AN912 Datasheet, PDF (4/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

AN912
FIGURE 4:
160
140
120
100
80
60
40
20
0
0
1
2
3
τ
4
5
EQUATION 3:
τ₌
tSETTLE
ln(Vo/VSETTLE)
tSETTLE = 200 µS
VO = 150V
VSETTLE = 1 mV
Using Equation 3, τ was calculated to be 16.78 µs. The
question now is how will the data signal be affected by
the decoupling stage? The decoupling stage, shown in
Figure 3, is also a high-pass filter and it was calculated
that the RC time constant needs to be 16.78 µs to
satisfy the transient response requirement. The 3 dB
cutoff frequency, for a τ, of 16.78 µs is calculated as
9.48 kHz using Equation 4. This means that the
decoupling stage will only pass one quarter of the
original data signal at 2.5 kHz, which is not desirable
from a signal-to-noise ratio perspective.
EQUATION 4:
1
FC = _2πτ
From a data signal conservation, or high-pass filter
point-of-view, τ should be at least 64 µs. The conflicting
τ requirement shows that a basic high-pass filter is not
sufficient as a decoupler unless either dynamic
response or data signal strength is sacrificed.
DS00912A-page 4
 2004 Microchip Technology Inc.