Filter should be apply after or before amplify?

Thread Starter

Xin_99

Joined Nov 18, 2022
4
If using LDR with wheatstone bidge and needs to use a filter. Where shound i apply the filter to? before or after
If the input signal is DC source

Did DC source contain frequency? How i know the cut-off frequency of the filter should be? close to 0Hz?
 

crutschow

Joined Mar 14, 2008
31,146
The filter should typically be after the LDR to suppress it's noise.
The required filter characteristics will depend upon the nature of the noise and how much noise you can tolerate on the bridge.
 

Ian0

Joined Aug 7, 2020
6,724
It looks like you need a comparator. So don't use an instrumentation amplifier, use a comparator.
The filter can be as simple as a capacitor across the LDR
 

Thread Starter

Xin_99

Joined Nov 18, 2022
4
It looks like you need a comparator. So don't use an instrumentation amplifier, use a comparator.
The filter can be as simple as a capacitor across the LDR
oh...I need to use this is because that is the requirement for my assignment. haha
 

Ian0

Joined Aug 7, 2020
6,724
The AD620 is 100 times as expensive as a comparator. Perhaps you ought to get the opinion of your economics teacher!
Seriously - if you short out Rg it becomes more like a comparator.
For the filter - you need to know how fast a flashing light the circuit has to be able to resolve (which may be limited by the response time of the LDR - look in its datasheet); and what the signal is that you are trying to remove.
 

ericgibbs

Joined Jan 29, 2010
16,819
hi Xin,
The filter should be across the two inputs pins of the IA.

As it is an LDR is it the ambient illumination that is considered as interference.?

This PDF is a little advanced for you, but save it in your database. Page # 1-6
E
 

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