| Filter automatically synchronizes to any
frequency between 47 to 70 Hz. |
| Attenuates the fundamental line interference
and its harmonics through the frequency range DC to 1,200HZ |
| Optical coupled interfacing to the reference
frequency. |
| Small size – only 2” x 4” |
| Dynamic Range – 10000 |
| Notch Width – 0.3 Hz |
| |
| Introduction about the multi-notch
(comb) reject filter: |
| The EDR 82534 is a low cost, multi-notch (comb) reject
filter for low frequency signal applications. A Power interference
or band stop filter is an important and needed sub-component to
design and build medical recording systems where minute bioelectric
signals, on the order of mill volts to microvolts are measured.
In the operation room, the cath lab, or in research facilities there
is always a substantial level of electrical interference produced
by local power lines. |
| |
| The EDR 82534 is more than just a notch filter. It
represents a significant improvement on other methods of signal
filtering. |
| |
| Features for the multi-notch (comb)
reject filter: |
| q
The filter allows a “clean” bandwidth of bioelectric
signals right up through the frequency range used in His bundle
measurement. In addition to the fundamental frequency of 50 or
60 Hz, all the upper harmonics. |
| q
All useful bioelectric information is permitted to
pass unaltered. The EDR 82534 will not filter out every 50 or 60
Hz signal, but waits to be sure that a signal is constant before
filtering it. A one-time occurrence such as a 60 Hz H-wave does
not get filtered out. |
| q
If there is no ac interference, the Filter does not
affect the signal. There is no unnecessary alteration of the output
waveform, as only the noise is filtered out. (By contrast, a normal
notch filter is always “on”) |
| |
In a biomedical establishment substantial electrical
interference is often induced on a subject by local power lines.
Most of the power lines noise can be reduced to acceptable levels
by common mode rejection circuits if a differential configuration
of input amplifier is appropriate and can be applied. Some common
mode rejection cannot always be removed because the applied noise
is unbalanced.
Also noise can be picked up from other equipment located near the
source of a signal, or from interference caused by the location
or positioning of the equipment installed. The situation is hopeless
when a single-ended input must be used and only a notch filter is
the solution. Analog-based notch filters can be built and applied
to remove AC interference but the usefulness of it is limited to
just a handful of applications. Analog filters are unstable in
time due to the deterioration of components in the circuitry. An
analog type comb-filter (multi-notch) is very expensive to build
and maintain. A filter with a notch of Q: 200 and notch depth greater
than 40 dB is almost impossible to build for a practical application.
The EDR 82534 overcomes all typical problems of an analog only based
filter. |
| |
| Theory of Operation: |
| The principle behind the EDR 82534 is to reduce line
interference using multiplexed sampling of “hum” with a series of
capacitors charging to the hum level, and subtracting that level
from the overall signal. |
| The main circuitry includes: |
| 1)
A multiplexer that commutates the signal across 64 storage
capacitors in a cycle synchronized to reference frequency. |
| 2)
Input for a reference signal is optical-coupled and any 5
Volt AC signal can be used. |
| 3)
A phase-locked loop oscillator and counter that provide line
synchronization. |
| 4)
A post filter to remove residual high frequency components
(greater than 1,200Hz) from the commutated signal. |
| |
| During each cycle of the hum, the storage capacitors
are charged to a portion of the difference between the charge voltage
built up during previous cycles and the hum voltage present at the
Input. After a number of cycles, the capacitors are charged up
to various voltages. Each capacitor now holds the average value
of the hum voltage occurring during the interval of the hum cycle
when the capacitor is connected into the circuit. Since an input
signal not in timing relationship with a reference signal build
up little or not average charge on the storage capacitors, single
events – even those with frequency components equal to the reference
frequency 50/60 Hz – pass to the output virtually unaffected. |
| |
| Because the polarity of the voltages across the respective
capacitors opposes the hum voltage in the signal, the portion of
the hum voltage appearing at the output is the hum voltage minus
the previously “stored” capacitor voltages. The remaining interference
consists of high frequency signal components. |
| |
| Specifications for
EDR 82534 and EDR 82534A |
| A very narrow band-stop of only .3Hz allows
removing the unwanted frequency with its entire highest harmonics
with surgical precision. |
| |
| Electrical specification
applied to both models or otherwise indicated |
| Analog |
| Input signal (bipolar) maximum
6 V p-p |
| Input Impedance 100 K |
| Reference input |
| AC signal (voltage) minimum 5
V AC / maximum 25 V AC |
| AC signal (current) minimum 1mA;
maximum 50mA |
| |
| Reference input comprised of an optical-couple
device (TLP127 by Toshiba) and 1.0K resistor installed between terminal
1 and LED of the photo coupler. |
| |
