el400 Intersil Corporation, el400 Datasheet - Page 7

• Manufacturer Part Number: el400
• Description: 200mhz Current Feedback Amplifier
• Manufacturer: Intersil Corporation

the feedback resistor is 250:, which is the value used for
production testing. This value guarantees stability. For a
given 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
IRE
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
2V
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.
PP
PP
, with each back-terminated load at 150:. The EL400 is
reference signal is applied to the device with 0V DC
7
EL400
Offset Adjustment Pin
Output offset voltage of the EL400 can be nulled by tying a
10k potentiometer between +V
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.
S
and -V
S
with the slider