Passive low-pass filtering question for a transducer output

Question

I'm having ringing-noise from a pressure transducer analog output. Here is the only information about the instrument: http://www.trescal.be/pdf/PPC500_GB_SI.pdf

Let me briefly explain the device and the issue:

This device has an internal analog to digital conversion at some stage and outputs that information on its display. The output from digital data displayed on the screen can be sent via RS-232 directly or as an analog output which is created using digital to analog conversion. The both RS-232 and analogue output sends the averaged voltages from the transducer at a rate of around three times per second.

For some reason I have to use the analog output. I record the readings from the analog output by a data-acquisition device which has 1GigaOhm input impedance. The DAQ device has many analog inputs and one of the analog inputs comes from this pressure transducer via a BNC cable which is between 10 up-to 15 meters long. All the analog inputs share the same ground and only at this transducer's channel I'm observing ringing-like noise. Same noise is observed if I hook the BNC up to an oscilloscope.

Below is an example of the transducer's analog signal in question. It shows the signal in time series for 60 seconds which is sampled at 8000 Hertz and plotted in LTspice:

Here below if I zoom the signal, please see the three steps of discrete voltage sent by the transducer’s analog output per second:

It seems like there is no reconstruction filter on the output of the transducer’s DAC. But actually it doesn’t matter in my case. What bothers is mostly those crazy jumps/ringings on the edges which sometimes exceeds one volt. And here below I'm directly zooming to the crazy ringing-edge and by using LTspice's cursors I find out that the ringing-noise is around 1kHz:

So I decided to use a RC passive low-pass filter but only checked in LTspice and the simulation results are below (V1 is the output from the traducer; C1 and R2 represents the low-pass filter; R1 is the input impedance of the data acquisition channel; green plot shows after filtering):

And for a better comparison before(blue) and after the filter(green) here:

Here are my concerns and questions:

1-) I found the solution by filtering. It seems to me the loading is negligible due to the huge input impedance of the DAQ channel here. So do you agree in this case to use these R2 C1 values are optimum for a passive low-pass filter to eliminate 1kHz?

2-) Do you think this passive filter enough here? Is active filter necessary(I haven’t built any yet)?

3-) What could be the reason for this ringing? Could it be length of BNC or inductive effect? (It seems like we have an DAC outputting 3Hz discrete signal sent via a BNC coax cable)

Answer 1

1). Confirm output impedance of Analog sensor output by load test for ~50% drop

• complementary emitter followers used in Op Amps or discrete are notorious for resonance with capacitive loads from 100pF/m cables and current limiting causes low out followed by overshoot followed by ringing at loop gain BW of Op Amp
  • This may be what is happening
• this is where current mode converters work better 4-20mA with separate return wire, shielded or twisted pair and CM ferrite sleeve.
• add 100-300 R series at source to verify reduction in edge pulse
• then load destination with 1k to reduce stray noise coupling and verify voltage drop if any. You may need a power op amp buffer with 120 Ohm source and low impedance load to reject stray coupling or whatever cable Z you are using.

• check with proper high speed measurements for RF noise and LPF as required to rule out RF aliasing noise at 8kHz sampling rate

  • consider 10mH CM choke and RF cap across output for 50/60Hz and up noise

  • example of your CM 50Hz noise showing up as a differential mode (DM) signal

  • due to unbalanced source and load Z on 50 Ohm coax.

  • Is a shame to degrade an instrument with > 60dB SNR down to 20 dB with improper analog cable maybe 30~40dB SNR with filter

Answer 2

Assuming that the output of the instrument (used as a sensor) has some really noisy pressure input and the internal instrument workings do produce this kind of jumpy output, the answers are:

1. Yes, a simple RC filter with 1kHz cut-off is an acceptable solution

2. Yes, the data show that this is good enough;

3. Most likely reason for the ringing at 1kHz is a flaw in instrument's design of output stage of internal DAC. The 1 kHz ringing cannot be explained by cable/load mismatches.

SIDE NOTE: It looks quite obvious that the selection of this sensor is inadequate to the nature of input pressure signal. The instrument states "settling time" of ~0.5s, which limits the spectrum of input signals to pretty much static pressure levels.

Answer 3

Transducers are very sensitive instruments. Your signal is probably not the actual transducer signal that the device is processing. I know that magnetic transducers on fathometers and fish finders use amps, not filters to straighten out the signal pre-processing. The fact is I don't know a lot about the processors used with transducers, but I do know that if you filter a tranducer signal, you lose the signal rather than reduce the noise in the line.

I remember once when I was playing with a transducer booster, and the thing started squealing and stopped only after I pulled the plug! I also know that the reason transducers work is because they are powered, and not passive. Filters are generally used in passive circuits, so you will want to use an active filter too.

Good luck, don't fry the transducer... they are very sensitive to their input signal, and can't take any extra resistance at the output, or they will blow their filaments.