Understanding a current excitation circuit using Op Amps

[faiz18]

Hi All,

I am working on a project to sense the temperature using RTDs (Resistance Temperature detectors). I need some help in understanding the circuit attached. I am using this to excite a RTD sensor in order to measure the voltage across it that I can later correlate to temperature.

Also are there any other ways to produce a constant current source like this one.

Thank you for your time and appreciate all the help.

Faizan

 

[FvM]

It's a Howland current source variant. Many previous threads about it can be found at Edaboard.

 

[faiz18]

Hi FvM,

Thank you for the quick reply. I had a look at the previous posts but I still have some missing pieces.

I am trying to get a constant current source using the above schematic. To output a current of 1 mA from an input voltage of 3.3V, I believe I need to have matching resistors for R6, R7, R9 and R10 = 33KOhms and my R8 should be around 3.3Kohms. So my question is, the resistors R6-10, will they influence the current running through the circuit or the only resistor that matters is the R8?

Sorry if its a naive question, I am still learning. Thank you in advance for the help.

 

[FvM]

Your calculation is correct for four equal resistors, other resistor combinations are however possible, e.g. R6=R9, R7=R10, Iout = Vref/R8*(R7/R6). The circuit has the disadvantage of output resistance depending on good resistor matching.

I would prefer a simpler current source like below. Vref can be a simple 1.2V shunt reference or a Vcc voltage divider. In many RTD measurement circuits, there's no need for a precise voltage reference. Instead use a ratiometric circuit, same reference (e.g. logic supply voltage) for current source and ADC.