- Datasheets
- Accessories
- CB15
- CB25
- CB37 V1.2
- CB37 V2.1
- EB37
- EI-1022
- EI-1034
- EI-1040
- EI-1050
- IDCA-10
- LJTick-CurrentShunt
- LJTick-DAC
- LJTick-DAC Testing Utility
- LJTick-DigitalOut5V
- LJTick-Divider
- LJTick-InAmp
- LJTick-InBuff
- LJTick-LVDigitalIO
- LJTick-OutBuff
- LJTick-Proto
- LJTick-RelayDriver
- LJTick-Resistance
- LJTick-VRef
- Mux80 AIN Expansion Board
- PS12DC
- RB12
- RB16
- T-Series Datasheet
- Preface: Warranty, Liability, Compliance
- 1.0 Device Overview
- 2.0 Installation
- 3.0 Communication
- 4.0 Hardware Overview
- 5.0 USB
- 6.0 Ethernet
- 7.0 WiFi (T7-Pro only)
- 8.0 LEDs
- 9.0 VS, Power Supply
- 10.0 SGND and GND
- 11.0 SPC
- 12.0 200uA and 10uA (T7 Only)
- 13.0 Digital I/O
- 13.1 Flexible I/O (T4 Only)
- 13.2 DIO Extended Features
- 13.2.1 EF Clock Source
- 13.2.2 PWM Out
- 13.2.3 PWM Out with Phase
- 13.2.4 Pulse Out
- 13.2.5 Frequency In
- 13.2.6 Pulse Width In
- 13.2.7 Line-to-Line In
- 13.2.8 High-Speed Counter
- 13.2.9 Interrupt Counter
- 13.2.10 Interrupt Counter with Debounce
- 13.2.11 Quadrature In
- 13.2.12 Interrupt Frequency In
- 13.2.13 Conditional Reset
- 13.3 I2C
- 13.4 SPI
- 13.5 SBUS
- 13.6 1-Wire
- 13.7 Asynchronous Serial
- 14.0 Analog Inputs
- 15.0 DAC
- 16.0 DB37 (T7 Only)
- 17.0 DB15
- 18.0 Internal Temp Sensor
- 19.0 RTC (T7 Only)
- 20.0 Internal Flash
- 21.0 SD Card (T7 Only)
- 22.0 OEM Versions
- 23.0 Watchdog
- 24.0 IO Config, _DEFAULT
- 25.0 Lua Scripting
- Appendix A - Specifications
- Appendix B - Drawings and CAD Models
- Appendix C - Firmware Revision History
- Appendix D - Packaging Information
- Appendix E - Software Options
- U3 Datasheet
- Preface
- 1 - Installation
- 2 - Hardware Description
- 2.1 - USB
- 2.2 - Status LED
- 2.3 - GND and SGND
- 2.4 - VS
- 2.5 - Flexible I/O (FIO/EIO)
- 2.6 - AIN
- 2.6.1 - Channel Numbers
- 2.6.2 - Converting Binary Readings to Voltages
- 2.6.3 - Typical Analog Input Connections
- 2.6.3.1 - Signal from the LabJack
- 2.6.3.2 - Unpowered Isolated Signal
- 2.6.3.3 - Signal Powered By the LabJack
- 2.6.3.4 - Signal Powered Externally
- 2.6.3.5 - Amplifying Small Signal Voltages
- 2.6.3.6 - Signal Voltages Beyond 0-2.44 Volts (and Resistance Measurement)
- 2.6.3.7 - Measuring Current (Including 4-20 mA) with a Resistive Shunt
- 2.6.3.8 - Floating/Unconnected Inputs
- 2.6.3.9 - Signal Voltages Near Ground
- 2.6.4 - Internal Temperature Sensor
- 2.7 - DAC
- 2.8 - Digital I/O
- 2.9 - Timers/Counters
- 2.9.1 - Timer Mode Descriptions
- 2.9.1.1 - PWM Output (16-Bit, Mode 0)
- 2.9.1.2 - PWM Output (8-Bit, Mode 1)
- 2.9.1.3 - Period Measurement (32-Bit, Modes 2 & 3)
- 2.9.1.4 - Duty Cycle Measurement (Mode 4)
- 2.9.1.5 - Firmware Counter Input (Mode 5)
- 2.9.1.6 - Firmware Counter Input With Debounce (Mode 6)
- 2.9.1.7 - Frequency Output (Mode 7)
- 2.9.1.8 - Quadrature Input (Mode 8)
- 2.9.1.9 - Timer Stop Input (Mode 9)
- 2.9.1.10 - System Timer Low/High Read (Modes 10 & 11)
