EL4450
The maximum dissipation a package can offer is:
inputs between 200mVRMS and 1VRMS. The traditional use
of the EL4450 as an AGC detector and control loop would
be:
P ,max = (T ,max–T ,max)/θ
D
J
A
JA
Where
T ,max is the maximum junction temperature, 150°C for
J
reliability, less to retain optimum electrical performance
T ,max is the ambient temperature, 70°C for commercial
A
and 85°C for industrial range
θ
is the thermal resistance of the mounted package,
JA
obtained from data sheet dissipation curves
The more difficult case is the SO-14 package. With a
maximum junction temperature of 150°C and a maximum
ambient temperature of 85°C, the 65°C temperature rise and
package thermal resistance of 120°/W gives a dissipation of
542mW at 85°C. This allows the full maximum operating
supply voltage unloaded, but reduced if loaded significantly.
FIGURE 1. TRADITIONAL AGC DETECTOR/DC
FEEDBACK CIRCUIT
The EL4450 simply provides an output equal to the square
of the input signal and an integrator filters out the AC
component, while comparing the DC component to an
amplitude reference. The integrator output is the DC control
voltage to the variable-gain sections of the AGC (not shown).
If a negative polarity of reference is required, one of the
multiplier input terminal pairs is reversed, inverting the
multiplier output. Input bias current will cause input voltage
offsets due to source impedances; putting a compensating
resistor in series with the grounded inputs of the EL4450 will
reduce this offset greatly.
Output Loading
The output stage is very powerful. It typically can source
85mA and sink 120mA. Of course, this is too much current to
sustain and the part will eventually be destroyed by
excessive dissipation or by metal traces on the die opening.
The metal traces are completely reliable while delivering the
30mA continuous output given in the Absolute Maximum
Ratings table in this data sheet, or higher purely transient
currents.
Gain accuracy degrades only 0.2% from no load to 100Ω
load. Heavy resistive loading will degrade frequency
response and video distortion for loads < 100Ω.
This control system will attempt to force:
2
V
,RMS /4=V
REF
IN
Capacitive loads will cause peaking in the frequency
response. If a capacitive load must be driven, a small-valued
series resistor can be used to isolate it. 12Ω to 51Ω should
suffice. A 22Ω series resistor will limit peaking to 2.5dB with
even a 220pF load.
Mixer Applications
Because of its lower distortion levels, the Y input is the better
choice for a mixer’s signal port. The X input would receive
oscillator amplitudes of about 1V RMS maximum. Carrier
suppression is initially limited by the offset voltage of the Y
input, 20mV maximum, and is about 37dB worst-case. Better
suppression can be obtained by nulling the offset of the X
input. Similarly, nulling the offset of the Y input will improve
signal-port suppression. Driving an input differentially will
also maximize feedthrough suppression at frequencies
beyond 10MHz.
AC Level Detectors
Square-law converters are commonly used to convert AC
signals to DC voltages corresponding to the original
amplitude in subsystems like automatic gain controls (AGCs)
and amplitude-stabilized oscillators. Due to the controlled AC
amplitudes, the inputs of the multiplier will see a relatively
constant signal level. Best performance will be obtained for
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