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CB37 V2.1 Datasheet

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LabJack CB37 USB, Ethernet, WiFi DAQ Screw Terminal Accessory board

The CB37 terminal board connects to the DB37 connector on the LabJack U6, UE9, or T7, and provides convenient screw terminal access. The CB37 is designed to connect directly to the LabJack (see image below), but can also connect via a 37-line 1:1 male-female cable (not included). Since the CB37 V2.1 has screw-terminals with the same IO/IO/GND/VS arrangement as the U6/UE9/T7 itself, it is compatible with LJTick signal conditioning modules.

The green LED on the CB37 is directly powered by the 5-volt supply (Vs) from the LabJack, so it should be lit whenever the CB37 is connected to a powered LabJack.

The CB37 PCB is mounted to a piece of Snaptrack. The Snaptrack is DIN rail mountable using TE Connectivity part #TKAD (not included).

On older CB37 boards, PIN2 was labeled TX0, and PIN20 was labeled RX0.

On the U6/T7, PIN2 and PIN20 are current sources. On the UE9, PIN2 and PIN20 are UART connections.

The table below shows the pinout of the DB37 connector on the LabJack.  Pins 1, 8, 10, 19, and 30, are all connected to GND on the LabJack.  On the CB37, all GND terminals connect to a single ground plane and this ground plane is connected to pins 1, 8, 10, and 19, of the DB37 connector.  The AGND terminal is simply connected to pin 30 of the DB37 connector.  That means the GND and AGND terminals connect to the same single ground plane on the LabJack, but they have different paths to get there.


Table 1. DB37 Pinout

Pin # Pin Name Pin # Pin Name Pin # Pin Name
1 GND 14 AIN9 27 Vs
2 PIN2 (200μA) 15 AIN7 28 Vm+
3 FIO6 16 AIN5 29 DAC1
4 FIO4 17 AIN3 30 GND
5 FIO2 18 AIN1 31 AIN12
6 FIO0 19 GND 32 AIN10
7 MIO1/CIO1 20 PIN20 (10μA) 33 AIN8
8 GND 21 FIO7 34 AIN6
9 Vm- 22 FIO5 35 AIN4
10 GND 23 FIO3 36 AIN2
11 DAC0 24 FIO1 37 AIN0
12 AIN13 25 MIO0/CIO0
13 AIN11 26 MIO2/CIO2

Ground Offsets

When using the analog connections on the CB37, the effect of ground currents should be considered, particularly when a cable is used and substantial current is sourced/sunk through the CB37 terminals. For instance, a test was done with a 6 foot cable between the CB37 and a LabJack UE9, and a 100 ohm load placed from Vs to GND on the CB37 (~50 mA load). A measurement of CB37 GND compared to UE9 GND showed 5.9 mV. If a signal was connected to AIN0 on the CB37 and referred to GND on the CB37, the UE9 reading would be offset by 5.9 mV. The same test with the CB37 direct connected to the UE9 (no cable) resulted in an offset of only 0.2 mV. In both cases (cable or no cable), the voltage measured between CB37 AGND and UE9 GND was 0.0 mV.

When any sizeable cable lengths are involved, a good practice is to separate current carrying ground from ADC reference ground. An easy way to do this on the CB37 is to use GND as the current source/sink, and use AGND as the reference ground. This works well for passive sensors (no power supply), such as a thermocouple, where the only ground current is the return of the input bias current of the analog input. Another option is to use a separate ground wire for loads requiring substantial current.  Another option is to use a differential measurement, rather than a single-ended measurement which is taken versus ground at the built-in instrumentation amp.
See the Mux80 Datasheet for more information about ground offset errors. 





From this description I take that the AGND and GND pins are connected to different nets in the PCB layout of the board. Are these joint at any point? I fear for ground loops between the labjack and the monitored equipment.

AGND and GND are kept separate on the CB37 and any cable connecting the CB37 to the LabJack, but at the LabJack are both connected to the same GND.  You can see from the DB37 pinouts that it has 5 GND pins and no other ground pins.  AGND connects to pin 30.  All GND terminals are the same and connect to pins 1, 8, 10 and 19.