HA5024_06 Datasheet, PDF (7/16 Pages) Intersil Corporation – Quad 125MHz Video Current Feedback Amplifier with Disable

HA5024
Application Information
Optimum Feedback Resistor
The plots of inverting and non-inverting frequency response,
see Figure 11 and Figure 12 in the Typical Performance
Curves section, illustrate the performance of the HA5024 in
various closed loop gain configurations. Although the
bandwidth dependency on closed loop gain isn’t as severe
as that of a voltage feedback amplifier, there can be an
appreciable decrease in bandwidth at higher gains. This
decrease may be minimized by taking advantage of the
current feedback amplifier’s unique relationship between
bandwidth and R_F. All current feedback amplifiers require a
feedback resistor, even for unity gain applications, and R_F,
in conjunction with the internal compensation capacitor, sets
the dominant pole of the frequency response. Thus, the
amplifier’s bandwidth is inversely proportional to R_F. The
HA5024 design is optimized for a 1000Ω R_F at a gain of +1.
Decreasing R_F in a unity gain application decreases stability,
resulting in excessive peaking and overshoot. At higher
gains the amplifier is more stable, so R_F can be decreased
in a trade-off of stability for bandwidth.
The table below lists recommended R_F values for various
gains, and the expected bandwidth.
GAIN (A_CL)
-1
+1
+2
+5
+10
-10
R_F (Ω)
750
1000
681
1000
383
750
BANDWIDTH (MHz)
100
125
95
52
65
22
PC Board Layout
The frequency response of this amplifier depends greatly on
the amount of care taken in designing the PC board. The
use of low inductance components such as chip resistors
and chip capacitors is strongly recommended. If leaded
components are used the leads must be kept short
especially for the power supply decoupling components and
those components connected to the inverting input.
Attention must be given to decoupling the power supplies. A
large value (10µF) tantalum or electrolytic capacitor in
parallel with a small value (0.1µF) chip capacitor works well
in most cases.
A ground plane is strongly recommended to control noise.
Care must also be taken to minimize the capacitance to
ground seen by the amplifier’s inverting input (-IN). The
larger this capacitance, the worse the gain peaking, resulting
in pulse overshoot and possible instability. It is
recommended that the ground plane be removed under
traces connected to -IN, and that connections to -IN be kept
as short as possible to minimize the capacitance from this
node to ground.
7
Driving Capacitive Loads
Capacitive loads will degrade the amplifier’s phase margin
resulting in frequency response peaking and possible
oscillations. In most cases the oscillation can be avoided by
placing an isolation resistor (R) in series with the output as
shown in Figure 6.
100Ω
V_IN
+_-
R
V_OUT
R_T
C_L
R_F
R_I
FIGURE 6. PLACEMENT OF THE OUTPUT ISOLATION
RESISTOR, R
The selection criteria for the isolation resister is highly
dependent on the load, but 27Ω has been determined to be
a good starting value.
Power Dissipation Considerations
Due to the high supply current inherent in quad amplifiers, care
must be taken to insure that the maximum junction temperature
(T_J, see Absolute Maximum Ratings) is not exceeded. Figure 7
shows the maximum ambient temperature versus supply
voltage for the available package styles (Plastic DIP, SOIC). At
±5V_DC quiescent operation both package styles may be
operated over the full industrial range of -40°C to 85°C. It is
recommended that thermal calculations, which take into
account output power, be performed by the designer.
130
120
110
100
PDIP
90
80
70
SOIC
60
50
5
7
9
11
13
15
SUPPLY VOLTAGE (±V)
FIGURE 7. MAXIMUM OPERATING AMBIENT TEMPERA-
TURE vs SUPPLY VOLTAGE
Enable/Disable Function
When enabled the amplifier functions as a normal current
feedback amplifier with all of the data in the electrical
specifications table being valid and applicable. When
disabled the amplifier output assumes a true high
impedance state and the supply current is reduced
significantly.
3550.6
February 8, 2006