LMH6704MF datasheet(8/12 Pages) NSC

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Application Information (Continued)

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 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:

For example, if an A

V = +2 configuration is used with a

source impedance of 37.5

Ω (parallel combination of 75Ω

source and 75

Ω termination impedances), where “I

BN”is

18.5pA/

and the output referred voltage noise (exclud-

ing non-inverting input noise current) can be found in Table 1

below. The total noise (E

O) at the output can be calculated

as:

TABLE 1. Measured Output Noise Voltage

Gain (A

Output Referred Voltage Noise

(nV/

), excluding non-inverting

noise current

10.5

9.3

+1, alternate

method shown in

Figure 4

10.5

-1

10.5

Note: f

≥ 100 kHz

ENABLE/DISABLE

The LMH6704 has a TTL logic compatible disable function.

Apply a logic low (<.8V) to the DS pin and the LMH6704 is

disabled. Apply a logic high (>2.0V), or let the pin float and

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

+/2 with single supply biasing). Doing so

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

20103632

FIGURE 5. Unity Gain Frequency Response

20103612

FIGURE 6. DIS Pin Simplified Schematic

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