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# 信号经过高通滤波之后存在直流偏置

Category: Hardware

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• Hi LANVictory,

I'll have to answer in English, Outlook's translation of your question pasted below. Note that there is  中文社区 as well.

The translation:

Hello engineers!
I encountered a problem when using LTspice to simulate, specifically related to the high-pass filter, and the simulation circuit is as follows:

The circuit includes a voltage follower, a high-pass filter, an inverting amplifier, and a leading correction circuit. The original input is set to a sine wave with a frequency of 21kHz and an amplitude of 10mV, and V(n208) is the output after passing through the voltage follower, which shows that it tracks the original input very well and has almost no DC bias. Instead, after passing through the high-pass filter, V(n211) has a DC bias of about -3mV, as shown in the figure below

MarkThoren: We can't see the node names that you're referrring to. But I take it that the output of U6 is correct - low offset?  Indeed C18 will remove any DC offset, or more accurately, will allow an arbitrary DC voltage to appear between its terminals. If you are measuring the output of U7, then have you considered the offset of that part? It is not manufactured by Analog Devices so we cannot comment on its performance. Where did the LTspice model come from? Have you considered the datasheet offset of that part, as well as the effect of its bias current? If your signal frequency is 21 kHz, why do you need a 2 GHz amplifier?

Subsequently, a DC bias of 10mV was added to the original input, and V(n208) was the output after passing through the voltage follower, which showed that it had a good tracking effect on the original input, and the DC bias was also 10mV. After passing through the high-pass filter, V(n211) still has a DC bias of about -3mV, as shown in the figure below:

The question is like a DC bias of -3mV after a high-pass filter no matter how much DC bias there is in the original signal? Theoretically, shouldn't the DC bias after a high-pass filter be very small?

MarkThoren: If you are adding the DC bias to V6, it will have no effect on the output of U7 because as noted earlier, this offset will simply appear across C18.

-Mark

• Hi MarkThoren,

Thank you for your answer, I think the reason for the DC bias is because the input bias current of the op amp behind it passes through the resistor R16 and causes a voltage drop