EL400

FN7156 Rev 0.00

Page 7 of 9

Sep 20, 2002

closed-loop gain the bandwidth may be increased by

decreasing the feedback resistor and, conversely, the

bandwidth may be decreased by increasing the feedback

resistor.

Reducing the feedback resistor too much will result in

overshoot and ringing, and eventually oscillations. Increasing

the feedback resistor results in a lower -3dB frequency.

Attenuation at high frequency is limited by a zero in the closed-

loop transfer function which results from stray capacitance

between the inverting input and ground. Consequently, it is

very important to keep stray capacitance to a minimum at the

inverting input.

Differential Gain/Phase

An industry-standard method of measuring the distortion of a

video component is to measure the amount of differential gain

and phase error it introduces. To measure these, a 40 IREPP

reference signal is applied to the device with 0V DC offset

(0IRE) at 3.58MHz for NTSC, 4.43MHz for PAL, and 30MHz for

HDTV. A second measurement is then made with a 0.714V DC

offset (100IRE). Differential Gain is a measure of the change in

amplitude of the sine wave, and is measured in percent.

Differential Phase is a measure of the change in phase, and is

measured in degrees. Typically, the maximum positive and

negative deviations are summed to give peak values.

In general, a back terminated cable (75

 in series at the drive

end and 75

 to ground at the receiving end) is preferred since

the impedance match at both ends will absorb any reflections.

However, when double-termination is used, the received signal

is reduced by half; therefore a gain of 2 configuration is

typically used to compensate for the attenuation. In a gain of 2

configuration, with output swing of 2VPP, with each back-

terminated load at 150

. The EL400 is capable of driving up to

4 back-terminated loads with excellent video performance.

Please refer to the typical curves for more information on video

performance with respect to frequency, gain, and loading.

Capacitive Feedback

The EL400 relies on its feedback resistor for proper

compensation. A reduction of the impedance of the feedback

element results in less stability, eventually resulting in

oscillation. Therefore, circuit implementations which have

capacitive feedback should not be used because of the

capacitor's impedance reduction with frequency. Similarly,

oscillations can occur when using the technique of placing a

capacitor in parallel with the feedback resistor to compensate

for shunt capacitances from the inverting input to ground.

Offset Adjustment Pin

Output offset voltage of the EL400 can be nulled by tying a 10k

potentiometer between +VS and -VS with the slider attached to

pin 1. A full-range variation of the voltage at pin 1 to ±5V

results in an offset voltage adjustment of at least ±10mV. For

best settling performance pin 1 should be bypassed to ground

with a ceramic capacitor located near to the package, even if

the offset voltage adjustment feature is not being used.

Printed Circuit Layout

As with any high frequency device, good PCB layout is

necessary for optimum performance. Ground plane

construction is a requirement, as is good power-supply and

Offset Adjust bypassing close to the package. The inverting

input is sensitive to stray capacitance, therefore connections at

the inverting input should be minimal, close to the package,

and constructed with as little coupling the ground plane as

possible.

Capacitance at the output node will reduce stability, eventually

resulting in peaking, and finally oscillation if the capacitance is

large enough. The design of the EL400 allows a larger

capacitive load than comparable products, yet there are

occasions when a series resistor before the capacitance may

be needed. Please refer to the graphs to determine the proper

resistor value needed.