I am working with the DC-LVDT Omega 620-25


Manual: http://www.omega.com/Manuals/manualpdf/M4345.pdf

Specs: http://www.omega.com/pressure/pdf/LD620.pdf

I am using the NI 9220 module on a compact DAQ for data acquisition. the module supports differential inputs and single ended mode.

NI 9220: http://www.ni.com/pdf/manuals/373920a_02.pdf

I am experience some trouble obtaining 3 digits of accuracy from the converted signal, the excitation voltage is 12.75 Volts (external adapter) and the reading is -5.9

Furthermore the second case was by using a single ended connection (as per the modules datasheet) single ended is achieved by connecting the signal - of the LVDT module to the COM input, I also placed a 100nF capacitor for noise filtering, I was able to get 2 digits of accuracy 1.12x.

Input is read with a continuous acquisition mode of 100 samples/second with a 500Hz frequency and a voltage range of -10 and 10 as device parameter options ( only differential mode is selectable)

In addition and since the LVDT is linear I should directly convert the voltage reading to distance but the output voltage is of different range the datasheet states that for an lvdt of -d/2 and d/2 it should outputs a -5 and 5 V readings.

I double checked on the sensitivity formula mv/V/mm Output Ripple: 0.02% FSO LD620-25 ±25 (±1.0)inch and with a range of -6 -> 6 the full LVDT range is (12V) therefore is its of 0.0024 mv/V * 12.75(excitation) = 0.0306mm

Example used: http://tinyurl.com/hrcshow

but i didn't use that as for calibration I am considering the initial read voltage to be the 0 reference and then subtracting it from each reading!

1) What is the difference between using a single- ended and differential modes for a DC output? and how can I achieve better accuracy

2)where do I use the calculated sensitivity and did I get the calculation correctly?

3)how can I eliminate additional noise (AC noise) that is affecting my measurement and what should I take into consideration for designing a filter and choosing it's type

when disconnecting the LVDT excitation voltage the modules outputs uV values that varies. considering the copper wire acts a receiver and generating additional noise I place the 100nF capacitor between the signal + and signal - terminals.

In addition, I am going to design a circuit to level shift the output voltage from -6 6 to 0 - 12 and make it a single ended output and connecting it to an arduino or a PIC (is it a good idea)?

Thank you in advance! I appreciate the help


1 Answer 1


Perhaps you have mixed some models. From your description I presume you have a +/-5V output from LVDT, the supply voltage is 10 to 30V, but this has no influence on ouptut signal.
1 Whenever possible use differential mode, but I don't see that you have this possiblity (you should point exatly what is the output/wiring of your model).
2. Probaly with calcualting 12V as reference, as said the output is +/-5V
3. Good shileded cable, sheild connected to the earth.

  • \$\begingroup\$ It is the same module but I am getting around -5.9 to 5.9 output the differential mode is not stable the only stable reading i got with 2 digits accuracy was the single ended mode in differential mode I tried : signal + to AI+ and signal - to AI- the LVDT white shield cable connected to COM, the second mode was single ended by connecting AI+ to signal + and AI- to COM and signal - as 1 point the datasheet of the 9220 suggest using a 1Mohm between AI- and COM but that didn't improve the reading and the LVDT suggested a 10Kohm resistor between SIG+ and SIG- but that did not improve it as well \$\endgroup\$
    – chaosmind
    Commented May 8, 2016 at 20:03
  • \$\begingroup\$ I would change the PSU unit first, with a stable voltage LDO. The problem in differential mode, could be related to the ground connection. As for -5.9 to 5.9V output, I think you should contact Omega. Try to ground the - of PSU. \$\endgroup\$ Commented May 8, 2016 at 20:17
  • \$\begingroup\$ I have an external regulated 12 VDC adapter connected to the excitation wires of the LVDT \$\endgroup\$
    – chaosmind
    Commented May 8, 2016 at 20:20
  • \$\begingroup\$ Then, I think you did everything as one engineer would do. Most be something wrong with the part. You can scarp a ferrite ring from some old equipment like mouse, keyboard, extern hdd,... then make few turns with both signal wires, this would work only in differential mode - it's a common mode choke, it would block noise if it's the same in both wires. \$\endgroup\$ Commented May 8, 2016 at 20:26
  • \$\begingroup\$ I tried a capacitor on the inputs! but i will try the ring wiring, i wanted to convert the differential mode to a single ended one and shift the voltage level then use a divider and connect that to a pic or arduino! In addition I have no idea on what to consider and how to design an AC filter for the input is it a low pass or high pass and how you get the RC parameters \$\endgroup\$
    – chaosmind
    Commented May 8, 2016 at 20:30

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