LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 20

• Manufacturer Part Number: LMH6601MG
• Description: 250 MHz, 2.4V CMOS Op Amp with Shutdown
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
The final set of values for R
which will result in the proper gain and correct video levels
(0V to 1V) at the output (V
AC COUPLED VIDEO
Many monitors and displays accept AC coupled inputs. This
simplifies the amplification and buffering task in some re-
spects. As can be seen in Figure 8, R
input to the center of the input linear range while C
couples the video onto the op amp’s input. The op amp is set
for a closed loop gain of 2 with R
make sure the device output is also biased at mid-supply.
SAG COMPENSATION
The capacitors shown in Figure 8 (except C
cially C
ably costly and take up valuable real estate on the board. It
is possible to reduce the value of the output coupling capaci-
tor, C
SAG compensation. SAG refers to what the output video
experiences due to the low frequency video content it con-
tains which cannot adequately go through the output AC
Estimate
O
R
, which is the largest of all, by using what is called
O
G
820
620
390
560
1k
, are the large electrolytic type which are consider-
(Ω)
(from Equation (2))
LOAD
Calculated
G
TABLE 1. Finding Figure 7 External Resistor Values by Iteration
R3 (Ω)
1.69k
1.56k
1.37k
1.30k
and R
239
).
F
and R
3
1
in Table 1 are values
and R
FIGURE 8. AC Coupled Video Amplifier/Driver
(Continued)
G
. C
2
G2
simply set the
G
), and espe-
is there to
Equation (1) LHS
IN
Calculated
AC
0.988
1.15
1.45
4.18
1.59
20
R
Because of the DC bias at the output, the load needs to be
AC coupled as well through C
ment a small valued ceramic capacitor (not shown) in paral-
lel with C
ceramic capacitor will tend to shunt the inductive behavior of
the Electrolytic capacitor at higher frequencies for an im-
proved overall low impedance output.
C
improve the video frequency response. This value is to be
set and trimmed on the board to meet the application’s
specific system requirements.
coupling scheme due to the low frequency limit of this circuit.
The −3 dB low frequency limit of the output circuit is given
by:
A possible implementation of the SAG compensation is
shown in Figure 9.
F
G2
= 620Ω
is intended to boost the high frequency gain in order to
f_low_frequency (−3 dB)= 1/ (2*pi* 75*2(Ω) * C
O
which is electrolytic. The reason for this is that the
Increase Equation (1) LHS by reducing R
Increase Equation (1) LHS by reducing R
Increase Equation (1) LHS by reducing R
Reduce Equation (1) LHS by increasing R
Close to target value of 1.5V/V for Equation (1)
(Compare Equation (1) LHS Calculated to
= ∼ 4.82 Hz For C
Comment
O
RHS)
20136449
. Some applications imple-
O
= 220 µF
O
G
G
G
G
)
(3)