Why is the output waveform of OP07 distorted?

天天1

Why is the output waveform of OP07 distorted? [Copy link]

 

As shown in the figure, this is the amplifier circuit diagram of OP07. The first stage uses AD620, and the input signal is a 10mV sine wave. Probe 1 measures the output waveform of AD620, and probe 2 measures the output waveform of OP07. At present, the waveform of AD620 is normal, but the output waveform of OP07 is wrong. There is only the negative part but no positive part of the waveform. The output circuit of OP07 is now disconnected and nothing is connected.

chunyang

With 0.5V input and 200x gain, can the positive and negative 5V power supply output be normal? How did you measure it? Where is the test point? What is the signal frequency?

chunyang

Note that the MCU input has a clamp and does not allow AC signal input, otherwise the clamp will bypass the negative half cycle and thus clip the bottom.

If the signal exceeds the power rail of the MCU, whether it is the high-end or low-end, the clamp circuit action will inevitably cause the signal at the IO to be clipped to the top and/or bottom.

maychang

[As shown in the figure is the amplifier circuit diagram of OP07, the input is a 500mV sine wave]

500mV multiplied by 200 is 100V. Your power supply is plus or minus 5V, how can it output 100V?

haocede

Note that the MCU input has a clamp and does not allow AC signal input, otherwise the clamp will bypass the negative half cycle and thus clip the bottom.

If the signal exceeds the power rail of the MCU, whether it is the high-end or low-end, the clamp circuit action will inevitably cause the signal at the IO to be clipped to the top and/or bottom.

Leave a comment, learn the circuit by word, can the host explain this circuit chip?

天天1

haocede posted on 2023-12-14 08:42 Note that the MCU input has a clamp, and AC signal input is not allowed, otherwise the clamp will bypass the negative half cycle, thus cutting the bottom. If the signal exceeds the MCU...

The first stage is instrument amplification, which amplifies 50 times, and the second stage OP07 is inverting amplification, which amplifies 200 times.

天天1

chunyang posted on 2023-12-13 23:37 Input 0.5V, 200 times gain, positive and negative 5V power supply output can still be normal? How did you measure it? Where is the test point? What is the signal frequency?

I updated the following figure. The first stage of the whole circuit uses AD620, the input signal is 10mV sine wave, probe 1 measures the output waveform of AD620, and probe 2 measures the output waveform of OP07. At present, the output waveform of AD620 is normal, but the output waveform of OP07 is wrong. There is only the negative part but no positive part of the waveform. The output circuit of OP07 is now disconnected and nothing is connected.

天天1

chunyang posted on 2023-12-13 23:37 Input 0.5V, 200 times gain, positive and negative 5V power supply output can still be normal? How did you measure it? Where is the test point? What is the signal frequency?

Input signal 10mv signal frequency 3K

maychang

Tiantian1 posted on 2023-12-14 12:07 Input signal 10mv Signal frequency 3K

10mV, AD620 amplifies it 50 times, getting 500mV, which is still within the power supply voltage range. Then OP07 amplifies it 200 times, getting 100V, which is far beyond the power supply voltage range.

maychang

Tiantian1 posted on 2023-12-14 12:07 Input signal 10mv Signal frequency 3K

So, on the 7th floor you said that the AD620 output waveform is normal, that's because 500mV does not exceed the power supply voltage range.

The OP07 waveform is not [only negative part but no positive part], but only a little bit on the top (for the input signal, it is a little bit of negative peak), and the angle is obviously much smaller than 180°. This can be seen from the length of the flat bottom of the next line of yellow waveform.

天天1

maychang posted on 2023-12-14 12:25 So, in the 7th floor you said that the AD620 output waveform is normal, that is because 500mV does not exceed the power supply voltage range. The OP07 waveform is not [only negative...

The distortion of clipping is understandable, but the output of AD620 is a ±250mV sine wave. Logically, it should be a ±5V sine wave after the second stage amplification, and actually it should be a ±5V square wave. However, I don’t understand why there is only negative and no positive value at test point 2.

maychang

天天1 posted on 2023-12-14 13:43 The distortion of clipping is understandable, but the AD620 outputs a ±250mV sine wave, which should be ±5V after the second stage amplification...

[However, the output of AD620 is a ±250mV sine wave. Logically, it should be a ±5V sine wave after secondary amplification.]

It is not ±5V, but ±50V. The amplitude difference between the two is ten times.

maychang

天天1 posted on 2023-12-14 13:43 The distortion of clipping is understandable, but the AD620 outputs a ±250mV sine wave, which should be ±5V after the second stage amplification...

[But I don’t understand why test point 2 here only has negative results but no positive results. ]

This is caused by the AD620's misalignment.

Try to short-circuit the input signal (ground the left end of R80), and then measure the voltage between the output terminal of AD620 and ground (you can use a multimeter). Ideally, if the input is zero, the output should also be zero. In reality, you will know it only after you measure it.

chunyang

天天1 Published on 2023-12-14 12:06 I updated the following figure. The first stage of the entire circuit uses AD620. The input signal is a 10mV sine wave. Probe 1 measures the output waveform of AD620, and probe 2 measures...

The circuit is still wrong. OP07 cannot provide 200 times gain for your input signal amplitude, which is far beyond the power rail. Even if you lower the gain, OP07 still cannot output the power rail because it is not rail-to-rail output.

Let’s clarify the basic concepts first.

Gen_X

620 output is 10*50=500mv,

OP07 can only magnify 10 times, so how can you set it to 200 times!!!

It is recommended to try OP07 at 5 times the input impedance.

The R694 below needs to be calculated, it is not a simple 10K!!!

天天1

Gen_X posted on 2023-12-17 10:19 The output of 620 is 10*50=500mv, OP07 can only be magnified 10 times, how can you set it to 200 times!!! It is recommended to try OP07 at 5 times, and...

In my actual use, the signal may be 0.5mV to 10mV, so the amplification factor is a bit large. The following R694 needs to be calculated. The size of this resistor should be the resistance value after selecting R45 and R51, right? That means putting a 1K resistor?

Gen_X

天天1 posted on 2023-12-17 13:14 In my actual use, the signal may be 0.5mV to 10mV, so the amplification factor is a bit large. The following R694 needs to be calculated, ...

Right! According to other current resistors, it should be at least no greater than 1K

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Check the negative power supply of OP07. Without negative power supply, there will be no negative signal output.