EL4095CS INTERSIL [Intersil Corporation], EL4095CS Datasheet - Page 12

• Manufacturer Part Number: EL4095CS
• Description: Video Gain Control/Fader/Multiplexer
• Manufacturer: INTERSIL [Intersil Corporation]

100% Accuracies
When a channel gain is set to 100%, static and gain errors
are similar to those of a simple CFA. The DC output error is
expressed by
The input offset voltage scales with fed-back gain, but the
bias current into the negative input, I
dependent on gain. Generally, I
about seven.
The fractional gain error is given by
The gain error is about 0.3% for a gain of one, and increases
only slowly for increasing gain. R
of the input stage buffer, and R
the amplifier, 80kΩ and 350kΩ respectively.
Gain Control Inputs
The gain control inputs are differential and may be biased at
any voltage as long as V
3V above V-. The differential input impedance is 5.5kΩ, and
a common-mode impedance is more than 500kΩ. With zero
differential voltage on the gain inputs, both signal inputs
have a 50% gain factor. Nominal calibration sets the 100%
gain of V
-0.5V of gain control. V
V
-0.5V gain control sets 100% V
does not have a completely abrupt transition at the 0% and
100% points. There is about 10mV of “soft” transfer at the
gain endpoints. To obtain the most accurate 100% gain
factor or best attenuation of 0% gain, it is necessary to
overdrive the gain control input by about 30mV. This would
set the gain control voltage range as -0.565mV to +0.565V,
or 30mV beyond the maximum guaranteed 0% to 100%
range.
In fact, the gain control internal circuitry is very complex.
Here is a representation of the terminals:
For gain control inputs between ±0.5V (±90µA), the diode
bridge is a low impedance and all of the current into V
INA
V
E
OUT
GAIN
; +0.5V of gain control sets 0% gain at V
FIGURE 2. REPRESENTATION OF GAIN CONTROL
, Offset = V
INA
= (R
input at +0.5V of gain control voltage, and 0% at
F
+ A
INPUTS V
V
OS
*R
* A
IN
INB
GAIN
V
-) R
’s gain is complementary to that of
+ (I
G
12
F
is less than 2.5V below V+ and
AND V
B
OL
+ A
INB
B
-)*R
- dominates up to gains of
IN
is the transimpedance of
V
- is the input impedance
gain. The gain control
F
G
R
.
B
-, adds an error not
IN
)/R
OL
INB
and
G
flows
EL4095
back out through V
amount, the bridge becomes a high impedance as some of
the diodes shut off, and the V
from the nominal 5.5kΩ to over 500kΩ. This is the condition
of gain control overdrive. The actual circuit produces a much
sharper overdrive characteristics than does the simple diode
bridge of this representation.
The gain input has a 20MHz -3dB bandwidth and 17ns
risetime for inputs to ±0.45V. When the gain control voltage
exceeds the 0% or 100% values, a 70ns overdrive recovery
transient will occur when it is brought back to linear range. If
quicker gain overdrive response is required, the Force
control inputs of the EL4095 can be used.
Force Inputs
The Force inputs completely override the V
establish maximum attainable 0% and 100% gains for the
two input channels. They are activated by a TTL logic low on
either of the FORCE pins, and perform the analog switching
very quickly and cleanly. FORCEA causes 100% gain on the
A channel and 0% on the B channel. FORCEB does the
reverse, but there is no defined output state when FORCEA
and FORCEB are simultaneously asserted.
The Force inputs do not incur recovery time penalties, and
make ideal multiplexing controls. A typical use would be text
overlay, where the A channel is a video input and the B
channel is digitally created text data. The FORCEA input is
set low normally to pass the video signal, but released to
display overlay data. The gain control can be used to set the
intensity of the digital overlay.
Other Applications Circuits
The EL4095 can also be used as a variable-gain single input
amplifier. If a 0% lower gain extreme is required, one
channel’s input should simply be grounded. Feedback
resistors must be connected to both -V
EL4095 will not give the expected gain range when a
channel is left unconnected.
This circuit gives +0.5 to +2.0 gain range, and is useful as a
signal leveller, where a constant output level is regulated
from a range of input amplitudes:
G
. When gain control inputs exceed this
G
impedance rises sharply
IN
terminals; the
GAIN
setting and