I am trying measure a very small change in resistance (approximately .400 Ohms), due to the the effects of connecting a large resistance in parallel with a network of smaller resistors essentially looking like this
The voltage drop across A and B however cannot exceed 0.5V, I consider using a AD8276 in the following configuration to create a constant current source. I got this design from http://www.analog.com/library/analogDialogue/archives/43-09/current_source.html figure 5 without the transitor.
My problem is that the difference in current through the load (in my case the first diagram) when the measured R is connected and not connected is 1.388mA and 1.3912mA respectively (approximately). To achieve these currents the range of Vref is extremely small, when R2 =1K the swing is 1.354926V and 1.35726V. The range grows as you increases R2 and R1 but Vref also increase.
I want to do this measurement in house and it will only ever be done in this configuration so I don't want to have to buy expensive equipment, I was hoping to be able to build a circuit to do this. do you think my original idea would work (AD8276 current source) I just need a method to control Vref, or do you think there is another method.
Thanks for the second pair of eyes.