I need to measure both pH and the TDS in just one system for my salt water fish tank. But it is a known problem that the pH interface and the TDS interface interference when they are measuring in the same (water)tank without some extra adjustments in the circuits. The solution is to isolate the interfaces from each other; e.g. galvanically isolate the interfaces. Below a picture of my working test. 

TDS pH

(bit of a mesh with all the wiring…)

First of all, you need a seperate power supply for each interface and for each interface a 0-5V voltage isolation module. See the pictures at the end of this article.

The “interface” side:
The interfaces for pH and TDS both need 5Vdc. So the TDS and pH interface are now seperately powered by a mini 230Vac to 12Vdc/5Vdc supply. Then it is important to galvanic isolated the signals from the TDS an pH interface. This is done by the -on the photo 2 pieces of-  “voltage signal isolation modules 0-5V” boards. These boards need a 12Vdc power at the interface side (P1) and a 12Vdc power supply at the Arduino board side (P2). The 12Vdc power for the P1 side , is also supplied from the same mini supply as for the 5Vdc interface supply. The output of the interface is connected to the IN1 channel and is grounded on the G of the P1 side of the voltage signal isolation module.
Be aware that the circuits for the interface modules may never be grounded to each other (so you need 2 seperate voltage isolation modules)! 

The “arduino” side:
On the Arduino side the two 0-5Vdc signals, must have a common ground. So on the Arduino side the ground (P2 side) of both voltage signal isolation modules (G) is connected to the common Arduino ground. The needed 12Vdc supply for the P2 side of the modules is connected to a common 12Vdc power supply. The ground of the 12Vdc power supply is connected to the Arduino ground. The output of the voltage signal isolation modules, can now be connected to the analogue inputs of an Arduino board and are -on the interface side- galvanic isolated from each other.

Used components:


Power supply 230Vac to 5Vdc/100mA + 12Vdc/500mA

voeding230 5 12

Voltage signal isolation module 0-5V

SC03 voltage isolator

pH interface (whitout sensor on picture)

PHInterface

TDS interface (with sensor)

TDSInterface