EL4095CS datasheet(12/24 Pages) ELANTEC

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EL4095C

Video Gain ControlFaderMultiplexer

Applications Information

Contd

Stray capacitance at each bVIN terminal should

absolutely be minimized especially in a positive-

gain mode or peaking will occur Similarly the

load capacitance should be minimized If more

than 25 pF of load capacitance must be driven a

load resistor from 100

X to 400X can be added in

parallel with the output to reduce peaking but

some bandwidth degradation may occur

‘‘snubber’’ load can alternatively be used This is

a resistor in series with a capacitor to ground

150

X and 100 pF being typical values The ad-

vantage of a snubber is that it does not draw DC

load current A small series resistor low tens of

ohms can also be used to isolate reactive loads

Distortion

The signal voltage range of the aVIN terminals

is within 35V of either supply rail

One must also consider the range of error cur-

rents that will be handled by the bVIN termi-

nals Since the bVIN of a CFA is the output of a

buffer which replicates the voltage at aVIN er-

ror currents will flow into the bVIN terminal

When an input channel has 100% gain assigned

to it only a small error current flows into its neg-

ative input when low gain is assigned to the

channel the output does not respond to the chan-

nel’s signal and large error currents flow

Here are a few idealized examples based on a

gain of a1 for channels A and B and RF e 1kX

for different gain settings

Gain

VINA

VINB

I(bVINA)I (bVINB)VOUT

100%

1 mA

75%

250

750

075V

50%

500

05V

25%

750

250

025V

1mA

Thus either bVIN can receive up to 1 mA error

current for 1V of input signal and 1 k

X feedback

resistors The maximum error current is 3 mA for

the EL4095 but 2 mA is more realistic The ma-

jor contributor of distortion is the magnitude of

error currents even more important than loading

effects The performance curves show distortion

versus input amplitude for different gains

If maximum bandwidth is not required distor-

tion can be reduced greatly (and signal voltage

range enlarged) by increasing the value of RF

and any associated gain-setting resistor

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 e VOS AV a (IBb) RF

The input offset voltage scales with fed-back

gain but the bias current into the negative input

adds an error not dependent on gain Gener-

ally IB

b dominates up to gains of about seven

The fractional gain error is given by

EGAIN e (RF a AV RIN b)RF

AV RIN)ROL

The gain error is about 03% for a gain of one

and increases only slowly for increasing gain

RINb is the input impedance of the input stage

buffer and ROL is the transimpedance of the am-

plifier 80 k

X and 350 kX respectively

Gain Control Inputs

The gain control inputs are differential and may

be biased at any voltage as long as VGAIN is less

than 25V below Va and 3V above Vb The dif-

ferential input impedance is 55 k

X and a com-

mon-mode impedance is more than 500 k

X 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

05V of gain control voltage and 0% at b05V

of gain control VINB’s gain is complementary to

that of VINA a05V of gain control sets 0% gain

at VINB and b05V gain control sets 100% VINB

gain The gain control does not have a complete-

ly abrupt transition at the 0% and 100% points

There is about 10 mV 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 30 mV This would set the gain control

voltage range as b0565 mV to a0565V

30 mV beyond the maximum guaranteed 0% to

100% range

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