- 2.9.1.11 - Period Measurement (16-Bit, Modes 12 & 13)
- 2.9.1.12 - Line-to-Line Measurement (Mode 14)
- 2.9.2 - Timer Operation/Performance Notes
- 2.9.1 - Timer Mode Descriptions
- 2.10 - SPC (… and SCL/SDA/SCA)
- 2.11 - DB15
- 2.12 - U3-OEM
- 2.13 - Hardware Revision Notes
- 3 - Operation
- 4 - LabJackUD High-Level Driver
- 4.1 - Overview
- 4.2 - Function Reference
- 4.2.1 - ListAll()
- 4.2.2 - OpenLabJack()
- 4.2.3 - eGet() and ePut()
- 4.2.4 - eAddGoGet()
- 4.2.5 - AddRequest()
- 4.2.6 - Go()
- 4.2.7 - GoOne()
- 4.2.8 - GetResult()
- 4.2.9 - GetFirstResult() and GetNextResult()
- 4.2.10 - DoubleToStringAddress()
- 4.2.11 - StringToDoubleAddress()
- 4.2.12 - StringToConstant()
- 4.2.13 - ErrorToString()
- 4.2.14 - GetDriverVersion()
- 4.2.15 - TCVoltsToTemp()
- 4.2.16 - ResetLabJack()
- 4.2.17 - eAIN()
- 4.2.18 - eDAC()
- 4.2.19 - eDI()
- 4.2.20 - eDO()
- 4.2.21 - eTCConfig()
- 4.2.22 - eTCValues()
- 4.3 - Example Pseudocode
- 4.3.1 - Open
- 4.3.2 - Configuration
- 4.3.3 - Analog Inputs
- 4.3.4 - Analog Outputs
- 4.3.5 - Digital I/O
- 4.3.6 - Timers & Counters
- 4.3.7 - Stream Mode
- 4.3.8 - Raw Output/Input
- 4.3.9 - Easy Functions
- 4.3.10 - SPI Serial Communication
- 4.3.11 - I²C Serial Communication
- 4.3.12 - Asynchronous Serial Communication
- 4.3.13 - Watchdog Timer
- 4.3.14 - Miscellaneous
- 4.4 - Errorcodes
- 5 - Low-level Function Reference
- 5.1 - General Protocol
- 5.2 - Low-Level Functions
- 5.2.1 - Bad Checksum
- 5.2.2 - ConfigU3
- 5.2.3 - ConfigIO
- 5.2.4 - ConfigTimerClock
- 5.2.5 - Feedback
- 5.2.5.1 - AIN: IOType = 1
- 5.2.5.2 - WaitShort: IOType=5
- 5.2.5.3 - WaitLong: IOType=6
- 5.2.5.4 - LED: IOType=9
- 5.2.5.5 - BitStateRead: IOType=10
- 5.2.5.6 - BitStateWrite: IOType=11
- 5.2.5.7 - BitDirRead: IOType=12
- 5.2.5.8 - BitDirWrite: IOType=13
- 5.2.5.9 - PortStateRead: IOType=26
- 5.2.5.10 - PortStateWrite: IOType=27
- 5.2.5.11 - PortDirRead: IOType=28
- 5.2.5.12 - PortDirWrite: IOType=29
- 5.2.5.13 - DAC# (8-bit): IOType=34,35
- 5.2.5.14 - DAC# (16-bit): IOType=38,39
- 5.2.5.15 - Timer#: IOType=42,44
- 5.2.5.16 - Timer#Config: IOType=43,45
- 5.2.5.17 - Counter#: IOType=54,55
- 5.2.5.18 - Buzzer: IOType=63
- 5.2.6 - ReadMem (ReadCal)
- 5.2.7 - WriteMem (WriteCal)
- 5.2.8 - EraseMem (EraseCal)
- 5.2.9 - Reset
- 5.2.10 - StreamConfig
- 5.2.11 - StreamStart
- 5.2.12 - StreamData
- 5.2.13 - StreamStop
- 5.2.14 - Watchdog
- 5.2.15 - SPI
- 5.2.16 - AsynchConfig
- 5.2.17 - AsynchTX
- 5.2.18 - AsynchRX
- 5.2.19 - I²C
- 5.2.20 - SHT1X
- 5.2.21 - SetDefaults (SetToFactoryDefaults)
- 5.2.22 - ReadDefaults (ReadCurrent)
- 5.2.23 - 1-Wire
- 5.3 - Errorcodes
- 5.4 - Calibration Constants
- Appendix A - Specifications
- Appendix B - Enclosure and PCB Drawings
- U3 Firmware Revision History
- U6 Datasheet
- Preface: Warranty, Liability, Compliance
- 1 - Installation
- 2 - Hardware Description
- 2.1 - USB
- 2.2 - Power and Status LED
