ISL59451 Datasheet, PDF (14/17 Pages) Intersil Corporation – Triple 4:1 DC Restored Single Supply Video Multiplexing Amplifier

ISL59451
V_IN
50Ω
OR
75Ω
ISL59451
x2
*C_L
2.1pF
R_L
150Ω
V_OUT
*C_L Includes PCB trace capacitance
FIGURE 35A. TEST CIRCUIT WITH OPTIMAL OUTPUT LOAD
ISL59451 _LCRIT
V_IN
x2
50Ω
R_S
OR
C_L
C_S
R_L
75Ω
FIGURE 35B. INTER-STAGE APPLICATION CIRCUIT
V_IN
50Ω
ISL59451
L_{CRIT RS}
x2
118Ω
*C_L
2.1pF
86.6Ω
TEST
EQUIPMENT
50Ω
*C_L Includes PCB trace capacitance
FIGURE 35C. 150Ω TEST CIRCUIT WITH 50Ω LOAD
ISL59451
L_{CRIT RS}
V_IN
x2
50Ω
50Ω or 75Ω
OR
*C_L
75Ω
2.1pF
TEST
EQUIPMENT
50Ω/75Ω
*C_L Includes PCB trace capacitance
FIGURE 35D. BACKLOADED TEST CIRCUIT FOR 50Ω/75Ω
VIDEO CABLE APPLICATION
FIGURE 35. AC TEST CIRCUITS
AC Test Circuits
Figures 35A and 35B illustrate the optimum output load for
testing AC performance at 150Ω loads. Figure 35C
illustrates how to use the optimal 150Ω load for a 50Ω cable.
Figure 35D illustrates the optimum output load for 50Ω and
75Ω cable-driving.
Application Information
General
The ISL59451 triple 4:1 MUX video driver features single
+5V supply operation, high bandwidth and TTL/CMOS logic
compatible gain select (AV2) of x1 (0dB) or x2 (+6dB). It also
includes a DC restore function to set the blanking level of the
output signal.
The ISL59451 implements the video DC-restore function
with a high performance gain-adjustable video amplifier and
a nulling, sample-hold amplifier to establish a user defined
DC reference voltage at the video amplifier output. A
detailed description of the DC-restore function implemented
in the ISL59451 can be found in application note AN1089,
EL4089 and EL4390 DC-Restored Video Amplifier. The
ISL59451 performs the same function with the exception that
it is designed for single supply operation. Each of the three
output channels feature DC restore functionality.
Video Amplifier Operation
(refer to “Functional Block Diagram” on page 12)
The ISL59451 video amplifier (A1) is a voltage-feed, high
performance video amplifier designed for +5V operation.
The output stage is capable of swinging to within 15mV of
the negative rail. The differential input stage contains an
internal voltage reference that positions the non-inverting
input DC level (V1) to ~1.6V higher than ground. This offset
ensures that the amplifier input DC level is maintained within
the common mode input voltage range. The amplifier
non-inverting gain is given in Equation 1.
V_OUT
=
(V_I
N
+
–
1.6 V )
•
⎛
⎜1
⎝
+
R^-R----_G^-F--_{⎠⎟⎞}
(EQ. 1)
DC-Restore Amplifier
(refer to “Functional Block Diagram” on page 12)
The DC-restore circuit contains a voltage reference amplifier
and an analog switch function that closes the DC-restore
loop under control of the CLAMP logic input. The A2
amplifier output stage operates in a current-feedback mode
with a source capability of 860µA (Typ).
A logic “0” at the CLAMP input closes switch S1, which
closes the DC-restore loop. The video input AC coupling
capacitor, CX1, acts as a DC hold capacitor (through the
75Ω termination resistor) to average the current-source
output of amplifier A2. When the DC-restore loop has
reached equilibrium, the DC voltage stored on CX1 will be
the value required to set the voltages at A1 (V_OUT) and A2
(V_IN+) according to Equations 2 and 3:
V_OUT(DC) = V_REF
(EQ. 2)
V_IN+ = V_OUT(DC) + 1.6V
(EQ. 3)
14
FN6253.0
September 24, 2007