Designing Wheatstone Bridge circuit for a biosensor

[kookie91]

Hello

Im designing an analog front end circuitry for a biosensor.
The sensor was observed to give an output current of 200uA when a 0.1v bias was applied.

So, the best way to model this would be a wheatstone bridge.

I designed the bridge in the following manner
excitation voltage= 5v
Fixed resistances = 350 ohm
Varying resistance ( sensor) = 500 ohms ( simulating the behavior of sensor ; 0.1v/200uA= 500 ohms)

I would like to know if this design would yield optimum results or not. I am a beginner with circuit design. Any help would be greatly appreciated.
Thank you

 

[FvM]

I guess there's a reason why the sensor has been operated with a rather small voltage of 0.1 V. Will it survive several volts at all?

 

[kookie91]

Yes. The sensor is biased at 0.1 v for a reason. It cannot survive higher bias voltages. The sensor output which is very small will further be amplified using an instrumentation amplifier.
Before that, i just wanted to know if the bridge circuit design is optimally correct or not.

 

[FvM]

Obviously not a bridge with 5V excitation and not necessarily a bridge circuit at all.

 

[BradtheRad]

If you do not wish to allow more than 200 uA through the sensor...

Then it will be wise if you install a resistor with a value higher than 350 ohms between it and the 5V power supply.

Perhaps 33k, if this version of a wheatstone bridge resembles your own plan.



The potentiometers are to indicate that you will need to make several adjustments, in order to obtain the desired response.

 

[kookie91]

This is how i designed the bridge



So in balanced condition V1-V2=0
when an analyte is dropped on sensor(R2), R2 changes and (V1-V2) also changes.

Can anyone please tell me if this design is correct or not?

 

[BradtheRad]

Yes, that is the correct way to set up a wheatstone bridge. A wheatstone bridge is excellent for detecting any slight change from a particular state of voltage/current (balanced condition).

I assume your sensor is R1? It has 54 uA going through it. Because I notice that R2 (470 ohm) has 232 uA going through it, which exceeds your specified 200 uA limit.

As you know, real resistors are not exactly the value they are labelled. That is why I predict you'll need a potentiometer to make fine adjustments.

 

[kookie91]

Ok i will use pot to make adjustments. Used resistors to make the circuit design in spice easier.

The sensor is R2. The reason y i chose that is coz the sensor needs to be biased with a constant 0.1v.
So, 1.2*470/(470+4.7k)= almost 0.1v in one of the arms of the bridge.
The 200 uA current is not a stringent condition.