- 2.3 - GND and SGND
- 2.4 - VS
- 2.5 - 10UA and 200UA
- 2.6 - AIN
- 2.6.1 - Channel Numbers
- 2.6.2 - Converting Binary Readings to Voltages
- 2.6.3 - Typical Analog Input Connections
- 2.6.3.1 - Signal from the LabJack
- 2.6.3.2 - Unpowered Isolated Signal
- 2.6.3.3 - Signal Powered By the LabJack
- 2.6.3.4 - Signal Powered Externally
- 2.6.3.5 - Amplifying Small Signal Voltages
- 2.6.3.6 - Signal Voltages Beyond ±10 Volts (and Resistance Measurement)
- 2.6.3.7 - Measuring Current (Including 4-20 mA) with a Resistive Shunt
- 2.6.3.8 - Floating/Unconnected Inputs
- 2.6.4 - Internal Temperature Sensor
- 2.6.5 - Signal Range
- 2.7 - DAC
- 2.8 - Digital I/O
- 2.9 - Timers/Counters
- 2.9.1 - Timer Mode Descriptions
- 2.9.1.1 - PWM Output (16-Bit, Mode 0)
- 2.9.1.2 - PWM Output (8-Bit, Mode 1)
- 2.9.1.3 - Period Measurement (32-Bit, Modes 2 & 3)
- 2.9.1.4 - Duty Cycle Measurement (Mode 4)
- 2.9.1.5 - Firmware Counter Input (Mode 5)
- 2.9.1.6 - Firmware Counter Input With Debounce (Mode 6)
- 2.9.1.7 - Frequency Output (Mode 7)
- 2.9.1.8 - Quadrature Input (Mode 8)
- 2.9.1.9 - Timer Stop Input (Mode 9)
- 2.9.1.10 - System Timer Low/High Read (Modes 10 & 11)
- 2.9.1.11 - Period Measurement (16-Bit, Modes 12 & 13)
- 2.9.1.12 - Line-to-Line Measurement (Mode 14)
- 2.9.2 - Timer Operation/Performance Notes
- 2.9.1 - Timer Mode Descriptions
- 2.10 - SPC (or VSPC)
- 2.11 - DB37
- 2.12 - DB15
- 2.13 - OEM Connector Options
- 3 - Operation
- 4 - LabJackUD High-Level Driver
- 4.1 - Overview
- 4.2 - Function Reference
- 4.2.1 - ListAll()
- 4.2.2 - OpenLabJack()
- 4.2.3 - eGet() and ePut()
- 4.2.4 - eAddGoGet()
- 4.2.5 - AddRequest()
- 4.2.6 - Go()
- 4.2.7 - GoOne()
- 4.2.8 - GetResult()
- 4.2.9 - GetFirstResult() and GetNextResult()
- 4.2.10 - DoubleToStringAddress()
- 4.2.11 - StringToDoubleAddress()
- 4.2.12 - StringToConstant()
- 4.2.13 - ErrorToString()
- 4.2.14 - GetDriverVersion()
- 4.2.15 - TCVoltsToTemp()
- 4.2.16 - ResetLabJack()
- 4.2.17 - eAIN()
- 4.2.18 - eDAC()
- 4.2.19 - eDI()
- 4.2.20 - eDO()
- 4.2.21 - eTCConfig()
- 4.2.22 - eTCValues()
- 4.3 - Example Pseudocode
- 4.3.1 - Open
- 4.3.2 - Configuration
- 4.3.3 - Analog Inputs
- 4.3.4 - Analog Outputs
- 4.3.5 - Digital I/O
- 4.3.6 - Timers & Counters
- 4.3.7 - Stream Mode
- 4.3.8 - Raw Output/Input
- 4.3.9 - Easy Functions
- 4.3.10 - SPI Serial Communication
- 4.3.11 - I²C Serial Communication
- 4.3.12 - Asynchronous Serial Communication
- 4.3.13 - Watchdog Timer
- 4.3.14 - Miscellaneous
- 4.4 - Errorcodes
- 5 - Low-level Function Reference
- 5.1 - General Protocol
- 5.2 - Low-Level Functions
- 5.2.1 - Bad Checksum
- 5.2.2 - ConfigU6
- 5.2.3 - ConfigIO
- 5.2.4 - ConfigTimerClock
- 5.2.5 - Feedback
- 5.2.5.1 - AIN: IOType = 1
- 5.2.5.2 - AIN24: IOType = 2
- 5.2.5.3 - AIN24AR: IOType = 3
- 5.2.5.4 - WaitShort: IOType=5
- 5.2.5.5 - WaitLong: IOType=6
- 5.2.5.6 - LED: IOType=9
- 5.2.5.7 - BitStateRead: IOType=10
