12
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
VOUT, Offset = VOS* AV + (IB-)*RF.
The input offset voltage scales with fed-back gain, but the
bias current into the negative input, IB-, adds an error not
dependent on gain. Generally, IB- dominates up to gains of
about seven.
The fractional gain error is given by
EGAIN = (RF + AV*RIN -) RF + AV RIN)/ROL
The gain error is about 0.3% for a gain of one, and increases
only slowly f or increasing gain. RIN- is the input impedance
of the input stage buffer, and ROL is the transimpedance of
the amplifier, 80kΩ and 350kΩ respectively.
Gain Control Inputs
The gain control inputs are differential and may be biased at
any v oltage as long as V GAIN is less than 2.5V below V+ and
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 VINA input at +0.5V of gain control voltage, and 0% at
-0.5V of gain control. VINB’ s gain is complementary to that of
VINA; +0.5V of gain control sets 0% gain at VINB and
-0.5V gain control sets 100% VINB gain. The gain control
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 necessar y 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 G flows
back out through VG. When gain control inputs exceed this
amount, the br idge becomes a high impedance as some of
the diodes shut off, and the VG impedance rises sharply
from the nominal 5.5kΩ to over 500kΩ. This is the condition
of gain control overdrive . The actual circuit produces a much
sharper ov erdrive 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 v oltage
exceeds the 0% or 100% values, a 70ns overdrive recovery
transient will occur when it is brought back to linear range. If
quick er gain o verdrive response is required, the F orce
control inputs of the EL4095 can be used.
Force Inputs
The Force inputs completely override the VGAIN setting and
establish maximum attainable 0% and 100% gains for the
two input channels. They are activ ated b y a TTL logic low on
either of the FORCE pins, and perform th e 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 creat ed text data. The FORCEA input is
set low normally to pass the video signal, but released to
display o v erlay data. The gain control can be used to set the
intensity of the digi tal 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. Feedbac k
resistors must be connected to both -VIN terminals; the
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 le vel is regulated
from a range of input amplitudes:
FIGURE 2. REPRESENTATION OF GAIN CONTROL
INPUTS VG AND VG
EL4095