LMH6714 Datasheet, PDF (11/30 Pages) National Semiconductor (TI) – Wideband Video Op Amp; Single, Single with Shutdown and Quad

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LMH6714, LMH6720
LMH6722, LMH6722-Q1
SNOSA39G – NOVEMBER 2002 – REVISED APRIL 2013
In the Figure 32 and Figure 33 charts, the recommended value of R_F is depicted by the solid line, which starts
high, decreases to 200Ω and begins increasing again. The reason that a higher R_F is required at higher gains is
the need to keep R_G 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 180Ω and 50Ω is a
practical lower limit for R_G. Due to the limitations on R_G the LMH6714/LMH6720/LMH6722 begins to operate in a
gain bandwidth limited fashion for gains of ±5V/V or greater.
450
400
350
300
250
200
150
100
50
0
12
3 4 5 67
GAIN (-V/V)
8 9 10
Figure 33. R_F 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, V_CC/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