Frequently Asked Questions

1. WHY EXTERNAL SYSTEM RATHER THAN COMPUTER BOARD LEVEL PRODUCT?

Transducer level signals are usually in the mV range so they require amplification and filtering. Computer power supplies and circuits create a noisy environment. As a result, at least some of the system must be external. With the entire system provided, there are no matching problems to be solved.

2. HOW ABOUT VXI?

VXI offers attractive solutions for some measurement and control functions but is weak in a data acquisition application. Noise is again a problem. Channel cost is so high that important components are omitted in favor of high channel count: Components such as remote sensing of bridge excitation, effective normal mode filtering and protection against high frequency common mode noise. Adequate amplifier stability is sometimes neglected.

3. WHAT MINIMUM INFORMATION DO I NEED WHEN LOOKING FOR A SYSTEM?

You should know the approximate transducer output levels, the signal frequency and possibly the waveshape. If the signal is dynamic what parameters are important: waveshape, peak, minimum, skew, channel-to-channel phase match.

4. HOW FAST SHOULD I SAMPLE? IS FASTER BETTER?

The minimum sample rate can be based on aliasing considerations. Remember that sampling rate must be at least twice the highest frequency present. Beyond that, it is based on reproducing the waveshape. Faster may not be better if it does nothing but expand the data file.

5. CAN I CALCULATE THE REQUIRED SAMPLE RATE TO AVOID ALIASING?

Yes, NEFF offers an application note.

6. WITH SO MANY SPECIFICATIONS HOW DO I KNOW WHAT TO LOOK FOR AND HOW TO COMPARE PRODUCTS?

NEFF offers an application note which describes how to combine sources of error and evaluate the measurement uncertainty.

7. I HAVE DECIDED I NEED FILTERS. NOW, DO I SPECIFY 2, 4, 6 OR 8 POLES? IS MORE BETTER? WHAT ARE THE DRAW BACKS TO MORE POLES?

More is better to a point. A higher rolloff rate is achieved with more poles. This results in a lower rate to passband; which results in less data to transfer and to store. The draw back is higher cost.

8. WHAT TYPE OF FILTER IS BEST?

This depends on the application. If yours is one without sharp transitions, a Butterworth filter, known as Maximally Flat, exhibits practically no attenuation below 50% of the cutoff frequency, fc. Step inputs, however, result in overshoot with a little ringing. A Bessel response has no overshoot but attenuation of the input signal starts at DC and at 50% of fc it is -7.4% and at 90% of fc it is -24%. Terminal, or final, rolloff rate of both is 6dB per octave per pole and in the Butterworth it is reached at frequencies a little above fc. With a Bessel, however, the knee; is softer and the final slope occurs at much higher frequencies. This means that attenuation of 120dB is achieved at frequencies greater than 6fc through a Butterworth and greater than 20fc through a Bessel filter. Other types of filters include the Chebyshev with very high rolloff rates and ripple (or variations) in the passband.

9. EXCEPT FOR THE OCCASIONAL SITUATION SUCH AS THERMOCOUPLES MOUNTED ON ELECTRIC MOTOR WINDINGS, DO COMMON-MODE VOLTAGES REALLY EXIST?

Absolutely - both DC and AC common mode voltages are found everywhere with varying magnitudes. In fact they are probably not constant even in static environments.

10. WHAT IS A GROUND LOOP?

This is a term given to an arrangement which connects one of the signal leads, usually ground, at the transducer and that same lead to ground at the measuring instrument. If a voltage exists between those two points, a loop is formed.

11. WHAT IS THE EFFECT OF A GROUND LOOP?

Current in the signal lead produces a voltage drop which adds to the transducer signal. Since it is likely to vary there is no way to compensate even if it were somehow possible to measure at some particular time.

12. ARE THERE GOOD GROUND LOOPS?

Are there good plagues?

13. MY TRANSDUCER IS GROUNDED, HOW DO I AVOID GROUND LOOPS?

Use a measuring device with differential; rather than (one terminal grounded) inputs.

14. WHAT IS COMMON-MODE VOLTAGE?

A voltage applied to both input terminals relative to ground.

15. WHAT IS COMMON-MODE REJECTION?

The measure of sensitivity of a differential amplifier to a common-mode
voltage.

16. ARE ALL DIFFERENTIAL AMPLIFIERS EQUAL IN TERMS OF COMMON-MODE REJECTION?

No. An amplifier's ability to minimize the effect of common mode voltage is calculated as the ratio of the common mode voltage at the input to the resultant output voltage. It is normally expressed in dB and should include a source impedance mismatch such as 350 Ohms.

17. WHY DON'T YOU USE SIGMA/DELTA CONVERTERS?

This is great technology but as of now these devices have sufficient accuracy at DC or AC frequencies but not both. Digital filtering, frequently associated with these converters can be implemented using other ADCs1 using the oversampling technique.

18. WHAT ARE THE ADVANTAGES OF A DIGITAL FILTER?

1. Tight phase match between channels.
2. Low sample rate to cutoff frequency ratio.
3. Very low noise.
4. Stable over time and temperature.