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AD620 And Noise on output with grounded input

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nima_1981

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Hi Friends.
I have a big problem with this ic , I grounded two input of ad620 ,but i have a big noise on output
please help me to solve this problem ,
Thank a you so much .

I use exactly this schematic :
Ashampoo_Snap_2015_05_21_04h09m26s_001_.jpg

And i have this on output :
rsz_ad620.jpg
 
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crutschow

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The problem is not with the IC, it's with your circuit.
Grounding both inputs is not an acceptablel mode of operation for an op amp. With that you have the op amp open loop gain at the output amplifying any noise and offset.
Why did you do that?
 

nima_1981

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The problem is not with the IC, it's with your circuit.
Grounding both inputs is not an acceptablel mode of operation for an op amp. With that you have the op amp open loop gain at the output amplifying any noise and offset.
Why did you do that?
Thanks for your reply , i what to test my design . did you have a better way to test this ic ?
 

Audioguru

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The instrumentation amplifier does not have an important resistor between pin 1 and pin 8 to set its gain so its gain is extremely high at DC and low frequencies.
Your 'scope does not show the frequency of its timebase. Is the interference caused by the AC to DC rectifiers charging the main filter capacitors?
 

nima_1981

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The picture of oscilloscope on first post is edited ,
did you have any method to test ad620 ?
 

Audioguru

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The signal on the 'scope occurs every 24us so the fundamental frequency of the interference is about 1/23u= about 43.5kHz but I do not know why the 'scope says half at 21.55kHz.
each blip occurs at about 87kHz.

The datasheet for the aD620 shows you all about it. Set its gain and feed an input signal and measure and observe the output signal to test it.
 

FvM

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The instrumentation amplifier does not have an important resistor between pin 1 and pin 8 to set its gain so its gain is extremely high at DC and low frequencies.
Not true. Connecting no gain setting resistor sets the instrumentation amplifer gain to unity, a reasonable option.

The interfering signal shown in post #1 has it's dominant frequency components in the MHz range. Looking at the AD620 datasheet, you'll realize that AD620 has effectively no power supply (PSR) or common mode rejection (CMR) in this frequency range. So any high frequent noise present at the power supply or at the input will just pass to the output.

But what's your actual test setup? How about power supply bypass capacitors or ground wiring? How did you connect the oscilloscope? What's the interference source? Are you sure sure at all that the signal is transmitted by the AD620 and no simple measurement artefact by unsuitable probe connection?

Please give an informative test circuit and a sketch or photo of the full test setup.
 

Audioguru

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I admit that I was wrong. No gain setting resistor results in a gain of 1.
I stated the repetition rate and its frequency of the interference.
 

dick_freebird

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I would suggest the poster look at the positive and negative
supplies, AC coupled, same timebase & vertical settings, and
see how much those terminals resemble the output behavior.
Supply ripple is suspect #1, prove or disprove that first.

I have played some with this part and never seen this sort
of thing.
 
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