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    CN0288: LVDT Signal Conditioning Circuit

    CFTL

      If you have any questions or comments about this circuit please post them below.

        • 1. Re: CN0288: LVDT Signal Conditioning Circuit
          Horbach

          It is mentioned that CN0288 is made for inductive LVDT. I want to apply this circuit for a capacitive sensor. Is it possible to use this circuit with capacitive sensor for measuring a displacement? I attach the scheme of capacitive sensor as an image. Sensor looks like two cylindrical variable capacitors which can be used in bridge.

          sensor.jpg

          • 2. Re: CN0288: LVDT Signal Conditioning Circuit
            JFitzger

            Are you planning on using the AD598?  I'm not sure if this will work.  What is the sensor you are planning on using?

            • 3. Re: CN0288: LVDT Signal Conditioning Circuit
              Horbach

              As far as I understand AD598 is a part of CN0288 and it has an analog signal at the out. For me it is better to have logic out. But if AD598 will suit for my sensor it will be OK.

              Sensor that I want to apply is hand-made device and it looks like inductive LVDT, but it is capacitive. Value of capacitance is approximately 4 pF. It consist of two coaxial cylinders. Internal cylinder can move back and forth so the capacitance changes.

              • 4. Re: CN0288: LVDT Signal Conditioning Circuit
                JFitzger

                The AD598 drives an AC voltage on the Excitation Pins, if you need a DC signal it might not work.

                • 5. Re: CN0288: LVDT Signal Conditioning Circuit
                  Scott Jenney

                  I would like to use the CN0288 but I need a high pass filter for the LVDT secondary coils to remove the primary signals. Are there details for the AD598 A/B filtering. The LVDT is used for detection more than position.

                  Do you have any suggestions?

                  • 6. Re: CN0288: LVDT Signal Conditioning Circuit
                    tribologist

                    I have problem getting the bandwidth I need out of the demo board. We are looking at integrating the position sensing into a system rather than having a purchased LVDT signal conditioner so the CN-0288 looked like a good start. I calibrated the output using gage blocks and I have a secondary position sensor (Keyence laser position sensor) At DC and up to as fast as I can jerk the core in the system they track perfect but as I start to excite the system I loose output very quickly and at 120 Hz I get a 0.00035" output where my true motion is 0.005"

                     

                    I also hooked up a very small LVDT (Measurement specialty MHR025) and put a stainless screw through it and two nuts as travel stops allowing me to flick it with a finger and the output wave form show a rise time of about 30-40ms. I tried to change the three caps that set the bandwidth to 0.1uF but that did not make a difference. I also attached a lead after the AD8615 thinking my DAQ system or the scope loaded the AD598 but that signal is identical to the one at the JC6 SMB output.

                     

                    I'm at a loss here. Any advice is greatly appreciated.

                     

                    Ulf

                    • 7. Re: CN0288: LVDT Signal Conditioning Circuit
                      Ryan_Curran

                      Hi Ulf,

                       

                      CN0288 is designed specifically to work with an E100 LVDT.  If the LVDT that you are using does not have the exact same operating characteristics (Voltage Transformation Ratio (VTR), target excitation frequency, etc.), then the passive components that set the operating characteristics of the AD598 will need to be recalculated to configure the system for your specific transducer.

                       

                      The A and B inputs can never see a signal greater than 3.5 Vrms.  If your LVDT has a more sensitive VTR than an E100 LVDT, then you may be saturating the inputs.

                       

                      Also, C11 and C12 of CN0288 should be DNI.  We're in the process of updating the BOM.

                       

                      -- Ryan

                      • 8. Re: CN0288: LVDT Signal Conditioning Circuit
                        tribologist

                        Thanks Ryan,

                         

                        In the application I use a HR050 and it had quite similar characteristic. it has 290mV/V output, 2.5kHz, Slightly higher input and output impedance. The scaling in v/inch would differ but I cant see anything in the datasheet that is pointing on how the bandwidth could be reduced to ~25Hz vs 250Hz. As far as I can read the C2-C3 and C4 are the primary for setting the bandwidth and the other components deals mainly with the gains.

                         

                        Am I missing something?

                        • 9. Re: CN0288: LVDT Signal Conditioning Circuit
                          tribologist

                          Should I remove C11 and C12?

                          • 10. Re: CN0288: LVDT Signal Conditioning Circuit
                            Ryan_Curran

                            Hi Ulf,

                             

                            Yes, please remove C11 and C12.  These are incorrectly populated and should be DNI.  However, these components have no association to system bandwidth.

                             

                            You are correct that data sheet capacitors C2,C3 and C4 are the bandwidth capacitors.  For a 250 Hz BW, they should be set to 400 nF, so 100 nF should result in a bandwidth even greater than 250 Hz.  CN0288 also features a shunt capacitor across the output scaling resistor (C8 and C10) that will influence output settling time.  There is also a low-pass RC filter between the AD598 and the AD7992 that will limit system bandwidth.  These may require adjustments to achieve your bandwidth of interest.

                             

                            What is the VTR of the HR050 in V/V/inch?

                             

                            -- Ryan

                            • 11. Re: CN0288: LVDT Signal Conditioning Circuit
                              tribologist

                              Thanks Ryan!

                               

                              The sensitivity is about 2X. It is 5.8V/V/in since it is a 0.050 stoke unit. Input impedance is 430Ohm vs 660 on the E100 and output impedance is 4000Ohm vs 960Ohm for the E100. Its hard for me to see how any of the LVDT parameters would change the rise time/bandwith that much. I took the little MHR025 I used to measure the rise time and hooked it to a Measurement Specialties LVM-110 conditioner as we currently are using  and the typical rise time I see when flicking the core about 0.010" is ~1ms on that card while I get about 25-30ms on the CN0288. I also have a CN0301 and that show the same slow response.  I also get the exact same rise time at the J6 port as I get at the R14-C31 junction leading into the A/D. I will pull the C8/C10 and let you know what I find.

                               

                              Thanks again! I reapply appreciate it!

                               

                              Ulf

                              • 12. Re: CN0288: LVDT Signal Conditioning Circuit
                                tribologist

                                You nailed it Ryan, The C10 is gone and now I am at 1ms. The ripple is up to 75mV but I will readjust the BW caps and see what I get.

                                 

                                THANKS!!!!

                                • 13. Re: CN0288: LVDT Signal Conditioning Circuit
                                  tribologist

                                  After going back to original caps on the BW pins i'm down to 40mV ripple but after the  R14-C31its only 12mV. Its mainly composed of a low level 400kHz signal and a 2.5kHz and 5 kHz so it should be easy to get rid of.

                                   

                                  Are there by any chance a set of VI's available for CN0288 or CN0301?

                                   

                                  Ulf

                                  • 14. Re: CN0288: LVDT Signal Conditioning Circuit
                                    Ryan_Curran

                                    Hi Ulf,

                                     

                                    I'm happy to have helped and glad to hear you had success!  I had asked for the VTR parameters so that I could independently perform the passive component calculations.  I wanted to ensure that there were no clipping issues in the system that manifested as a bandwidth issue. 

                                     

                                    The purpose of C8 and C10 is to provide additional ripple filtering, but as you found out, it comes at a trade off of reducing system bandwidth.

                                     

                                    Are you looking for LabView VIs for processing the data from the AD7992?  I would need to speak with the group who developed the board and the GUI to get the files used to make the CN0288 executable.

                                     

                                    Please let me know if this is what you are looking for,

                                    Ryan

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