Ocean Optics NeoFox - Manual (Page 70)

C : Analog Output
60 013-20000-009-02-201203
Interfacing to the 4-20mA Output
The NeoFox includes a non-isolated 4-20 mA current sink that can be used to transmit oxygen,
temperature, or tau data. It is extremely important that you set up the interface for this output correctly, or
else irreparable damage to the NeoFox unit can occur.
The figure below shows a complete 4-20 mA loop and its 3 main components: the NeoFox unit, an
ammeter, and a voltage supply. In the diagram, the NeoFox unit is shown on the left and the customer’s
interface circuitry (voltage supply and ammeter) is on the right. Although they are drawn close together,
the NeoFox unit and the customer’s interface circuitry may be physically located far apart, connected by a
2-wire cable.
Typical 4-20 mA Interface
4-20 mA Loop Voltage Supply
As a simple current sink, NeoFox does not provide its own power for the current loop. The user must
provide an isolated power supply between 11 V and 30 V (measured at the input to the NeoFox unit) to
power the loop. This power supply must be completely separate from the NeoFox and its power supply;
the grounds of the Neofox and the supply cannot be connected together.
Most customers will use separate AC adapters to power their NeoFox unit and their interface circuitry.
Usually this will result in the two circuits floating with respect to each other so that the circuit will work
properly. However, it is important to note that NeoFox has a grounded metal case that is connected to its
circuit ground and its USB shell. Hence, customers who connect the NeoFox’s case ground to their
interface circuit’s case ground will find that the measurements are not correct. Some common situations
that could lead to unintended ground connections are the following:
A
Neofox
Unit
PIN 4
PIN 3
Loop
Voltage
Supply
Ammeter