Eliminating Noise When Using a DI-145/149 Starter Kit to Acquire Data

A common complaint among DI-145/149 users; “noisy” signals in WinDaq are sometimes attributed to grounding, or lack thereof.  Because the DI-145/149 draws its power from the USB port, it shares a ground with the PC.

Under these unique conditions, where noise is present, the DI-145/149 should be grounded:

• You’re connected to a laptop that is running off of battery power
• You’re laptop/desktop is powered via a two-prong plug (no ground prong)
• You’re laptop/desktop does have a ground prong, but you’re using a two-prong “cheater”

• Any other situation in which the PC is not grounded

Under these circumstances, the laptop, and consequently the DI-145/149, are free to float at some unknown potential. Due to the limited common-mode rejection ratio of the differential amplifiers, the potential difference between the floating DI-145/149 and the ground of the device under test (common mode voltage) can appear in WinDaq as “noise”. And that’s only if the measurement can be made at all. Often, the sum of the common mode voltage, and the signal that you’re trying to acquire, will exceed the full scale measurement range of the instrument.

Although DI-145/149 instruments don’t have a ground pin (AGnd) like older DI-148/158 starter kits (now obsolete); a ground connection can be made on the negative (-) side of any of the digital inputs.

While grounding your DI-145/149 will squelch the common mode voltage, it does come with risk. There is a 4.7K ohm, 1/8 watt, current limiting resistor on the negative side of the digital inputs. If too much current flows through the resistor, POOF, you have a damaged unit. How much is too much?

P (Watts) = I² * R

0.125 = I² * 4.7K

so  Imax = 5.2 mA

Since we know that that the maximum current is 5.2mA, we can use Ohm’s Law to calculate the maximum voltage (assuming that it can supply 5.2mA of current).

V = I * R

V = 5.157mA * 4.7K ohms

so  Vmax = 24V

In the case of an ungrounded PC, the common mode voltage is typically generated from a high impedance source. As a result, you can expect that the current (I) will be very small. Consequently, grounding your DI-145/149 in the manner described above shouldn’t be a problem.

If you’re unsure, connect your DI-145/149 to another power ground of your choosing.

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