amplifying and conditioning

[corushtd]

Can anyone tell me the essential different between a line drive supply and a charge amplifier for acc. sensors.? Whene to use what? And why is there a change in phase angle using charge?

 

[GregLocock]

They are for different instruments. You use a charge amp with a pure piezo type device, whereas (guessing that you are talking about an ICP driver) the other requires a 4 mA current supply.

I don't know why there is a phase error associated with a charge amp. There wasn't on the ones I used, the entirely wonderful B&K 2635. There may be a signal inversion. Your calibration procedure will tell you if that is the case.



Cheers

Greg Locock

 

[corushtd]

thanx Greg, We also use the B&K2635 and a college of mine found a diverend in phase angle using chagre or ICP. I did read somthing about is on the internet but I can't seem to find it anymore. I did find some papers about using charge or voltage amplifiers but I don't really know if this is the same because most of these papaers are from the 70's.
I was looking in the B&K Master Catalogue and found that de line drive supply is basicly the same as a charge if used together with a 2644 or special line drive sensors.

 

[MikeyP]

The issue of charge vs voltage amplifiers in the 70's was concerning purely charge accelerometers. If you were using a voltage amplifier with a charge accelerometer in the bad old days then you had to calibrate the accelerometer with a particular length of a particular type of cable to account for the cable capacitance (which would otherwise cause a phase error). The same problem does not occur with ICP which has an on-board impedance convertor that renders the capacitance of the cable unimportant.

When you compared the two amplifiers, did you do it on the same input of the same data acquisition system/scope?

M

--
Dr Michael F Platten

 

[corushtd]

thanx Michael, We where not really comparing the sensors but during balancing we changed sensors and found a phase difference. Later I heard that this can happen using charge amplifiers. But if I understand you correct this only happens using old fashion voltage amp's when changing cables. Could you get a phase change switching between ICP and charge? Would you know some literature/web-sites that have information on ICP and charge amp's?

 

[GregLocock]

Talk to your B&K rep, or read the manual for the charge amps.

More practically, set the two accelerometers up on a shaker and look at the outputs. You will learn many things.



Cheers

Greg Locock

 

[ftm2]

When routing a high impedance charge accelerometer signal through a charge amplifier, the signal is first converted to a low impedance voltage signal, via the charge converter stage. This will cause a 180° pahse shift in the signal. If the charge amplifier is a dual stage amplifier, the second stage will cause another 180° pahse shift in the signal. If this is the case, you will not see a phase shift in the final output signal.
If you are running the high impedance signal through a single stage charge converter(line driver/ remote charge converter), you will see a 180° phase shift, as most RCC's are single stage converters.