VSP1000: What is the input impedance of vsp1000

I have apply offset of 4v and input voltage 1v p-p.

Input current configuration is 0.408mA.

Is there is any theoritical method to measure it.

Please give me the equation to measure it.

My vsp1000 supply voltage is 12v.

I already mentioned the equation in word form in my previous post.

R = delta V / delta I

Apply a DC voltage of 5V and 5.1V and measure the current at these 2 voltages and use the above equation to calculate the input impedance.

In my board their is no provisional for connecting multimeter in series. Have you ever calculate the then say me the input impedance.

What is the value of input impedance.

Unfortunately we do not have a board to measure this parameter and if this parameter was tested before device release, it would be listed in the datasheet.