14.0 AIN

T7 AIN Overview

Analog Inputs: 14

Voltage Ranges: ±10V, ±1V, ±0.1V, and ±0.01V (Appendix A-3 and Appendix A-3-2)

Effective Resolution: 11-bit to 22-bit (Appendix A-3-1)

Max Data Rate: 100 ksamples/second in stream mode (Appendix A-1)

Available Inputs

The LabJack T7/T7-Pro has 16 total built-in analog inputs. Two of these are connected internally (AIN14=Temperature & AIN15=GND), leaving 14 user-accessible analog inputs (AIN0-AIN13). The first 4 analog inputs, AIN0-AIN3, appear both on the screw terminals and on the DB37 connector.  The remaining 10 analog inputs appear on the DB37 only.

Table 14.0-1. Positive Channel Numbers

Table 14.0-2. Negative Channel Numbers

Extended channels 16 through 127 are created with external multiplexers or the Mux80 accessory.  See Section 14.2 for more information.

Resolution Index

The analog inputs have variable resolution.  Sample time increases as the resolution index increases.  Resolution indices range:

• from 1 to 8 for the T7
• from 1 to 12 for the T7-Pro

Setting resolution index to 0 sets the default resolution index, which is:

• 1 for stream mode
• 8 for command-response mode on a T7
• 9 for command-response mode on a T7-Pro

See Appendix A-3-1 more details about resolution index.

Impedance, Gain, Single-ended vs. Differential, and Floating Inputs

The analog inputs are connected to a high-impedance instrumentation amplifier, as shown in Figure 4.2-2. 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).

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 analog 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 related floating input application note.

Extended Features

Various extended features are available for analog inputs to allow them to do thermocouple math, RMS calculations, and more.  See Section 14.1.

Analog Input App Note

For information regarding typical analog input connections, please see the Analog Input App Note.

  Register Listing

Some Examples

Analog Input Example: To read a voltage connected to AIN2, perform a read of AIN2 (address 4).  The result will be in the form of a floating point number, like 8.82332V.

Range Example: If the voltage source connected to AIN1 has a known range of 0 to 0.7V, the appropriate range for AIN1 is the ±1V range. Configure this by writing 1.0 to AIN1_RANGE (address 40002).

Differential Analog Input Example:  To take a differential reading on AIN2, set AIN3 as its negative channel so that the measurement is AIN2-AIN3.  Configure this by writing a value of 3 to AIN2_NEGATIVE_CH (41002).  To set it back to single-ended (AIN2-GND), write 199 to AIN2_NEGATIVE_CH (address 41002).

Resolution Index Example: Change the AIN1 resolution index to 5 by writing a value of 5 to AIN1_RESOLUTION_INDEX (address 41501).

Settling Example: Change the settling time of AIN3 to 500 µs by writing 500 to AIN3_SETTLING_US (address 42006). The value 0 sets automatic settling, which is recommended for most applications.

Address Increment Size

Addresses of the FLOAT32 type increment in steps of 2 because FLOAT32 uses two sets of 16-bits. FLOAT32 registers include AIN#, AIN#_RANGE, and AIN#_SETTLING_US.

Addresses of the UINT16 type increment in steps of 1 because UNIT16 uses only one set of 16-bits. UINT16 registers include AIN#_NEGATIVE_CH and AIN#_RESOLUTION_INDEX.

Extra Details

The AIN#(0:13)_RANGE and AIN_ALL_RANGE parameters are controlling the gain of the internal instrumentation amplifier. The in-amp supports gains of x1, x10, x100, and x1000. If you set range=10, you get gain=x1, and the analog input range is ±10 volts. If you set range=1, you get gain=x10, and the analog input range is ±1 volts. Note that the device 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. Write range=10 to get a range of ±10V (default), range=1 to get a range of ±1V, range=0.1 to get a range of ±0.1V, or range=0.01 to get a range of ±0.01V. If you write a value in between the valid ranges, the larger range will be used.

The AIN#(0:13)_NEGATIVE_CH and AIN_ALL_NEGATIVE_CH parameters pertain to differential versus single-ended readings (not to be confused with bipolar and unipolar ... the T7 is always bipolar).  On the T7, 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.  Only an even channel can have an associated negative channel, so you will never write to odd channels of this register (e.g. never write to AIN3_NEGATIVE_CH/41003).  Channel numbers in the extended range (above AIN15), are connected to AIN0-AIN13 and those dictate the even/odd rule, not the extended channel numbers (see the Mux80 datasheet).

The AIN#(0:13)_RESOLUTION_INDEX and AIN_ALL_RESOLUTION_INDEX parameters affect command-response readings from the ADC.  A higher Resolution_Index results in lower noise and thus higher effective & noise-free resolution, with the tradeoff of longer sample times.  See the Noise and Resolution Appendix.  The value passed for Resolution_Index is from 0-8, where 0 corresponds to default, 1 is roughly 16-bit resolution (RMS or effective), and 8 is roughly 19-bit resolution. The T7-Pro has additional Resolution_Index settings 9-12 that use the alternate high-resolution converter (24-bit sigma-delta) and correspond to roughly 19-bit to 22-bit resolution.  For command-response readings, the default value of 0 corresponds to Resolution_Index=8 on a T7 and Resolution_Index=9 on a T7-Pro.  For typical noise levels and sample times at different combinations of resolution index and gain see Appendix A-3-1 Noise and Resolution.  For general discussion on the meaning of resolution see the Noise and Resolution App Note.  This register is for command-response and AIN-EF only.  Stream mode has its own resolution index register documented in the Stream Section.

The AIN#(0:13)_SETTLING_US and AIN_ALL_SETTLING_US parameters are setting the time from a step change in the input signal to when the signal is sampled by the ADC, measured in microseconds. 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 impedance up to at least 1000 ohms. This parameter applies to command/response mode and AIN EF.  Stream mode has its own settling parameter. The timings in Appendix A are measured with “auto” settling.  This register is for command-response and AIN-EF only.  Stream mode has its own settling register documented in the Stream Section.

Duplicated Terminals on the T7 (AIN0-AIN3)

AIN0-AIN3 appear on the built-in screw-terminals and also on the DB37 connector. Users 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.