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synchronous acquisition 2 channels with different signal amplitude



I'm having some issues with my VI. I'm using NI-DAQ 6212 and I'm trying to acquire 2 different signals (ECG with 1mV amplitude and Respiration with 100mV).

When I'm using a VI that only acquires 1 channel, I have no problem acquiring both signals. But when I use the other VI the ECG signal is acquired, but on the other hand the Respiration isn't, it's mostly just noise.


I've made acquisition of different ECG Leads synchronously but I'm not being able to do the same with these 2 signals.


I'm sending the file to see if anyone could help me find the problem.



Thank you for your attention,



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Message 1 of 10

@fredjmartins wrote:

When I'm using a VI that only acquires 1 channel, I have no problem acquiring both signals.

Are you saying that if you acquire one or the other individually, then it works?




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Message 2 of 10


When I acquire each signal indidually I obtain what is supposed to acquire in both signal. When I try both at the same time it appears a different signal with the 2nd channel (respiration signal) but the 1st channel the ECG signal is exactly the same acquired individually.

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Message 3 of 10

This may be ghosting. Without seeing the signals that is just a guess. Ghosting occurs in multiplexed analog input devices when two conditions occur: 1. The two signals have significant differences in amplitude.  2. One or both of the input signals come from a source with a high source impedance.


Ghosting is a hardware issue and is best solved by driving the DAQ input with a low output impedance amplifier or buffer.


One trick which may help is to acquire 4 channels of data. ECG on ai0. Short ai1 to AI GND. Respiration on ai2. Short ai3 to AI GND. This helps to isolate the two signals from each other. An alternative is to connect ECG to both ai0 and ai1 and Respiration to both ai2 and ai3. The signals on ai1 and ai3 should be largely free of ghosting.



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Message 4 of 10

That's what I suspected. The signal amplitude difference is about 100x (ECG 1mV and RESP 100mV).


Thanks for all the help.

I will try the sugested configuration as soon as possible.



Thank you again,


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Message 5 of 10

Hello again,


Sorry for the late reply, but I've been on holiday and only this week I got back to work with the DAQ-6212.


I didn't totally understood the design you suggested. If I connect in two separated channels how can I get the differential acquisition of the signals?



Anyway I'm sending the pictures of both signals, hope it will help somehow.



Thanks again for all your help,


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Message 6 of 10

If you need differential connections, then you need to ground both inputs of the second and fourth input pairs. I did not look up the connections for your device.


Are you connecting the DAQ device directly to the electrodes? This is not a good idea for several reasons. The most important reason is SAFETY. General purpose DAQ devices do not meet the safety standards for leakage current for direct connections to patients. The electrodes reduce the impedance across the skin to the point that dangerous leakage currents are possible. Another reason is that the source impedance (while low from a safety point of view) is high enough to aggravate the ghosting issue. Third is that such a connection is likely to be noisier than you would have with a well-designed medical signal conditioner.


Use of a signal conditioner with medical grade isolation, differential inputs, low input noise, gain, and filtering will eliminate any safety concerns and  probably reduce the ghosting substantially.



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Message 7 of 10

That's the part where I start to have big problems.

I'm aware of using signal conditioning for safety issues, but I don't know how to do it and I think I will have some problems getting the right materials to do it.


Wouldn't a expensive equipment like this be more "this problem-free"?





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Message 8 of 10


Wouldn't a expensive equipment like this be more "this problem-free"?



Are you refering to the NI DAQ device or signal conditioning?  The NI DAQ devices are general purpose devices and as such cannot be expected to  meet all the requirements of a specialized field such as medical equipment. Even to eliminate ghosting would require an amplifier per channel before the multiplexer and possibly some changed in the design of the multiplexer. That would require a larger circuit board, more power, and a price increase. Very little, if any, of NI's DAQ equipment is designed to meet the standards required of medical equipment, and even where it may meet those standards technically, I doubt that NI has gone to the expense of getting certification for it.


Apparently many of NI's customers find the DAQ devices they offer useful or NI would be designing, building and selling other devices or be out of business.



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Message 9 of 10

Yes, you are absolutely right. It makes perfect sense since bio-signals acquisition is a bit more complicated than regular signals.

My question was more of a outburst because I suspected I needed to have amplifiers and more material to make the signal conditioning and it will be a bit difficult to get them. I was praying that would exist a way of surrounding it.



Thanks again for all the help,


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Message 10 of 10