AC Coupling

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AC Coupling

Postby Peter704070 » Mon Oct 24, 2011 6:52 pm

Hello I work as a college lecturer and trying to get students to make the most out of using our Picoscope. My question relates to ABS, we get the students to do all the basic checks using a meter. We then get them using the scope. While using the scope to check a Renault Scenic ABS sensor we struggled to get a reading.

These sensors are the Magneto type so we told the students to expect to get a digital pattern, for at least half an hour they struggled and then asked for help. It was lunch time so I had a look in peace. I also struggled to get any reading until I removed the black lead and changed the voltage from DC to AC and got a pattern.

I am struggling to explain this to the students in any real detail, I am not someone who just gets them to scope and not understand what they have done.

Can you help me to explain how I got the pattern and how changing the voltage from DC to AC made a difference, is this something to do with AC Coupling. Thanks in advance Pete.
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Re: AC Coupling

Postby Alan » Mon Oct 24, 2011 7:12 pm

Hi,

Please could you upload the waveform then we should be able to comment.

There are several types of these sensors so without knowing which sort you were looking at my guess is that they are of the current switching type (switch between 5mA and 15mA ish). When viewed as a voltage you see typically just 5V or 12V but in fact its switching up / down a tiny bit but its too small to see on a 20V range.

When you AC coupled it takes out the steady 5V or 12V and then you can use a smaller voltage range (eg 50mV) which reveals the signal.

As above take this with a pinch of salt as without a waveform I can not be sure.
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Re: AC Coupling

Postby Autonerdz » Mon Oct 24, 2011 7:19 pm

Peter,

I can't explain why using AC coupling (filtering out the DC components of the signal) would have helped at all. We really do not want to AC couple unless we specifically desire to filter out the DC. In this case you would not. Perhaps you used a different scaling and were able to see something.

I am not familiar with that specific application but you could be dealing with current based magneto resistive type wheel speed sensors.. You are not likely to get much of anything looking at voltage on these. However, you can check them with a micro amp probe.

Here is a ink to a discussion the first time a micro amp probe was used to sample this type of sensor:

http://www.autonerdz.com/cgi/yabb2/YaBB ... 1276215303


Edit; Ha....You beat me to it Alan. :shock:
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Re: AC Coupling

Postby Alan » Mon Oct 24, 2011 7:31 pm

I just found an example off a Aston Martin V8 Vantage that is probably similar (possibly the only time I have said that a Renault Scenic is similar to an Aston Martin :wink: ) . Here we are DC coupled so on a 20V range:

ABS.gif


Now flip to AC coupling

ABS AC.gif


Tom is correct of course though (he usually is) - with some of these sensors there is no voltage signal to see.
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Re: AC Coupling

Postby Robski » Tue Oct 25, 2011 12:04 pm

A few more details of the model/system would help too.
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