# Circuit for current injection and voltage reading

Discussion in 'The Projects Forum' started by tifania, Jun 27, 2016.

1. ### tifania Thread Starter New Member

Mar 14, 2016
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0
Hello everyone,

I am Stefania and I am working on a project for developing a sensor based on the principle of EIT (Electrical Impedeance Tomography).

For the project, I need to inject into the material I am using, a constant current between two electrodes placed at the periphery of the sensor and then read the corresponding voltages at the other electrodes (I have 16 electrodes). Then the current has to be switched between the remainig electrode pairs in a scanning cycle (as you can see in the picture-only 8 electrodes in the picture).

I am using 2 demultiplexer with 16 outputs to control the current injection. One multiplexer serves to control the current source and the other one for the ground in order to switch all the electrodes between ground and current source.

The MUX are controlled by the digital outputs of a NI DAQ-6353 card which also serves for reading the voltages in a differential mode.(you can see it in the schematic)

The circuit is powered with 5V from the DAQ and the resistance of the sensor is around 60K Ohm.

I am using a 10uA constant current. The power source will be external, as my DAQ can only supply 5mA and I will need to change the current amplitude in the future.

My question is this: is the circuit going to work? I don't see any problem, I have also tested the constant current generator and it works up to a load of 100K, but maybe someone with a more expert eye could see if there are some errors.

Thanks a lot,

Stefania

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
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What is the purpose of the two opamps?

3. ### tifania Thread Starter New Member

Mar 14, 2016
4
0
Hi, they are there for the constant current generator. One of them is for the voltage divider giving in output a voltage Vref. The second one is for having on the terminal D of the mux a constant current equal to Vref/R1, indipendent from the load.

4. ### OBW0549 Well-Known Member

Mar 2, 2015
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941
I see several issues that could prevent this design from doing what you want it to do.

First, the OP297 is an "old-style" opamp designed back when +/- 15V power supply voltages were the norm. You can see this from the Input Voltage Range (Vcm) and Output Voltage Swing (Vout) specs; neither one goes all the way to the (+) and (-) supply rails (Vcc and Gnd in your case), making this part ill-suited for operating off a single 5V supply. In my opinion, a low-voltage "rail-to-rail I/O" opamp like the LT1490A or LMC6482 would be a better choice, but many others are available.

Second, the ADG1406 analog MUX is characterized for +/-15V, +12V, and +/-5V operation, but not for operation off a single +5V supply; while it's possible that it will work in this circuit, I wouldn't count on it. I think you will either need to find a low-voltage substitute for this part, or generate a +12V supply voltage for it using some kind of step-up converter or an auxiliary 12V power source.

Other than those two issues, the design looks OK to me.

tifania likes this.
5. ### tifania Thread Starter New Member

Mar 14, 2016
4
0

Thanks a lot for your very useful feedback. I am thinking to use different power supplies during the test, 5V and 12V. So The idea is to use an ADG1606 as mux (which also has a very small ON resistance and small current as power requirement, that are some requirements of my circuit) and LT1490A as you suggested. Also I think TS912 can be ok. Do you think these are a good choice?

Thanks,

Stefania

6. ### OBW0549 Well-Known Member

Mar 2, 2015
1,395
941
Those components look like they should work fine.