LMH6720MA datasheet(11/29 Pages) TI1

11 / 29 page

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GAIN (-V/V)

LMH6714, LMH6720

LMH6722, LMH6722-Q1

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SNOSA39G – NOVEMBER 2002 – REVISED APRIL 2013

In the Figure 32 and Figure 33 charts, the recommended value of RF is depicted by the solid line, which starts

high, decreases to 200

Ω and begins increasing again. The reason that a higher RF is required at higher gains is

the need to keep RG from decreasing too far below the output impedance of the input buffer. For the

LMH6714/LMH6720/LMH6722 the output resistance of the input buffer is approximately 180

Ω and 50Ω is a

practical lower limit for RG. Due to the limitations on RG the LMH6714/LMH6720/LMH6722 begins to operate in a

gain bandwidth limited fashion for gains of ±5V/V or greater.

Figure 33. RF vs. Inverting Gain

ACTIVE FILTERS

When using any current feedback Operational Amplifier as an active filter it is important to be very careful when

using reactive components in the feedback loop. Anything that reduces the impedance of the negative feedback,

especially at higher frequencies, will almost certainly cause stability problems. Likewise capacitance on the

inverting input needs to be avoided. See Application Notes OA-07 (SNOA365) and OA-26 (SNOA387) for more

information on Active Filter applications for Current Feedback Op Amps.

Figure 34. Enable/Disable Operation

ENABLE/DISABLE OPERATION USING ±5V SUPPLIES (LMH6720 ONLY)

The LMH6720 has a TTL logic compatible disable function. Apply a logic low (<.8V) to the DS pin and the

LMH6720 is disabled. Apply a logic high (>2.0V), or let the pin float and the LMH6720 is enabled. Voltage, not

current, at the Disable pin 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, VCC/2 with

single supplies) doing so could cause transistor Q1 to Zener resulting in damage to the disable circuit. The core

amplifier is unaffected by this, but disable operation could become slower as a result.

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Product Folder Links: LMH6714 LMH6720 LMH6722 LMH6722-Q1

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