Safely measuring high voltage defibrillation with DAQ.

[mattoeding]

Attached is my proposed project.

The aim is to measure the leakage current through a patient from different positions while undergoing defibrillation.

We are resistively attenuating the biphasic signal from up to 4000V to a +-10V range for input into the DAQ module yet we require additional safeguarding to protect the DAQ module if the attenuation may fail.

The following are what i am looking for:

• Is it possible for the NI-DAQ to safely operate without the use of an isolation amplifier? (See Figure 1 in attachment)
• Is it possible to use a series of bi-directional varistor’s or transorb’s be used to shunt the voltage instead of an isolation amp? (see Figure 2 in attachment)
• Will there be a problem with isolation between channels if isolation amplifiers aren’t used?

 

[Steve.J]

I assume that you are planning on doing this testing on some kind of phantom body or cadaver?  Correct?

I think you will need an isolated system to have the best chances at getting useful information in a situation like this - and it will help protect the inputs of the measurement system.  We have economical choices for isolated DAQ products from integrated solutions like the USB-6216/6218 series or more modular solutions like CompactDAQ where the C-series plug-in modules are isolated.

[mattoeding]

Initally, it will be used on a mannequin and then if all goes well it will be implimented in live trials on patients undergoing cardioversion.

The system we are using is a NI-9215 module in a cDAQ-9191 chassis and using Wi-Fi to transmit the signal. We are aware we need some form of electrical isolation but we are unsure whether to acheive galvanic isolation from an isolation amplifier or we can just get away with some other, less expensive, solution.

It will be a battery powered system, therefore there will be minimul risk to the patient and are more concerned with damage to the DAQ module.

Any advice will be appreciated:)

[Igo · Accepted solution]

Hi

Firstly, the main point is a patient safety. And I see, that you understand this problem and I don't take it into account. One note, only: You can encounter a EMC problem, because a delivery of defibrillation pulse may cause to Wi-Fi channel down.

It's possible to build your system without isolation amplifiers. But your circuit is wrong.
1. You must consider that a high voltage may be applied between any patient electrodes. For the case as you suggest, applying high voltage between any wires connected to negative inputs of DAQ will damage it. To prevent it you can insert a high-impedance resistor in the each patient electrode net, not only for a half of nets.
2. You must consider right creepage distances and air clearances in the high voltage part.
These decisions can help to achieve enough reliability for your system as research one (If you choose a right type of high voltage resistors).

For increasing reliability:
3. Commonly, each high voltage resistor is replaced by series connection of two resistors which have full voltage rating and 1/2 resistance. If one resistor fails (shorted), then the other protects your system. You can detect this failure by periodically testing a gain of channels.
4. You can use a voltage suppressor, as you suggest. But you must connect a protection resistor between the high voltage resistive divider and the suppressor. You must calculate the power that the protection resistor and the suppressor dissipate in the case of failure. The idea is that the protection resistor must burn as fuse earlier than the suppressor. Voltage rating of the protection resistor must be full to prevent its breakdown.