- 5.2.5.8 - BitStateWrite: IOType=11
- 5.2.5.9 - BitDirRead: IOType=12
- 5.2.5.10 - BitDirWrite: IOType=13
- 5.2.5.11 - PortStateRead: IOType=26
- 5.2.5.12 - PortStateWrite: IOType=27
- 5.2.5.13 - PortDirRead: IOType=28
- 5.2.5.14 - PortDirWrite: IOType=29
- 5.2.5.15 - DAC# (8-bit): IOType=34,25
- 5.2.5.16 - DAC# (16-bit): IOType=38,39
- 5.2.5.17 - Timer#: IOType=42,44,46,48
- 5.2.5.18 - Timer#Config: IOType = 43, 45, 47, 49
- 5.2.5.19 - Counter#: IOType = 54, 55
- 5.2.6 - ReadMem (ReadCal)
- 5.2.7 - WriteMem (WriteCal)
- 5.2.8 - EraseMem (EraseCal)
- 5.2.9 - SetDefaults (SetToFactoryDefaults)
- 5.2.10 - ReadDefaults (ReadCurrent)
- 5.2.11 - Reset
- 5.2.12 - StreamConfig
- 5.2.13 - StreamStart
- 5.2.14 - StreamData
- 5.2.15 - StreamStop
- 5.2.16 - Watchdog
- 5.2.17 - SPI
- 5.2.18 - AsynchConfig
- 5.2.19 - AsynchTX
- 5.2.20 - AsynchRX
- 5.2.21 - I²C
- 5.2.22 - SHT1X
- 5.2.23 - 1-Wire
- 5.2.24 - StreamAddChannels
- 5.3 - Errorcodes
- 5.4 - Calibration Constants
- Appendix A - Specifications
- Appendix B - Noise and Resolution Tables
- Appendix C - Enclosure and PCB Drawings
- U6 Firmware Revision History
- Digit Datasheet (Discontinued)
- UE9 Datasheet
- Preface
- 1 - Installation
- 2 - Hardware Description
- 2.1 - USB
- 2.2 - Ethernet
- 2.3 - Vext (Screw Terminals and Power Jack)
- 2.4 - Comm and Control LEDs
- 2.5 - GND and SGND
- 2.6 - VS
- 2.7 - AIN
- 2.7.1 - Channel Numbers
- 2.7.2 - Converting Binary Readings to Voltages
- 2.7.3 - Typical Analog Input Connections
- 2.7.3.1 - Signal from the LabJack
- 2.7.3.2 - Unpowered Isolated Signal
- 2.7.3.3 - Signal Powered by the LabJack
- 2.7.3.4 - Signal Powered Externally
- 2.7.3.5 - Amplifying Small Signal Voltages
- 2.7.3.6 - Signal Voltages Beyond ±5 Volts (and Resistance Measurement)
- 2.7.3.7 - Measuring Current (Including 4-20 mA) with a Resistive Shunt
- 2.7.3.8 - Floating/Unconnected Inputs
- 2.7.4 - Internal Temperature Sensor
- 2.8 - DAC
- 2.9 - Digital I/O
- 2.10 - Timers/Counters
- 2.10.1 - Timer Mode Descriptions
- 2.10.1.1 - PWM Output (16-Bit, Mode 0)
- 2.10.1.2 - PWM Output (8-Bit, Mode 1)
- 2.10.1.3 - Period Measurement (32-Bit, Modes 2 & 3)
- 2.10.1.4 - Duty Cycle Measurement (Mode 4)
- 2.10.1.5 - Firmware Counter Input (Mode 5)
- 2.10.1.6 - Firmware Counter Input With Debounce (Mode 6)
- 2.10.1.7 - Frequency Output (Mode 7)
- 2.10.1.8 - Quadrature Input (Mode 8)
- 2.10.1.9 - Timer Stop Input (Mode 9)
- 2.10.1.10 - System Timer Low/High Read (Modes 10 & 11)
- 2.10.1.11 - Period Measurement (16-Bit, Modes 12 & 13)
- 2.10.2 - Timer Operation/Performance Notes
- 2.10.1 - Timer Mode Descriptions
- 2.11 - SCL and SDA (or SCA)
- 2.12 - DB37
- 2.13 - DB15
- 2.14 - OEM Connector Options
- 3 - Operation
- 4 - LabJackUD High-Level Driver
- 4.1 - Overview
- 4.2 - Function Reference
- 4.2.1 - ListAll()
- 4.2.2 - OpenLabJack()
- 4.2.3 - eGet() and ePut()
- 4.2.4 - eAddGoGet()
- 4.2.5 - AddRequest()
