Wiring and Noise Reduction Methods
365
Reducing Common
Mode Rejection
Using Tri-Filar
Transformers
One HP E1413 customer determined that greater than 100 dB CMR to 10
MHz was required to get good thermocouple (TC) measurements in his test
environment. To accomplish this requires the use of tri-filar transformers
which are an option to the HP E1586A Remote Rack Terminal Panel. (This
also provides superior isothermal reference block performance for
thermocouple measurements.) This works by virtue of the inductance in the
shield connected winding presenting a significant impedance to high
frequency common mode noise and forcing all the noise voltage to be
dropped across the winding. The common mode noise at the input amplifier
side of the winding is forced to 0 volts by virtue of the low impedance
connection to the HP E1415 ground via the selectable short or parallel
combination of 1 k
Ω
and 0.1
µ
F. The short can’t be used in situations where
there is a very high common mode voltage, (DC and/or AC) that could
generate very large shield currents.
The tight coupling through the transformer windings into the signal Hi and
Low leads, forces the common mode noise at the input amplifier side of
those windings to 0 volts. This achieves the 110 dB to 10 MHz desired,
keeping the high frequency common mode noise out of the amplifier, thus
preventing the amplifier from rectifying this into an offset error.
This effectively does the same thing that shielded, twisted pair cable does,
only better. It is especially effective if the shield connection to the HP E1415
ground can’t be a very low impedance due to large DC and/or low frequency
common mode voltages.
The tri-filar transformers don’t limit the differential (normal mode) signal
bandwidth. Thus, removing the requirement for "slowly varying signal
voltages". The nature of the tri-filar transformer, or, more accurately,
common-mode inductor, is that it provides a fairly high impedance to
+
-
E
NM
shielded twisted pair cable
removable
jumper
5 uF
jumper
configured
Common Mode
Noise Source
Note: RC filter is a series 5.11KOhm resistor with
a 220 pF capacitor to GND
Normal Mode
(differential)
Noise Source
thermocouple
E
CM
RC filter
RC filter
E1419 Ground
DUT Ground
HP E1586
with Filter Option 001
Hi
Lo
G
SCSI cable
Op
ti
o
n
A
3
F
SC
S
I
to
D
IN
HP E1419
110 dB CMR to 10 MHz
Figure 6-8. HF Common Mode Filters
Summary of Contents for VXI 75000 C Series
Page 2: ......
Page 16: ...16 ...
Page 18: ......
Page 30: ...30 Getting Started Chapter 1 Notes ...
Page 32: ...32 Field Wiring Chapter 2 Figure 2 1 Channel Numbers at SCP Positions ...
Page 44: ...44 Field Wiring Chapter 2 Figure 2 11 HP E1415 Terminal Module ...
Page 54: ...54 Field Wiring Chapter 2 Notes ...
Page 61: ...Programming the HP E1415 for PID Control 61 Chapter 3 Programming Overview Diagram ...
Page 136: ...136 Creating and Running Custom Algorithms Chapter 4 Notes ...
Page 152: ...152 Algorithm Language Reference Chapter 5 Notes ...
Page 304: ...304 HP E1415 Command Reference Chapter 6 Command Quick Reference Notes ...
Page 308: ...308 Specifications Appendix A Thermocouple Type E 200 800C SCPs HP E1501 02 03 ...
Page 309: ...Specifications 309 Appendix A Thermocouple Type E 200 800C SCPs HP E1508 09 ...
Page 310: ...310 Specifications Appendix A Thermocouple Type E 0 800C SCPs HP E1501 02 03 ...
Page 311: ...Specifications 311 Appendix A Thermocouple Type E 0 800C SCPs HP E1509 09 ...
Page 312: ...312 Specifications Appendix A Thermocouple Type E Extended SCPs HP E1501 02 03 ...
Page 313: ...Specifications 313 Appendix A Thermocouple Type E Extended SCPs HP E1508 09 ...
Page 314: ...314 Specifications Appendix A Thermocouple Type J SCPs HP E1501 02 03 ...
Page 315: ...Specifications 315 Appendix A Thermocouple Type J SCPs HP E1508 09 ...
Page 316: ...316 Specifications Appendix A Thermocouple Type K SCPs HP E1501 02 03 ...
Page 317: ...Specifications 317 Appendix A Thermocouple Type R SCPs HP E1501 02 03 ...
Page 318: ...318 Specifications Appendix A Thermocouple Type R SCPs HP E1508 09 ...
Page 319: ...Specifications 319 Appendix A Thermocouple Type S SCPs HP E1501 02 03 ...
Page 320: ...320 Specifications Appendix A Thermocouple Type S SCPs HP E1508 09 ...
Page 321: ...Specifications 321 Appendix A Thermocouple Type T SCPs HP E1501 02 03 ...
Page 322: ...322 Specifications Appendix A Thermocouple Type T SCPs HP E1508 09 ...
Page 323: ...Specifications 323 Appendix A 5K Thermistor Reference SCPs HP E1501 02 03 ...
Page 324: ...324 Specifications Appendix A 5K Thermistor Reference SCPs HP E1508 09 ...
Page 325: ...Specifications 325 Appendix A RTD Reference SCPs HP E1501 02 03 ...
Page 326: ...326 Specifications Appendix A RTD SCPs HP E1501 02 03 ...
Page 327: ...Specifications 327 Appendix A RTD SCPs HP E1508 09 ...
Page 328: ...328 Specifications Appendix A 2250 Thermistor SCPs HP E1501 02 03 ...
Page 329: ...Specifications 329 Appendix A 2250 Thermistor SCPs HP E1508 09 ...
Page 330: ...330 Specifications Appendix A 5K Thermistor SCPs HP E1501 02 03 ...
Page 331: ...Specifications 331 Appendix A 5K Thermistor SCPs HP E1508 09 ...
Page 332: ...332 Specifications Appendix A 10K Thermistor SCPs HP E1501 02 03 ...
Page 333: ...Specifications 333 Appendix A 10K Thermistor SCPs HP E1508 09 ...
Page 334: ...334 Specifications Appendix A Notes ...
Page 346: ...346 Glossary Appendix C Notes ...
Page 388: ...388 Generating User Defined Functions Appendix F Notes ...