EL4095CS datasheet(12/19 Pages) INTERSIL

12 / 19 page

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 for increasing gain. RIN- is the input impedance

of the input stage buffer, and ROL is the transimpedance of

the amplifier, 80k

Ω and 350kΩ respectively.

Gain Control Inputs

The gain control inputs are differential and may be biased at

any voltage as long as VGAIN is less than 2.5V below V+ and

3V above V-. The differential input impedance is 5.5k

Ω, and

a common-mode impedance is more than 500k

Ω. 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 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 VG flows

back out through VG. When gain control inputs exceed this

amount, the bridge becomes a high impedance as some of

the diodes shut off, and the VG impedance rises sharply

from the nominal 5.5k

Ω 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 VGAIN setting and

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 -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 level is regulated

from a range of input amplitudes:

FIGURE 2. REPRESENTATION OF GAIN CONTROL

INPUTS VG AND VG

EL4095

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