connecting an opamp to a daq card, difficulties

[6street]

Dear All,

Previously I had some piezoelectric sensors (pressure) connect to a PCI-1200 in the following configuration

 Signal ----------------

 Shield ----------------

[6street]

Sorry about the first post, I hit submit somehow and didnt finish my edit in time.

Dear All,

Previously I had some piezoelectric sensors (pressure) connect to a PCI-1200 in the following differential configuration this was working fine with multiple sensors ACH0-ACH5

 Signal -------------------------------------ACH0

                        l

                     100k

                        l_________________AGND

                        l

                     100k

                        l

 Shield -------------------------------------ACH1

The signal from the sensors is minute so i've added an op-amp to each sensor circuit connected as follows, i've tried various ways of connecting this to the PCI 1200 and so far have had relatively unsatisfactory results. The manual suggests I should still be using a differential connection as the signal level is quite low. If someone could suggest the best way to interface it with the DAQ card it would be much appreciated. Below I have detailed one of the way in which i connected it and the results.

                        _____________________________________________________AGND

                        l                                                                                   l             l

                     100k                                                                               l             l____ACH1

                        l                                                                                 100k

 Signal -10k--------------------------------Op Amp +\                               l

GND                                                                     \_______________l___________ACH0

                                                                            /               l

 Shield -10k--------- ----------------------Op amp -/                 l

                        l                                                                    l

                     100k                                                                l

                        l__________________________________l

The results in this configuration sometimes just show 50hz noise from wires that are close by and show none of the vibration when i tap on the wood to which they are attached. Other times they show the vibration as negative voltage only spikes which is a little strange as I would expect a conventional looking acoustic wave as with the non op amp configuration.

Thanks for any help.

[6street]

Another thing to note is I plan on adding a passive lowpass filter to the circuit after the amplification stage.

[Ian Colman]

Hi,

How you connect your AI signals depends on how you configure your PCI-1200, but for this application I reccomend using a Differential Input configuration. The AI Channels 0 & 1 can be used as a differential pair and should produce desired results providing you conform to the differential grounding reccomendations on page 3-8 (29/93) of the manual (http://www.ni.com/pdf/manuals/320942d.pdf). If implimented correctly, this method should largely illiminate the 50Hz noise that you are currently seeing.

If you have already tried this to no avail please give more details of the hardware connections used (in words please since the ASCII images seem to be displaying incorrectly on my browser) and also details of the software that you are using to acquire the information, for example if you are using a LabVIEW Virtual Instrument please attach the .vi file along with an explanation and I will do my best to help you out.

Best Regards,

Ian Colman
Applications Engineer
National Instruments UK & Ireland

[6street]

Hi Ian,

Thank you for replying.  I have sucessfully used the sensors in a differential configuration using two 100k resistors tied to AGND. The sensors also work (but with added noise) in RSE mode with the shielding from the sensor tied directly to earth and the input signal tied to earth through a 100k resistor.

My issue is that the output from the sensors is very small and so I would like to add an amplification circuit to utilise the full range of the DAQ card. The attached word file (fig2) describes the basic amplifying circuit I am trying to implement however when I do this I only seem to get a 50hz noise signal.

In case the word diagram displays incorrectly. It is a simply inverting amplifier opamp arrangement with the input signal arriving at the negative terminal of the opamp through a 10k resistor. Also connected to the negative terminal is the feedback loop from the opamp out to the negative through a 100k resistor. The positive input of the amp is tied to ground. I have the PCI 1200 configured in RSE mode with the AIGND input tied to a common ground with the op amp psu and sensor shielding cable.

[6street]

Sorry I almost forgot, here is the vi.

[Ian Colman]

Hi,

Try the op-amp circuit in differential configuration as oppose to RSE. You must also make sure that you comply with the wiring diagram of page 3-8 (29/93) of the manual (http://www.ni.com/pdf/manuals/320942d.pdf)

Best Regards,

Ian