lmh6601mgx National Semiconductor Corporation, lmh6601mgx Datasheet - Page 20

• Manufacturer Part Number: lmh6601mgx
• Description: 250 Mhz, 2.4v Cmos Op Amp With Shutdown
• Manufacturer: National Semiconductor Corporation

www.national.com
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
ples the video onto the op amp’s input. The op amp is set for
a closed loop gain of 2 with R
sure the device output is also biased at mid-supply. Because
SAG COMPENSATION
The capacitors shown in Figure 8 (except C
C
costly and take up valuable real estate on the board. It is pos-
sible to reduce the value of the output coupling capacitor,
C
compensation. SAG refers to what the output video experi-
ences due to the low frequency video content it contains
O
O
, are the large electrolytic type which are considerably
, which is the largest of all, by using what is called SAG
Estimate
R
820
620
390
560
G
1k
(Ω)
(from Equation 2)
LOAD
Calculated
G
TABLE 1. Finding Figure 7 External Resistor Values by Iteration
F
R3 (Ω)
and R
1.30k
).
1.69k
1.56k
1.37k
239
and R
3
1
G
in Table 1 are values
and R
. C
FIGURE 8. AC Coupled Video Amplifier/Driver
G
G2
is there to make
), and especially
2
simply set the
IN
AC cou-
Equation 1 LHS
Calculated
0.988
1.15
1.45
4.18
1.59
20
of the DC bias at the output, the load needs to be AC coupled
as well through C
valued ceramic capacitor (not shown) in parallel with C
which is electrolytic. The reason for this is that the ceramic
capacitor will tend to shunt the inductive behavior of the Elec-
trolytic capacitor at higher frequencies for an improved 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.
which cannot adequately go through the output AC 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.
G2
f_low_frequency (−3 dB)= 1/ (2*pi* 75*2(Ω) * C
is intended to boost the high frequency gain in order to
(Compare Equation 1 LHS Calculated to RHS)
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
=
∼
4.82 Hz For C
O
. Some applications implement a small
Comment
O
20136449
= 220 μF
O
G
G
G
)
G
(3)
O