Figure 1 is my single-cell flow resistance detection system based on the Coulter principle. The data acquisition card output 1v DC to one electrode (Ag) of the chip, and the response current is measured from the other electrode (Agcl). The current is converted by I-V and amplified by 100,000 times. The result is read by the data acquisition card. The problem is that when the DC is loaded, the test results get a signal with positive and negative interleaving like Figure 2（MATLAB）. Why is there a negative value? (I used a standard 1M resistor instead of the chip to verify that my test system works properly). The ideal result like Figure 3.
Sorry for the error. The second picture is Figure 1 , the first picture is Figure 2 , and the third picture is Figure 3.
Dear Peng Zhang,
First of all I need to know the order of resistance change which you are getting for particle count or at least expected order. What are the details of I to V converter?. Did you design it by yourself or procured? The basic difference between the two figures of particle pulse response shows that you have acquired the signal for long duration. That means the sampling rate and x axis scale. The data acquisition card is probably in ac coupled mode which indicates negative as well as positive. Make it DC coupled setting. If it shows the same positive and negative then the output of I to V itself is generating dual polarity. So, you need to solve this problem. I suggest to first acquire data for short duration just to see the separate pulses. If some more details are provided then probably we can estimate the exact problem associated with the system.
Good luck!
P. K. Dubey Thanks for your answer. I may forget to emphasize that if I replace the chip with a standard resistor (1MΩ), the collected signal is positive and consistent with the theoretical value(see the added figure.4). That is, my test system works properly. I will add some necessary details:
My problem is why there is a negative value like figure 2?
I appreciate your help!
Dear Peng Zhang,
It is obvious that the I to V converter is generating dual polarity output. You need to check the possible reason. Sometimes dual power supply may be uneven resulting in one side output for no input. For this, check the output response with no input current (zero) and note the polarity of output. It may be negative fixed value which may result in dual polarity due to input. You need to make it zero with no input current. This may help
P. K. Dubey Thank you. I hope what you said is right. If It is true, how to explain the result in Figure.4? I mean the signal is always positive when I use a 1MΩ resistor. Thank you very much.
Dear Peng Zhang ,
According to me it should not make any difference whether you get the polarity only positive or going from positive to negative. The ultimate objective is to count the particles and a pulse should be generated in response to a particle pass through. You should be able to count the number of pulses irrespective of the polarity.
The negative / positive polarity generation by the amplifier or I to V converter is due to the input offset current in the circuit. which ultimately results in the finite output voltage of either polarity depending on the direction of input offset current. This current flows through feedback resistor which changes the output voltage to compensate. Generally if the feed back resistor is too high (M ohm) it will generate higher output. When the input signal is applied to produce current in opposite direction and the amount exceeds the threshold it changes the polarity at the output.
P. K. Dubey Thank you. I will try to exceed my experiment with the problem to see if I can get the pulses generated by particles. Thank you a lot.
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