LMH6704 Datasheet, PDF (9/20 Pages) National Semiconductor (TI) – 650 MHz Progammable Gain Buffer with Disable

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LMH6704
SNOSAD0C – FEBRUARY 2005 – REVISED MARCH 2013
V_IN
R_IN
+5V _{6.8 µF}
.01 µF
C_SS
0.1 µF
3
6 ^CPOS
+
1
LMH6704
-
4 ₂ CNEG
.01 µF
V_OUT
6.8 µF
-5V
Figure 20. Alternate Unity Gain Configuration
UNITY GAIN COMPENSATION
With a current feedback Selectable Gain Buffer like the LMH6704, the feedback resistor is a compromise
between the value needed for stability at unity gain and the optimized value needed at a gain of two. In standard
open-loop current feedback operational amplifiers the feedback resistor, R_F, is external and its value can be
adjusted to match the required gain. Since the feedback resistor is integrated in the LMH6704, it is not possible
to adjust it’s value. However, we can employ the circuit shown in Figure 20. This circuit modifies the noise gain of
the amplifier to eliminate the peaking associated with using the circuit shown in Figure 18. The frequency
response is shown in Figure 21. The decreased peaking does come at a price as the output referred voltage
noise density increases by a factor of 1.1.
4
3 ^{STANDARD CIRCUIT}
(FIGURE 2)
2
1
0
-1
ALTERNATE CIRCUIT
-2
(FIGURE 4)
-3
-4
-5
-6
1
10
100
FREQUENCY (MHz)
1000
Figure 21. Unity Gain Frequency Response
OUTPUT VOLTAGE NOISE
Open-loop operational amplifiers specify three input referred noise parameters: input voltage noise, non-inverting
input current noise, and inverting input current noise. These specifications are used to calculate the total voltage
noise produced at the output of the amplifier. The LMH6704 is a closed loop amplifier with internal resistors, thus
only the non-inverting input current noise flows through external components. All other noise sources are internal
to the part. There are four possible values for the noise at the output depending on the gain configuration as
shown in Table 2. For more information on calculating noise in current feedback amplifiers see Application Notes
OA-12 and AN104 available at www.ti.com.
The total noise voltage at the output can be calculated using Equation 1:
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