Re: Relationship between filter roll off and response time
- From: "Ban" <bansuri@xxxxxx>
- Date: Tue, 19 Apr 2005 13:01:40 GMT
acannell@xxxxxxx wrote:
> Hello,
>
> Could someone explain the relationship between RC filter roll off
> steepness and overall response time? Lets say I have a signal at 10khz
> from a photodiode and I want to block stray 60hz light (not emi). If I
> add an RC high pass 6db/decade filter after the transimpedance amp,
> how do I figure out what the response time of the circuit is for the
> 10khz signal? In other words, if I pulse the 10khz off and on (100%
> AM) how can I predict how fast I can pulse it on and off?
>
> Thanks,
>
> Asa
When you use the filter(HP 10k 1st order) in the signal path, its rise and
fall time will be identical to the original signal, but the flat top of the
square input signal will be differentiated and becomes like needles with
*almost double* the amplitude.
+------+ +
| | |
| | |
| | | Input
| | |
| | |
+ +------+ ||
o-||----+------o
+ + ||C |
| | .-.
| | | |
| | | |R
|\ | '-'
| | |
| | o-------+------o
| \ |
| |
| \ |
| \ |
| \ |
| \+ |
| | | Output
+ | /+
| /
| /
| /
|
| /
|
|/
|
|
+
It might be that your receiving logic interprets this as a signal with
double the frequency. You then have to lower the pole frequency to get more
straight top lines.
There will be no delay, unless you filter out the high frequencies with an
additional lowpass. If your driving signal is a sine wave, it effectivly
zero-crosses even 12us before the original, due to the differentiation!
--
ciao Ban
Bordighera, Italy
.
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