[NI USB 6341] Analog Input shows 60 Hz noise in measurement and automation explorer

[uclaria]

Hello, 

I have the following circuit hooked up to my DAQ via the DIP relay. Essentially in this circuit, when the wires labled Lick Tube and Chamber Floor are each held in different hands so that I'm completing the circuit through my body, there should be a voltage drop that shows up on the Measurement and Automation Explorer. However, when I do this, I'm getting no signal and instead a lot of 60 Hz noise. I have tried this circuit on a USB-6008 and got perfect results. Does anyone have experience with this board? Do I need to ground the board itself somehow? Any input is appreciated. 

[johnsold]

What points in that circuit are connected to the DAQ inputs? How is the DAQ input configured? If differential , do you have a bias path to AI ground?  I suspect that this is the problem. The USB-6008 has a built-in bias path and much lower input impedance, which could explain the difference.

Lynn

[uclaria]

The points in the circuit that are connected are on the DIP relay. Also, I am not too familiar with what a bias path is-- could you explain what I would have to do to get the 6341 to output the same results as the 6008, since I'm not too sure which one has built in features that would require me to make additional modifications on the other DAQ. 

Regards,

Aria

[johnsold]

Aria,

The bias current is current which is required to flow in the input circuit, typically, to turn on the transistor in the input stage of the amplifier insdie the DAQ device. The current must flow from the inputs to the DAQ device ground.  For the USB-6341 it is specified as +/-100 pA. While a 100 pA current is quite small, without it the device will not work properly.

In your battery powered circuit there is no connection to ground so the bias current has no source. Typically connecting a large value resistor from each input to ground works. For your circuit the value will not be critical. I suggest trying 1 Mohm.

The USB-6008 has internal resistors connecting from power supply to input to ground so it does not have this problem. Its low input impedance and internal voltage source often cause other problems however. The USB-6341 and most other high-performance DAQ devices do not have internal bias sources so that they can have very high input impedances and no "pull" toward some internal voltage source.

Lynn

[uclaria]

Hello, 

I really appreciate your help. I am rebuilding the circuit that I originally sent you due to some technical difficulties, but was wondering if the picture attached to this post exemplifies what you said. Also, I was wondering if you can explain to me how creating this bias current path can prevent the 60 hz noise that I'm getting in my signal when the circuit gets completed (between the points labeled lick tube and chamber floor)? I'm suspecting that the noise is due to some sort of AC grounding error and while your explanation of bias current makes sense, I'm having trouble relating it to how it prevents this noise in my signal. Are we essentially emulating the 6008's "built in" resistors by adding in these resistors in the aforementioned configuration?

Thank you greatly for your insight!  

[johnsold]

Aria,

The circuit shown in your image is what I described.

The 60 Hz noise is a little more complicated. Power line frequency electric and magnetic fields are ubiquitous. When you touch the circuit, the fields intercepted by your body introduce voltages or currents into the circuit. The input resistance of the USB-6008 is ~1E5 ohms while that of the USB-6341 is ~1E10 ohms. These are in parallel with a capacitive reactance ~26E6 ohms. So the 6341 will "load" any induced signal less than 1% of the loading by the 6008 input. Probably both devices have similar amounts of 60 Hz introduced by your body at the electrodes, but because your body and the circuit represents a relatively high source impedance, you see a lot more at the 6341 output than at the 6008 output.

If you use 270000 ohm resistors with the 6341, you would have an input impedance similar to the 6008. Try it with both devices set to the +/-10 V range and see how they compare. Recall that the 6008 has a 12-bit converter, while the 6341 has a 16-bit converter, so the 6341 can measure smaller signals while set to the same range.

Even with smaller value resistors, you are not really emulating the input circuit of the 6008 because you are not connecting to an equivalent 1.4 V source. You really do not want to do that, and I have never been able to figure out why the designers of the 6008 did that.

Lynn