and correspond to roughly 19-bit to 22-bit resolution.  The default value of 0 corresponds to 8 (U6 command-response), 9 (U6-Pro command-response), or 1 (stream mode).  See Appendix B for more details about ResolutionIndex.

The analog inputs are connected to a high-impedance instrumentation amplifier. This in-amp buffers the signal for the internal ADCs, allows for single-ended or differential conversions, and provides gains of x1, x10, x100, and x1000 corresponding to ranges of ±10V, ±1V, ±0.1V, and ±0.01V. The U6 knows what the internal gain is set to and adjusts the return values to give the voltage at the input terminals, so if you connect a 0.8 volt signal to the input terminals, it will be amplified to 8.0 volts before being digitized, but the reading you get back will be 0.8 volts.

Differential channels are adjacent even/odd pairs only, such as AIN2-AIN3. Thus the positive channel must be even and the negative channel must be +1. The Windows UD driver has different IOTypes for single-ended or differential reads, but the differential IOType can always be used as a negative channel (x1 parameter) of 0/15/199 equates to a single-ended reading.

The inputs are not artificially pulled to 0.0 volts, as that would reduce the input impedance, so readings obtained from floating channels will generally not be 0.0 volts. The readings from floating channels depend on adjacent channels and sample rate and have little meaning. See Section 2.6.3.8.

Settling time is the time from a step change in the input signal to when the signal is sampled by the ADC. A step change in this case is caused when the internal multiplexers change from one channel to another. In general, more settling time is required as gain and resolution are increased. The default “auto” settling time ensures that the device meets specifications at any gain and resolution for source impedances up to at least 1000 ohms. In command/response mode, the effect of the SettlingFactor parameter is 0=Auto, 1=20us, 2=50us, 3=100us, 4=200us, 5=500us, 6=1ms, 7=2ms, 8=5ms, 9=10ms. Stream mode has its own settling parameter which is multiplied by 10 microseconds to determine settling time. The timings in Section 3 are measured with “auto” settling.

Duplicated Terminals (AIN0-AIN3)

AIN0-AIN3 appear on the built-in screw-terminals and also on the DB37 connector.  You should only connect to one or the other, not both at the same time.

To prevent damage due to accidental short circuit, both connection paths have their own series resistor.  All AIN lines have a 2.2k series resistor, and in the case of AIN0-AIN3 the duplicated connections each have their own series resistor, so if you measure the resistance between the duplicate terminals you will see about 4.4k.