NI 9205 Common

[clachankid]

I have a NI 9205 card and I am doing a differential measurement across a resistor and I get random jumps of about 40mV without a ground or common on the COM connection of the card. Can you explain the reason for this since the measurement is suppose to be a differential measurement and should not require a common connection?

[crossrulz]

Even differential measurements need a reference to ground.  Otherwise you are just opening yourself up to weird ground bound noise like you are seeing.

[clachankid]

If it is just noise wouldn't taking a differential measurement null that effect?

[crossrulz]

---

@clachankid wrote:

If it is just noise wouldn't taking a differential measurement null that effect?

---

No.  I recommend you just read this article: Noisy Readings on the NI 9205

.......

[DsX]

Hi,

Tie in a 1 MOhm resistor between AI- and COM and see if it helps.

This way you still have a diff. measurement because a lot of current cannot flow to COM and at the same time you are keeping the input within the common mode range.

Best regards

Matej Zorko

[clachankid]

Adding the ground to COM actually fixed the problem, but my understanding of differential measurement is that this is not necessary. Like a DMM does not need a ground to make a measurement. So I'm not sure of the benefit of taking a Differential measurement with this card you could simply take a Single ended and have more inputs.

[crossrulz]

---

@clachankid wrote:

Adding the ground to COM actually fixed the problem, but my understanding of differential measurement is that this is not necessary. Like a DMM does not need a ground to make a measurement. So I'm not sure of the benefit of taking a Differential measurement with this card you could simply take a Single ended and have more inputs.

---

Well, this card is actually taking two Single Ended inputs and the driver is doing the subtraction for you to make it differential.  It is a way of simplifying your software.

[clachankid]

Thank you that clarifies what is happening.

[Seth_B.]

It's also worth noting that Differential mode allows for measuring the difference of two signals with some common mode voltage on an input range of less than the full common mode, increasing the resolution of the measurement between the two signals.  For instance, you could use the ±1V input range to measure between a 8V and 8.5V signal instead of measuring each with a ±10V single ended measurement.  You'd just need to ground the COM to the ground of the common mode voltage.