RE: CN0273: High Speed FET Input Instrumentation Amplifier with Low Input Bias Current and High AC Common-Mode Rejection by mythos1453:
I wanted to know if I can use the CN0273 instrumentation amplifier board in the measurement a MEMS resonator device. The DUT has two output currents that show up on a negative and a positive electrode. The currents are in the order of ~100nA and are 180 degrees out of phase. The operating frequency is 10MHz and I would like to have to have a high common-mode rejection. (a gain of 14dB is sufficient for my needs)
I'm was wondering if I should transimpedance configuration instead. Any comments would be very helpful!
RE: CN0273: High Speed FET Input Instrumentation Amplifier with Low Input Bias Current and High AC Common-Mode Rejection by Walt_Kester:
Thanks for the question about CN0273.
You would be much better to use a transimpedance amplifier using a FET input op amp such as the ADA4817-2. This amplifier has a high GBW product, low input capacitance, and low input bias current.
If you have 100nA FS output, then a 20MΩ feedback resistor will give you a 2V output.
The input capacitance of the amp plus the capacitance of the current source will create a pole which will require a feedback capacitor to stabilize.
The following reference has the design equations for this:
This is the same problem you face with photodiode circuits, where you are trying to convert a current from a high impedance source into a voltage.
The problem is tougher when you are trying to achieve overall 10MHz bandwidth. The feedback capacitor should be as small as possible to give you at least 60 degrees of phase margin.
If you have any questions, let me know.
Thank you very much for the help, the reference you provided is very useful. Would you happen to have any reference on how to design high frequency, low noise PCBs?
The following are some good references on HF board layout:
A Practical Guide to High Speed Printed Circuit Board Layout
Techniques for High Speed ADC PCB Layout, AN-1142
High-Speed Time-Domain Measurements—Practical Tips for Improvement
An IC Users Guide to Decoupling and Grounding
Section 4: PC Board Layout and Design Issues
MT-031: Grounding Data Converters and Solving the Mystery of "AGND" and "DGND"