How to determaine the output impedance?

Question

Hello 
 I have a VCCS based on AD8130. After many googling and studying.. now I'm frustrated! 
 for checking the output capacitance usually I put a 1Kohm resistor on the output and check the 3-dB corner frequency. so the output capacitance is C=1/(2*pi*R*f3dB) 
 But I really I don't know how I should calculate the output impedance of the VCCS. 
 Regards

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JinoL · Answer

Hi Ali, 
 Few things. 
 1. The CMRR is dependent on how tightly matched the resistors are (RF/RG with RF'/RG'). For discrete components, the highest we can get is 66 dB CMRR for 0.1% tolerance. The external amplifier will have a little or no effect on the CMRR of the difference amplifier as we speak. 
 2. There are components, Difference Amplifiers, that offer higher CMRR but I don't think it's best suited for your application and a lot of factors are to be considered. 
 3. There is, however, a best viable solution for you, which is a Howland Current Source. I could help you design one if you could provide me the information I asked on my previous reply. The output impedance of HCS will be dependent only on the impedance you hook on the output. We also need to know the Output Compliance for your circuit. For example, for a 5V compliance, a maximum of 5 mA current will be allowed for a 1 kohm load. 
 Regards.

JinoL · Answer

Hi Ali, 
 The best example or reference for your application is the CN0151 . See figure 3. 
 
 The first AD8510 is a TIA to convert the current out of AD5443. So if you are in voltage mode, disregard the first OA. The computations are shown and explained the circuit note. 
 Should you have additional concerns, don't hesitate to let me know. 
 Regards, 
 Jino

JinoL · Answer

Hi Ali, 
 Few questions. 
 1. What made you decide to use AD8130, a differential amplifier? You can use an operational amplifier instead. 
 2. What is your current requirement? How much current do you need your amplifier to drive? 100mA, 300 mA, 1 A? 
 3. What frequency do you require to operate your VCCS? 
 4. Available supply, or required supply you want? 
 5. What other critical parameters we need to consider in your application? 
 6. What application do you intend to implement? 
 Regards, 
 Jino

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How to determaine the output impedance?

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