LMH6704MA National Semiconductor, LMH6704MA Datasheet - Page 8

• Manufacturer Part Number: LMH6704MA
• Description: BUFFER, PROGRAMMABLE, 6704, SOIC8
• Manufacturer: National Semiconductor

Specifications of LMH6704MA

• No. Of Amplifiers: 1
• Gain Bandwidth: 650MHz
• Slew Rate: 3000V/µs
• Supply Voltage Range: ± 4V To ± 6V
• Output Current: 90mA
• Amplifier Output: Single Ended
• Operating Temperature Range: -40°C To +85°C
• Rohs Compliant: Yes

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Application Section
UNITY GAIN COMPENSATION
With a current feedback PGB like the LMH6704, the feed-
back 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 opera-
tional amplifiers the feedback resistor, R
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 4. This circuit modifies the noise gain
of the amplifier to eliminate the peaking associated with
using the circuit shown in Figure 2. The frequency response
is shown in Figure 5. The decreased peaking does come at
a price as the output referred voltage noise density in-
creases by a factor of 1.1.
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 speci-
fications are used to calculate the total voltage noise pro-
duced 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 com-
ponents. 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 1. For
more information on calculating noise in current feedback
amplifiers see Application Notes OA-12 and AN104 available
at www.national.com.
The total noise voltage at the output can be calculated using
the following formula:
FIGURE 5. Unity Gain Frequency Response
(Continued)
F
, is external and its
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For example, if an A
source impedance of 37.5Ω (parallel combination of 75Ω
source and 75Ω termination impedances), where “I
18.5pA/
ing non-inverting input noise current) can be found in Table 1
below. The total noise (E
as:
TABLE 1. Measured Output Noise
Voltage
Note: f ≥ 100 kHz
ENABLE/DISABLE
The LMH6704 has a TTL logic compatible disable function.
Apply a logic low (
disabled. Apply a logic high (
the LMH6704 is enabled. Voltage, not current, at the Disable
pin (DS) determines the enable/disable state. Care must be
exercised to prevent the disable pin voltage from going more
than .8V below the midpoint of the supply voltages (0V with
split supplies, V
could cause transistor Q1 to Zener resulting in damage to
the disable circuit (See Figure 6 or the simplified internal
schematic diagram). The core amplifier is unaffected by this,
but the disable operation could become permanently slower
as a result.
method shown in
+1, alternate
Gain (A
Figure 4
FIGURE 6. DIS Pin Simplified Schematic
+2
+1
-1
V
and the output referred voltage noise (exclud-
)
+
/2 with single supply biasing). Doing so
<
.8V) to the DS pin and the LMH6704 is
Output Referred Voltage Noise
(nV/
noise current
V
= +2 configuration is used with a
O
) at the output can be calculated
>
2.0V), or let the pin float and
), excluding non-inverting
10.5
10.5
10.5
9.3
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” is