Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 48)

Page 30 PRINCIPLES OF OPERATION

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Pre-filter Input-referred Input-referred

Gain Range Resolution

x1 ±3.00 V 100 µV

x10 ±300 mV 10 µV

x100 ±30.0 mV 1 µV

Rather than adding it to the input, the DC offset is added after the pre-filter amplifier for two reasons.

First, it is technically difficult to add an offset at the input without compromising either the noise, the

common-mode rejection ratio, or the input impedance. Second, by adding the DC level after the pre-

filter amplifier, better resolution is available for small signals. For example, if the user is recording

temperature from a semiconductor temperature probe putting out 10 mV/°C, an offset control with

100 µV resolution and ±3 V range corresponds to a temperature resolution and range of 0.01°C and

300°C respectively. This is more than sufficient for most applications. On the other hand, if the user

is recording from a blood-pressure strain gauge that puts out 25 µV/mmHg, an offset control with

100 µV resolution corresponds to a blood-pressure resolution of 4 mmHg and would probably be

regarded as too coarse to adequately zero the output.

Notch Filter

The notch filter is often used in biological recording. This is a special filter that is designed to

eliminate a single fundamental frequency and very little else. Notch filters at 50 or 60 Hz are

commonly used to eliminate line-frequency pickup.

Notch filters remove a narrow section of the power spectrum and are often used in conjunction with

frequency-domain analysis. Nevertheless, many experimenters prefer to record the data "as is" and

then to digitally remove the offending frequency component from the power spectrum.

In the time domain, a notch filter must be used with caution. If the recording bandwidth encompasses

the notch filter frequency, signals that include a harmonic at the notch frequency will be grossly

distorted, as shown in Figure . On the other hand, if the notch filter is in series with a low-pass or

high-pass filter that excludes the notch frequency, distortions will be prevented. For example, notch

filters are often used during electromyogram (EMG) recording in which the line-frequency pickup is

sometimes much larger than the signal. The 50 or 60 Hz notch filter is typically followed by a

300 Hz high-pass filter. The notch filter is required because the high-pass filter does not adequately

reject the 50 or 60 Hz hum.

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Microsoft LPF-00004 Manuel de l'opérateur Page 48