I need to measure the impedance across a specialized electrochemical cell with as low noise as possible, for a research project. The impedance of the cell is about 30 Mohms, and the potential across the cell is a 50 mV 500 Hz sine wave. The circuit measures the current, from which the impedance can be determined.
I am using an AD797 op amp with a carbon 1 Mohm resistor in the feedback loop. Can the noise of the output signal be reduced by using a different op amp? What about replacing the feedback resistor with a metal film resistor? Is there a better overall circuit design that would provide lower noise? I am seeing a noise level of about 100 Kohms in the 30 Mohm measured impedance, and need to reduce the noise as much as possible.
Just want to confirm some items and you may agree.
The objective is to measure the Impedance of Electrochemical cell, so the 30Mohms here is an average or a datasheet value?
Given the 30Mohms and 50mV, the current will be 1.67nA which is too small as compared to the Bias current of AD797 which is around 0.9uA.
This means that the Vo is more on Bias current dependent rather than Input Current.
Try to check and consider ADA4530, for low Bias Current.
For the 1Mohm feedback resistor, I just wanted to know how it was derived.
The Feedback Resistor value is usually taken from the Max Voltage swing divided by the max Input Current.
We have also an Application Note which may help you with your design.
Hope this helps.
I see that you chose the AD797 for low noise reasons. A low noise voltage value is typical for an op-amp with a bipolar input, but this will only be achieved with low-impedance signal sources. But in your case, this is not implemented, and the noise current flowing through the feedback resistor comes to the fore. Here will have the advantages of a JFET amplifier.
I will also recommend a document to study
As for noise reduction methods, since the frequency of the useful signal is known in advance, it is logical to use filtering (bandwidth limitation).
Thank you, Kirill. I will try the AD795 op amp, and use a metal foil feedback resistor. I am doing bandwidth limitation, but I still need to get the noise lower.
Thank you for your response. The electrochemical cell is in a non-standard configuration, for use in research. The impedance has been empirically determined to be around 30 Mohms. We will try the ADA4530. The 1 Mohm feedback resistor was selected to attain as much gain as possible with a stable measurement.
I assumed that the 50mV is fixed, so I treated your circuit as TIA.
Your input current will be now 50mV divided by the Impedance which is 30M in your case.
Your Vo will be the input current multiplied by the feedback.
In this example, the Vo is relatively small when feedback R is 1M.
Again, this approach is on TIA.
Kindly proceed with the ADA4530 and if you can share the result, it will be much appreciated.