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| 型號： | VT900 |
| 廠商： | PerkinElmer Inc. |
| 英文描述： | Photoconductive Cells and Analog Optoisolators (Vactrols) |
| 中文描述： | 光導(dǎo)細胞和模擬光隔離器（Vactrols） |
| 文件頁數(shù)： | 70/76頁 |
| 文件大?。?/td> | 1240K |
| 代理商： | VT900 |
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65
Application Notes—Analog Optical Isolators
Figure 9 shows an AGC circuit which consists of three main elements:
a variable gain amplifier, full-wave active rectifier and a summng
amplifier. The variable gain amplifier consists of op-amp A1 with
potentiometric gain that is controlled by the resistance of the photocell
of the AOI. The gain of this amplifier is:
Gain = 1 + R
2
/ R
PHOTOCELL
With R
2
= 100k ohms, the mnimumgain is one since the cell “off”
resistance is several megohms. The maximumgain in only 100 since
the resistance of a typical VTL5C2 is 1000 ohms at an input current of
5.0 mA. If a range of 40 db (100:1) is not adequate, there are several
options. R
2
can be increased, the LED drive current for the AOI can be
increased or a lower resistance AOI such as the VTL5C4 can be used.
Amplifier A
2
together with diodes D
1
and D
2
and resistors R
3
, R
4
, and
R
5
forma full-wave rectifier. The amplifier has a gain of one so the
output is equal to the rectified input. There is no offset due to rectifier
forward drops so this circuit will rectify signals all the way down to zero
volts. Since the DC output of A
2
is not referenced to ground, op-amp
A
3
and resistors R
6
, R
7
, R
8
, and R
9
forma fully differential amplifier
which shifts the DC reference to ground.
Op-amp A
4
is used as an integrator. The signal fromthe full-wave
rectifier is summed with a reference voltage V
REF
and integrated. The
time constant of the integrator is selected to limt the bandwidth of the
control loop as well as assure stability of the loop. If the bandwidth is
too wide, the control loop will follow the signal on an instantaneous
basis. The AOI alone is not very fast, but signals with frequencies of 30
– 60 Hz could be distorted if there were no time delay in the integrator.
The AGC circuit operates as follows. When there is no signal, the
negative V
REF
causes A
4
to be at a maximumpositive output.
Maximumforward current is injected into the LED, driving the cell to a
low resistance and the gain of A
1
to the maximumwhere it stays until
there is a signal. A signal at the input termnal is amplified, rectified and
algebraically summed with V
REF
at the inverting termnal of the
integrator. The control loop will then act to make the absolute value of
the rectified signal equal to the reference voltage. V
REF
may be a fixed
value or a function of some other parameter.
Electrically Controlled Gain
The gain of an amplifier can be electrically programmed using the
circuit of Figure 10. An AOI with a center tapped photocell is used, one
side in the signal amplifier channel and the other in the control loop.
The signal amplifier consists of op-amp A
1
, resistors R
3
and R
2
which
set the gain and the input resistor R
5
. The gain of this amplifier is given
by:
e
e
in
Figure 9. AGC Circuit with Electrical Setpoint
G
--------
R
----------------------
R
+
R
2
(
)
=
=
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