Reply to the post "How to simulate weak signals?"

maychang

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Original post In biological measurements, such as ECG, EEG, EMG and other signals, these signals not only have extremely small voltage levels (ECG - milliampere level, EEG - microampere level...), but also have very small signal output impedance. How to simulate such signals? The simulation of weak signals basically uses the resistor voltage divider method to accurately divide large signals. This method is not only applicable to DC, but also to audio and even signals below tens of MHz. For example, if you need an accurate DC voltage of 1mV, you can use a 999:1 resistor voltage divider to divide the accurate 1V DC voltage. As for the size of the voltage divider resistor, as long as the resistor value of 1 is less than the internal resistance of the signal source you need. A 1V DC voltage can be measured very accurately and is easy to stabilize. The actual ECG and EEG signals have very low frequencies, and the internal resistance is not very small. Using the resistor voltage divider method, it is easy to obtain a signal source internal resistance of less than 1 ohm. Of course, the higher the frequency, the more difficult it is to make the voltage divider ratio accurate, and calibration is required. High-precision calibration can only be achieved by professional organizations such as the Technical Supervision Bureau. Self-calibration can only achieve a lower accuracy, such as 0.5%. The frequency is as high as hundreds of kHz or even higher, and each voltage divider resistor must be non-inductive and non-capacitive, and the shielding between the resistors is also very important. I don't know why I have to wait for review when I reply to your post. But after reviewing for a long time, there is no result, so I have to use this method to reply. @bigbat

chunyang

The questioner must have misunderstood the signal output internal resistance and said it exactly the opposite.

bigbat

Thank you: Mr. Maychang, this kind of signal is generally used with a "differential amplifier", and I don't know the specific range of current values. What I mean is that the power of this signal source is very small, and I also know that it is attenuated by a large signal. What I want to ask is, is this signal a "current source" or a "voltage source"?

maychang

bigbat posted on 2018-7-14 18:55 Thank you: Teacher Maychang, this kind of signal is generally used with a "differential amplifier", and I don't know the specific range of current values. What I mean is this...

"What I want to ask is, is this signal a "current source" or a "voltage source"? " After the resistor divides the voltage, the internal resistance of the signal source is very small, which is an approximate voltage source.

maychang

bigbat posted on 2018-7-14 18:55 Thank you: Mr. maychang, this kind of signal is generally used with a "differential amplifier", and I don't know the specific range of current values. I mean this...

The actual ECG signal should also be said to be an approximate voltage source with a relatively large internal resistance. When doing an electrocardiogram, the resistance of the skin between the electrode and the human body is still quite large, which may reach hundreds of ohms. In the actual electrocardiogram, I remember that it seems that the vertical axis is specified to represent 1mV per centimeter.