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AD8251 in CN0385 does not work when measuring the ac signal across a resistor at 1MHz

Question asked by liubenyuan on Apr 14, 2017
Latest reply on Jul 12, 2017 by Emman.A

Hi, I try to measure a AC signal across a resistor at 1MHz using reference design CN0385, where the difference signals (with relatively a large common mode voltage) are converted to single-ended using AD8251. I build this schematic using LTSpice.

However, I found that,

1. The voltage at the Vp node might be affected by the installation of AD8251. With AD8251 removed, V(vp) is a normal ac voltage signal with Vpp = 1.6V. However, when AD8251 is connected, V(vp) becomes,

which is centered above 0V and is around 2V. Although, V(vp) - V(vn) is the same regardless of AD8251.

2. The output of AD8251 is always tied to -5V. 

3. I found that the slew rate of AD8251 is only 20V/us, can AD8251 still be used in my applications where the frequency of ac signal is 1MHz, and the Vamp is around 3V to 7V ? It not, is there alternative ICs that can be used in this application ? (note that there is a large, high frequency common mode voltage when measuring ac signal across a resistor out of many resistors)

 

Thanks all in advance!

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