How to simulate weak signals?

bigbat

How to simulate weak signals? [Copy link]

 

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?

maychang

The basic method for generating weak signal sources is to use resistors to divide the voltage of stronger signals. For example, if a 1mV DC signal is needed, then use a 999:1 resistor to divide the 1V accurate DC signal. 1V low internal resistance DC is easy to generate and easy to measure, and can be measured very accurately. Therefore, the accuracy of the 1mV signal is basically determined by the accuracy of the voltage divider resistor. The requirement for internal resistance is that the part of the resistance that is 1 is smaller than the internal resistance of the signal source you need. This method can be extended to the audio range. Of course, the higher the frequency, the more difficult it is to be accurate, which requires various measures to calibrate. Calibration is a very troublesome thing, and it can generally only be done by professional organizations. Self-use can only achieve a relatively low degree of accuracy, such as an error of less than 0.5%. Up to the frequency of tens of MHz, resistor voltage division is the basic method.

maychang

The basic method of simulating weak signals is to use resistor voltage division. For example, if an accurate 1mV DC signal is required, a 999:1 resistor voltage division can be used to obtain a 1mV DC voltage from a 1V reference voltage. As for the internal resistance, as long as the resistance value of 1 is less than the requirement for the internal resistance of the 1mV signal, it will be fine. 1V DC voltage is easy to stabilize and easy to measure accurately. The accuracy of the 1mV signal after voltage division basically depends on the accuracy of the voltage division resistor. This method can be extended to audio. Of course, the voltage division ratio is not easy to be accurate at high frequencies. When the accuracy requirements are very high, calibration can only be completed by specialized institutions. Doing it yourself can only obtain a lower accuracy, such as 1%. The resistor voltage division method can be extended to tens of MHz. Calibration at this time is quite difficult.

xiaxingxing

maychang published on 2018-7-14 09:15 The basic method of generating a weak signal source is to divide the voltage of a stronger signal with a resistor. For example, if a 1mV DC signal is needed, then use 999:1...

Hello, "as long as the part of the resistance that is 1 is smaller than the internal resistance of the signal source you need", why? Isn't it sampling the voltage signal, the larger the sampling resistance, the smaller the impact on the signal source? ? ?

maychang

xiaxingxing posted on 2018-7-19 18:52 Hello, "as long as the part of the resistance that is 1 is smaller than the internal resistance of the signal source you need" Why? It is not sampling the voltage signal, the sampling resistance...

That's right! You hope that the internal resistance of the signal source is small (the smaller the closer to the ideal voltage source), for example, you hope that the internal resistance of the signal source is less than 1 ohm. Then you use 999 ohm and 1 ohm resistors to divide the 1V voltage, and you get a 1mV signal source with an internal resistance less than 1 ohm.

xiaxingxing

maychang posted on 2018-7-19 19:40 That's right! You want the internal resistance of the signal source to be small (the smaller the closer to the ideal voltage source), for example, you want the internal resistance of the signal source to be less than 1 ohm. Then you use 999 ohms and 1 ohm...

Oh, I understand... That 1 ohm resistor is used as the signal source, thank you!!!

maychang

xiaxingxing posted on 2018-7-22 21:59 Oh, I understand. . . The 1 ohm resistor is used as the signal source, thank you! ! !

Strictly speaking, it is 1 ohm and 999 ohm in parallel. Of course, the two resistance values differ by so many times, and the large resistor has little effect on the resistance value after parallel connection.

jxia1

Simple experiments can be done with resistor voltage division, but the noise is difficult to control. However, for quality inspection and certification, a dedicated signal generator is required. I recommend a signal generator from Hangzhou Huygens Technology, which is specially designed to output microvolt signals.

bigbat

jxia1 posted on 2019-9-19 15:20 Simple experiments can be realized by using the resistor voltage division method, but the noise is difficult to control. However, for quality inspection and certification, a dedicated signal generator is required. I recommend Hangzhou...

The principles we know so far are all voltage division through resistors, but the most difficult to imitate is impedance. I heard that the voltage signal can be more stable by using a standard electrode battery.