2.6 - AIN [U3 Datasheet]

AIN Overview

The LabJack U3 has up to 16 analog inputs available on the flexible I/O lines (FIO0-FIO7 and EIO0-EIO7). Single-ended measurements can be taken of any line compared to ground, or differential measurements can be taken of any line to any other line.

Analog input resolution is 12-bits. The range of single-ended analog inputs is normally about 0-2.44, and there is a “special” 0-3.6 volt range available. The range of differential analog inputs is typically ± 2.4 volts, but is pseudobipolar, not true bipolar. The difference (positive channel minus negative channel) can be -2.4 volts, but neither input can have a voltage less than -0.3 volts to ground. For valid measurements, the voltage on every low-voltage analog input pin, with respect to ground, must be within -0.3 to +3.6 volts. See Appendix A for voltage limits to avoid damage.

On the U3-HV, compared to the -LV, the first four flexible I/O are fixed as analog inputs (AIN0-AIN3), and have scaling such that the input range is a true bipolar ±10 volts normally, and -10 to +20 volts when using the “special” range. The input impedance of these four lines is roughly 1 MΩ, which is good, but less than the normal low voltage analog inputs. Analog/digital configuration and all other digital operations on these pins are ignored. FIO4-EIO7 are still available as flexible I/O, same as the U3-LV.

To get the special 0-3.6 volt or -10/+20 volt range, you do a differential reading request with the negative channel set to 32.  The U3 will then do a special range single-ended reading on the specified positive channel.  The U3 is using differential ADC techniques internally, but the external reading is single-ended.

Because the scaling on the high-voltage inputs on the U3-HV (AIN0-AIN3) is inherently single-ended, a factory calibration is not possible for differential readings. If a differential reading is requested where either channel is a high-voltage channel, the driver will return the raw binary reading and the user must handle calibration/conversion.  Note that 0 counts is about -20V and 65520 counts is about +20V, and no high-voltage channel can be less than -12.8V or more than +20.1V.

The analog inputs have a QuickSample option where each conversion is done faster at the expense of increased noise. This is enabled by passing a nonzero value for put_config special channel LJ_chAIN_RESOLUTION. There is also a LongSettling option where additional settling time is added between the internal multiplexer configuration and the analog to digital conversion. This allows signals with more source impedance, and is enabled by passing a nonzero value for put_config special channel LJ_chAIN_SETTLING_TIME. Both of these options are disabled by default.  This applies to command/response mode only, and the resulting typical data rates are discussed in Section 3.1.  For stream mode, see Section 3.2.

Note that sinking excessive current into digital outputs can cause substantial errors in analog input readings. See Section 2.8.1.4 for more info.