Datasheet

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SUGGESTED VALUE OF R

(:)

GAIN (V/V)

LMH6570

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SNCS104C –APRIL 2005–REVISED MAY 2013

FEEDBACK RESISTOR SELECTION

Figure 27. Suggested R

vs. Gain

The LMH6570 has a current feedback output buffer with gain determined by external feedback (R

) and gain set

) resistors. With current feedback amplifiers, the closed loop frequency response is a function of R

. For a

gain of 2 V/V, the recommended value of R

is 576Ω. For other gains see Figure 6. Generally, lowering R

from

the recommended value will peak the frequency response and extend the bandwidth while increasing the value

of R

will cause the frequency response to roll off faster. Reducing the value of R

too far below the

recommended value will cause overshoot, ringing and, eventually, oscillation.

Since all applications are slightly different it is worth some experimentation to find the optimal R

for a given

circuit. For more information see Application Note OA-13 which describes the relationship between R

and

closed-loop frequency response for current feedback operational amplifiers. The impedance looking into pin 8 is

approximately 20Ω. This allows for good bandwidth at gains up to 10 V/V. When used with gains over 10 V/V,

the LMH6570 will exhibit a “gain bandwidth product” similar to a typical voltage feedback amplifier. For gains of

over 10 V/V consider selecting a high performance video amplifier like the LMH6720 to provide additional gain.

MULTIPLEXER EXPANSION

With the SHUTDOWN pin putting the output stage into a high impedance state, several LMH6570’s can be tied

together to form a larger input MUX. However, there is a loading effect on the active output caused by the

unselected devices. The circuit in Figure 28 shows how to compensate for this effect. For the 8:1 MUX function

shown in Figure 28 below the gain error would be about 0.7% or −0.06dB. In the circuit in Figure 28, resistor

ratios have been adjusted to compensate for this gain error. By adjusting the gain of each multiplexer circuit the

error can be reduced to the tolerance of the resistors used (1% in this example).

Product Folder Links: LMH6570