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I have a differential Amplifier Problem on Oscope (50HZ)

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nima_1981

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differential Amplifier Problem on oscope(50HZ)

Hi , Friends
I need to ampifire a small Ac Signal that between (1 to 20 mv on 1000 HZ ) so i found this when i try to test this circuit with Dc input it work good

Ashampoo_Snap_2018.09.15_02h44m05s_013_.png

as ac signal add on dc for dc blocking i should to add 2 cap in input , after that i have a 50 HZ Signal On Oscope ??

Ashampoo_Snap_2018.09.15_02h49m39s_014_.png

result after add cap before Input:
result.jpg

** I have no any ac signal in circuit i have battery for supply .
could you please Help me
 

Re: differential Amplifier Problem on oscope(50HZ)

You are obviously picking up AC hum from the line. What's your signal source? How is it connected? Whats your Pcb layout look like?
 

Re: differential Amplifier Problem on oscope(50HZ)

Usually a shielded coax cable carries weak audio signals. Your 1 mV level resembles a weak microphone signal.

By adding two capacitors it creates high impedance in your input wiring. This makes it easy for it to pick up ambient mains hum (may be broadcast from nearby appliaces, etc). The hum is amplified along with your signal.

Your input wiring ought to be a low impedance loop overall. Try putting in only one DC blocking capacitor.
Try using shielded cable. Connect the shield directly to ground. This may require changes to input arrangement at your op amp.
 

Re: differential Amplifier Problem on oscope(50HZ)

Hi,

If your input is pure AC, then you don't need the capacitors.
If your input contains DC and you waont to get rid of it, then only one capacitor is sufficient.

You need a difference amplifier only if you need both AC signals to be floating.
I don't see a reason here why you can't connect one AC input to GND. Then use a simple noninverting circuit.

With a bit more explanation from your side we could give more detailed answers ... and don't need to guess around.

Klaus
 

Re: differential Amplifier Problem on oscope(50HZ)

Nothing has been said yet about the properties of the signal source. In some cases, a differential amplifier may be useful for a floating source, although the straightforward solution suggested by KlausST should be considered in the first place. But a floating source connected to a differential amplifier need an effective common mode termination and possibly better CMRR than the simple circuit provides.
 

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