- 4.2.6 - Go()
- 4.2.7 - GoOne()
- 4.2.8 - GetResult()
- 4.2.9 - GetFirstResult() and GetNextResult()
- 4.2.10 - DoubleToStringAddress()
- 4.2.11 - StringToDoubleAddress()
- 4.2.12 - StringToConstant()
- 4.2.13 - ErrorToString()
- 4.2.14 - GetDriverVersion()
- 4.2.15 - TCVoltsToTemp()
- 4.2.16 - ResetLabJack()
- 4.2.17 - eAIN()
- 4.2.18 - eDAC()
- 4.2.19 - eDI()
- 4.2.20 - eDO()
- 4.2.21 - eTCConfig()
- 4.2.22 - eTCValues()
- 4.3 - Example Pseudocode
- 4.3.1 - Open
- 4.3.2 - Configuration
- 4.3.3 - Analog Inputs
- 4.3.4 - Analog Outputs
- 4.3.5 - Digital I/O
- 4.3.6 - Timers & Counters
- 4.3.7 - Stream Mode
- 4.3.8 - Raw Output/Input
- 4.3.9 - Easy Functions
- 4.3.10 - SPI Serial Communication
- 4.3.11 - I²C Serial Communication
- 4.3.12 - Asynchronous Serial Communication
- 4.3.13 - Watchdog Timer
- 4.3.14 - Miscellaneous
- 4.4 - Errorcodes
- 5 - Low-level Function Reference
- 5.1 - General Protocol
- 5.2 - Comm Functions
- 5.3 - Control Functions
- 5.3.1 - BadChecksum
- 5.3.2 - ControlConfig
- 5.3.3 - Feedback (and FeedbackAlt)
- 5.3.4 - SingleIO
- 5.3.5 - TimerCounter
- 5.3.6 - StreamConfig
- 5.3.7 - StreamStart
- 5.3.8 - StreamData
- 5.3.9 - StreamStop
- 5.3.10 - ReadMem
- 5.3.11 - WriteMem
- 5.3.12 - EraseMem
- 5.3.13.1 - WatchdogConfig
- 5.3.13.2 - WatchdogRead
- 5.3.13.3 - Extended WatchdogConfig
- 5.3.13.4 - WatchdogClear
- 5.3.15 - Reset
- 5.3.16 - SPI
- 5.3.17 - AsynchConfig
- 5.3.18 - AsynchTX
- 5.3.19 - AsynchRX
- 5.3.20 - I²C
- 5.3.21 - SHT1X
- 5.3.22 - StreamDAC
- 5.3.23 - SetDefaults (SetToFactoryDefaults)
- 5.3.24 - ReadDefaults (ReadCurrent)
- 5.3.25 - 1-Wire
- 5.4 - Low-Level Errorcodes
- 5.5 - Modbus
- 5.6 - Calibration Constants
- 6 - Low-level Native Examples
- Appendix A - Specifications
- Appendix B - Noise and Resolution Tables
- Appendix C - Enclosure and PCB Drawings
- UE9 Firmware Revision History
- U12 Datasheet
- 1 - Installation
- 2 - Hardware Description
- 3 - Example Applications
- 4 - Programming Reference
- 4.1 - EAnalogIn
- 4.2 - EAnalogOut
- 4.3 - ECount
- 4.4 - EDigitalIn
- 4.5 - EDigitalOut
- 4.6 - AISample
- 4.7 - AIBurst
- 4.8 - AIStreamStart
- 4.9 - AIStreamRead
- 4.10 - AIStreamClear
- 4.11 - AOUpdate
- 4.12 - AsynchConfig
- 4.13 - Asynch
- 4.14 - BitsToVolts
- 4.15 - VoltsToBits
- 4.16 - Counter
- 4.17 - DigitalIO
- 4.18 - GetDriverVersion
- 4.19 - GetErrorString
- 4.20 - GetFirmwareVersion
- 4.21 - GetWinVersion
- 4.22 - ListAll
- 4.23 - LocalID
- 4.24 - NoThread
- 4.25 - PulseOut
- 4.26 - PulseOutStart
- 4.27 - PulseOutFinish
- 4.28 - PulseOutCalc
- 4.29 - ReEnum
- 4.30 - Reset (or ResetLJ)
- 4.31 - SHT1X
- 4.32 - SHTComm
- 4.33 - SHTCRC
- 4.34 - Synch
- 4.35 - Watchdog
- 4.36 - ReadMem
- 4.37 - WriteMem
- 4.38 - BuildOptionBits (ActiveX only)
- 4.39 - FourPack (ActiveX only)
- 4.40 - Description of Errorcodes
- 5 - Low-Level Function Reference
- Appendix A - Specifications
- Appendix B - Dimensions
