LMH6704MA/NOPB National Semiconductor, LMH6704MA/NOPB Datasheet - Page 8

• Manufacturer Part Number: LMH6704MA/NOPB
• Description: IC BUFFER PROG GAIN 650MHZ 8SOIC
• Manufacturer: National Semiconductor
• Series: LMH®r

Specifications of LMH6704MA/NOPB

• Amplifier Type: Buffer
• Number Of Circuits: 1
• Slew Rate: 3000 V/µs
• -3db Bandwidth: 650MHz
• Current - Input Bias: 5µA
• Voltage - Input Offset: 2000µV
• Current - Supply: 11.5mA
• Current - Output / Channel: 90mA
• Voltage - Supply, Single/dual (±): 8 V ~ 12 V, ±4 V ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• For Use With: LMH730227 - BOARD EVALUATION FOR SOIC PKGLMH730165 - BOARD EVALUATIONLMH730216 - BOARD EVAL HS MONO AMP SOT23
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Output Type: -
• Gain Bandwidth Product: -
• Other names: *LMH6704MA; *LMH6704MA/NOPB; LMH6704MA

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Application Information
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
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 4. This circuit modi-
fies 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 increases 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)
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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:
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.
Disabled, the LMH6704 inputs and output become high im-
pedances. While disabled the LMH6704 quiescent current is
method shown in
+1, alternate
Gain (A
Figure 4
TABLE 1. Measured Output Noise Voltage
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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