AD623 test voltage, how to solve interference?

bingtuohun

AD623 test voltage, how to solve interference? [Copy link]

 

I use AD623 to detect voltage signals. As shown in the figure: When the voltage signal is connected to TP3 and TP4 (using dry batteries for simulation), the output signal at the output end is completely abnormal, as shown in the figure: , in this case, there is no way to get the correct voltage value. I suspect it is power frequency interference, because the frequency is exactly 50hz. If I ground the negative pole of the input op amp, this situation will not occur. But this is not a completely differential input. It is equivalent to AD623 as a voltage follower and adder, which is not the result I want.

飞鸿浩劫

If it is really PCB power frequency interference, just add RC at the input of the op amp. In addition, does the 1M resistor in series with TP3 have any special effect? And does the REF pin connected in that way also have a special effect?

maychang

Your battery and the two wires from the battery to the op amp are too long and asymmetrical, and have high impedance to ground.

dsjsjf

What is your power supply like? Check if there is interference coming from the power supply.

bingtuohun

dsjsjf posted on 2017-12-11 20:46 What is your power supply like? Check if there is interference coming from the power supply

The positive and negative power supply used in the test is Zhaoxin's ZF-302D

bingtuohun

maychang posted on 2017-12-11 18:35 Your battery and the two wires from the battery to the op amp are too long and asymmetrical, and the impedance to ground is very high.

The wire prepared for the pH electrode is longer, and the internal resistance of the pH electrode is very high. However, the pH electrode wires are of equal length.

maychang

bingtuohun posted on 2017-12-11 22:56 Prepared for pH electrode, the wire is longer, and the internal resistance of pH electrode is very high. However, the pH electrode wires are of equal length.

pH value measurement cannot be very fast, so you can add low-pass filtering at the input to remove power frequency interference.

huaiqiao

I have used INA128 before. It needs dual power supply. When I designed this circuit, it was originally dual power supply, but the negative power supply was cut off during the review. As a result, the instrument amplifier could not work properly. Later, in order not to re-make the PCB, I found AD623, but I remember that AD8233 is better than AD623. AD8233 can be powered by a single power supply and is compatible with INA128 PIN to PIN. So far, it still works well. However, it should be reminded that for this kind of mixed analog-digital circuit, the analog ground and digital ground must also be handled well, including on the PCB.

bingtuohun

huaiqiao posted on 2017-12-12 14:40 I used ina128 before. This needs dual power supply. When I designed this circuit before, it was originally a dual power supply in the circuit, but during the review...

I used a single power supply before, which resulted in sudden large interference that caused unstable signal detection. It would occur several times at fixed times every day. This time, the problem did not occur when using dual power supplies. Moreover, AD623 failed to do pH detection because the leakage current of the op amp was too large.

bingtuohun

maychang posted on 2017-12-12 08:33 pH value measurement cannot be very fast, so you can add a low-pass filter at the input to remove power frequency interference.

In addition, the method of directly adding RC filtering at the input does not work. I have tried it, and the leakage current is too large to affect the system detection. In addition, it may be that the capacitor I chose is not good, and I may need a better capacitor. I put the RC filter at the back end of the op amp.

huaiqiao

bingtuohun posted on 2018-1-5 22:18 I used a single power supply before, which resulted in sudden large interference that caused unstable signal detection. It would occur several times a day at fixed times. ...

If dual power supplies are required, try to use dual power supplies. This is the problem when I used INA128. The data after amplification with a single power supply is not correct. . . . .

bingtuohun

huaiqiao posted on 2018-1-8 09:53 If dual power is required, try to use dual power. This is the problem when I use INA128. The data after amplification by a single power supply is not correct. . . .

In order to get dual power supply, I used the charge pump chip SGM3204. The power supply is an external adapter and there is no way to change it to a power supply that directly outputs positive and negative power.

huaiqiao

bingtuohun posted on 2018-1-10 10:05 In order to get dual power supplies, I used the SGM3204 charge pump chip. The power supply is an external adapter and there is no way to change it to a power supply that directly outputs positive and negative power supplies.

Yes, like your AD623, this instrumentation amplifier has a 3V~12V input. If the input is 3V, the negative power supply LM2776DBVT/SOT-23-6 chip can achieve -3V. I remember that this chip is from TI. I added it when I was working on ina128, but when they reviewed the drawings, they were empirical and cut off my chip. . . .

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