# Isolating Ac and Dc Signal

Discussion in 'The Projects Forum' started by mustafapk, Oct 16, 2008.

1. ### mustafapk Thread Starter New Member

Oct 16, 2008
3
0
Hello,
I am working on a project in which i have to isolate the Ac voltages of low amplitude and low frequency (100uV and 1Hz) from a continously decreasing Dc value. i have built just a smiple High pass filter type circuit. but the problem is that due to very low value of amplitude and frequency either the signal doesnt passes or when i reduce the cutoff freqency of the high pass filter the effect of Continously reducing dc voltages also prduces its effect on the output.
One more thing that i also mention here is, when i isolates these voltages they are effected from the Hum noise(50Hz Ac noise.) even i have used 8th order low pass filter of 10Hz cutoff frequency.
I want a technique through which i can completly isloate the Ac signal which isnt effected by the Dc reduction value.
Thanks

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
A variable DC is like a biased AC so it passes through filters if it has the proper frequency depending on the filter. A circuit would be helpful indeed.

3. ### kubeek AAC Fanatic!

Sep 20, 2005
4,632
787
How fast is the DC decreasing?

4. ### mustafapk Thread Starter New Member

Oct 16, 2008
3
0
Hi,
I think that the question was not clearly mentioned. Actually I am working on the design of Blood pressure monitor in which I am using pressure sensor to measure the pressure inside the cuff which gives output in Dc voltage. When we have to measure the systolic pressure we reduce the pressure inside the cuff, with the reduction of the pressure its Dc output voltages also decreases, but as the systolic level arrives small amount of pressure is increased in the cuff which in turns increases the Dc value again, and if we separate the Dc and pulsating Ac we will have small amount of Ac voltage with low amplitude and frequency.
The problem with my circuit is that I am unable to clearly isolate Dc and Ac voltages and the reducing dc voltages gives some impact on the ac voltages.
I think that now it is very clear to analyze the problem.

5. ### kubeek AAC Fanatic!

Sep 20, 2005
4,632
787
You could try two differentiators in series, or maybe just one and a blocking capacitor, assuming the DC decrease is linear. But I am not sure how this will affect the shape of the AC.