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# How to determaine the output impedance?

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

## Top Replies

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• 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.

• 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.

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