How to deal with the power frequency interference caused by 7815 7915

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How to deal with the power frequency interference caused by 7815 7915 [Copy link]

I use 7815 7915 positive and negative power supplies to power AD620. I input a 20mV DC signal and amplify it 15 times. The signal should be 300mV DC. But now the waveform is a square wave with a frequency of 50Hz, a duty cycle of 50%, and an amplitude of 300mV. What's going on? I use a battery +-15V to power it and it works perfectly fine!

maychang

Check the output of 7815 and 7915 with an oscilloscope

还我今生

It's a completely straight line and there doesn't seem to be anything wrong.

还我今生

It is said that there is 200mv interference. I don't know if it is really that serious.

将军令

If your power supply is normal, is there any problem with your signal input terminal: Input impedance is too high Input cable is too long and unshielded? Is the output impedance of the front-end model also high?

maychang

If there is interference between 7815 and 7915, such a large amplitude should be visible with an oscilloscope. If it is not visible now, it is not a problem with 7815 and 7915. Are you sure that the input is DC? You should also use an oscilloscope to check. If it is a dual-trace oscilloscope, one channel is used to view the input and the other channel is used to view the output. AD620 is an instrumentation amplifier. Is your input common-mode range exceeded? Bypass capacitors should be connected in parallel from the positive and negative power supplies of the amplifier to the ground. It is best to connect an electrolytic capacitor and a non-inductive film capacitor in parallel.

xwj

Obviously there is something wrong with your circuit. The ripple rejection ratio of the op amp is quite high.

awey

Common mode interference, try connecting the ground wire to the ground

maker

The output leads of the voltage regulator should be as short as possible and the filtering measures should be good.

davisson

I have used 7812 and 7915 power supplies, but the problem does not seem to occur. Is the input DC?

还我今生

First of all, the input impedance is a problem. The input impedance of AD620 itself is very large. The input impedance of my signal is only 20k, which should not affect the signal input of AD620. Secondly, the input cable is definitely not long, only 15cm. Is the output impedance of the front-end signal the input impedance of AD620? If so, the above answer has been given that the signal is a 200k series resistor with a 200 ohm resistor, and the 10V battery is powered. The 10mv signal is obtained from the 200 ohm resistor. So the input is definitely DC! I don't know if the impedance is considered large. The third answer is to use a dual-trace oscilloscope to view the input on one channel and the output on the other channel. This is not allowed because the ground of the two channels of the oscilloscope is the same. If you look at it in your way, the circuit itself is changed because the input is not in common with the output. Bypass capacitors should be connected in parallel from the positive and negative power supplies to the ground. It is best to connect an electrolytic capacitor and a non-inductive film capacitor in parallel. All of these have been done.

maychang

"Because the input is not grounded with the output" This is probably the problem. If you use AD620, the input must be a differential signal, otherwise such an expensive op amp would not be used. But "the input is not grounded with the output", how do you determine the common-mode level of the input signal? There is no electrical connection between the input and output, which can easily cause the common-mode to exceed the range.

ltmy125

"Because the input is not grounded with the output" The problem must be here!!! And why is it necessary to "The signal is from a 200k resistor in series with a 200 ohm resistor, powered by a 10v battery, and the 10mv signal is obtained from the 200 ohm resistor. So the input is definitely DC!" You have four 100K resistors to form a full bridge circuit, and connect a 51K adjustable resistor to any arm of the bridge, isn't it better! In addition, don't forget to ground your 10V bridge with the ±15V ground!!! I have made this circuit, and the problem you mentioned also occurred at that time. Here is how to solve it, haha

luvkyhj444468

Connecting the voltage input of 7815 7915 to a CLC ∏ type filter circuit can effectively filter out high frequency interference. If there is power frequency interference, it may be a problem with the operational amplifier circuit. Because 7815 7915 is a linear regulator, the internal transistor works in the amplification state, and the filtering of the voltage input and output are very important.