LMH6601_14 Datasheet, PDF (23/37 Pages) National Semiconductor (TI) – LMH6601/LMH6601Q 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

LMH6601, LMH6601-Q1
www.ti.com
SNOSAK9E – JUNE 2006 – REVISED MARCH 2013
Estimate
R_G (Ω)
1k
820
620
390
560
Table 1. Finding Figure 54 External Resistor Values by Iteration
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 respects. As can be seen in Figure 55, R₁ and R₂ simply set the input to the center of the input linear
range while C_IN AC couples 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 Electrolytic 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.
5V
V_IN
R_IN
75:
C_IN
0.47 PF
R₁
510 k:
R₂
510 k:
5V
+
LMH6601
- ^U1
R_O
75:
CABLE
C_O
220 PF
V_OUT
R_L
75:
R_F
620:
C_G2
R_G
620:
C_G ⁺
47 PF
Figure 55. AC Coupled Video Amplifier/Driver
SAG COMPENSATION
The capacitors shown in Figure 55 (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 possible 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 experiences 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
(4)
A possible implementation of the SAG compensation is shown in Figure 56.
Copyright © 2006–2013, Texas Instruments Incorporated
Submit Documentation Feedback
23
Product Folder Links: LMH6601 LMH6601-Q1