LMH6601_09 Datasheet, PDF (20/28 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

TABLE 1. Finding Figure 7 External Resistor Values by Iteration
Estimate
R_G (Ω)
1k
820
620
390
560
Calculated
(from Equation 2)
R3 (Ω)
1.69k
1.56k
1.37k
239
1.30k
Equation 1 LHS
Calculated
0.988
1.15
1.45
4.18
1.59
Comment
(Compare Equation 1 LHS Calculated to RHS)
Increase Equation 1 LHS by reducing R_G
Increase Equation 1 LHS by reducing R_G
Increase Equation 1 LHS by reducing R_G
Reduce Equation 1 LHS by increasing R_G
Close to target value of 1.5V/V for Equation 1
The final set of values for R_G and R₃ in Table 1 are values
which will result in the proper gain and correct video levels
(0V to 1V) at the output (V_LOAD).
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, R₁ and R₂ simply set the
input to the center of the input linear range while C_IN AC cou-
ples the video onto the op amp’s input. The op amp is set for
a closed loop gain of 2 with R_F and R_G. C_G is there to make
sure the device output is also biased at mid-supply. Because
of the DC bias at the output, the load needs to be AC coupled
as well through C_O. Some applications implement a small
valued ceramic capacitor (not shown) in parallel with C_O
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_G2 is intended to boost the high frequency gain in order to
improve the video frequency response. This value is to be set
and trimmed on the board to meet the application’s specific
system requirements.
FIGURE 8. AC Coupled Video Amplifier/Driver
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SAG COMPENSATION
The capacitors shown in Figure 8 (except C_G2), and especially
C_O, are the large electrolytic type which are considerably
costly and take up valuable real estate on the board. It is pos-
sible to reduce the value of the output coupling capacitor,
C_O, which is the largest of all, by using what is called SAG
compensation. SAG refers to what the output video experi-
ences due to the low frequency video content it contains
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:
f_low_frequency (−3 dB)= 1/ (2*pi* 75*2(Ω) * C_O)
= ∼ 4.82 Hz For C_O = 220 μF
(3)
A possible implementation of the SAG compensation is
shown in Figure 9.
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