- Appendix C - U12 Hardware Troubleshooting
- Appendix D - Maximum Data Rates for the LabJack U12
- Accessories
4 comments
I have 4 4-20 ma sensors
I have 4 4-20 ma sensors going into 2 current shunts going to a u6 pro. I am using this code to read those sensors. I am seeing a lot of variability in the readings that is not there. Would the resolution value in the code help with this? I tried running a line from the negative power supply which is connected to all my negative sensors to signal ground, but that didn't really help. Do you have any ideas how to decrease the signal fluctuation?
Thanks
First do a basic test to see
First do a basic test to see if you can reproduce the noise figures from Appendix B of the U6 User's Guide. Remove the LJTCS and instead place a small jumper wire from AINx to GND. View the voltage readings using the test panel in LJControlPanel. I would use the +/-10V range (BIP10V) as that is what you will likely use with the LJTCS, and you can try different resolution settings.
Once you confirm the ideal noise levels, remove the jumper and install the LJTCS, but instead of your signal use a simple/quiet voltage source with a resistor. For example, take a AA battery and connect the battery+ to one side of a 270 ohm resistor, connect the other side of the resistor to INA+ on the LJTCS, and connect INA- to both GND and battery-. Another source to try is connect DAC0 (set to 4.0 volts in the test panel) to a 1k resistor, connect the 1k resistor to INA+, and connect INA- to GND.
To continue along these lines I would post a comment on the LJTCS datasheet or the Noise and Resolution App Note.
If you find that everything is good, but just your signal has too much noise, then the noise is likely really there. The question is whether that is the best the sensor can do, or there is something that can be changed to improve it. To continue that, I would start a topic in our forums and provide more detail about the sensor and how it is connected. Do initial troubleshooting with just one sensor connected and nothing else.
Hello there, I am getting
By "errors", do you mean the
By "errors", do you mean the varying values (noise) or the difference from 1.0 (accuracy)?
http://labjack.com/support/app-notes/noise-and-resolution
http://labjack.com/support/app-notes/resolution-and-accuracy
Noise: From Appendix A of the U3 User's Guide, the typical noise of an analog input is +/-1 count (which is excellent BTW). On the +/-10V range, each count is 20/4096 => ~ 5mV. With your data you have a center value of 0.991 and then +/-1, so typical and expected.
Accuracy: From Appendix A of the U3 User's Guide, the accuracy is +/-0.13 % full-span, which on the +/-10V range equates to 20*0.0013 => +/-26mV. DAC0 has its own inaccuracy, but assume it is perfect and giving you 1.0 volts. If your average reading is 0.991 volts, you have a -9mV error which is within spec.