Biomedical Engineering Reference
In-Depth Information
The equation describing this relationship is
1
RC
V
out
=−
V
in
dt
+
V
c
(5.26)
where
V
c
is the initial charge on the capacitor at
t
=
0.
Real integrator circuits generally have a switch to short out the capacitor at the start
of an integration cycle and sometimes also a large value resistor across it to drain away
any residual voltage that may have accumulated due to leakage.
5.4.3.4 Envelope Detection
For an amplitude modulated signal, the envelope is a construct that joins the positive
peaks of the signal to recreate the original modulation waveform. The process of envelope
detection starts with rectification of the bipolar signal. If the signal amplitude is sufficiently
large, a conventional half- or full-wave rectifier can be used—notwithstanding the volt
drop across the diodes. A conventional envelope detector consisting of a diode and a
low-pass filter is shown in Figure 5-33.
If the signal is small, though, then a precision full-wave rectifier such as the one shown
in Figure 5-34 must be used.
This circuit is very common and has been around for many years. The tolerance of
R2, 3, 4, and 5 is critical for good performance, and all four resistors should be 1% or
FIGURE 5-33
Conventional
envelope detector.
FIGURE 5-34
Precision full-wave
rectifier.
