ADA4511-2ARMZ-R7 datasheet(21/28 Pages) AD

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Data Sheet

ADA4511-2

THEORY OF OPERATION

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Rev. A | 21 of 28

CAPACITIVE LOAD DRIVE CAPABILITY

The ADA4511-2 is stable with any capacitive load up to 1 nF. This

is accomplished by dynamically sensing the load-induced output

pole and adjusting the compensation at the internal gain node

of the amplifier. As the capacitive load increases, the bandwidth

decreases. The phase margin may increase or decrease with differ-

ent capacitive loads, so there may be overshoot in the transient

response for some capacitive loads (see Figure 59 and Figure 62).

Coaxial cable less than 1 nF can be driven directly, but, for best

pulse fidelity, the cable must be properly terminated by placing a

resistor of a value equal to the characteristic impedance of the

cable (for example 50 Ω) in series with the output. The other end

of the cable must be terminated with the same value resistor to

ground.

Figure 59 and Figure 62 show the overshoot of the ADA4511-2 with

various capacitive loads in unity gain and gain of −1 configurations.

To further improve the capacitive load drive of the ADA4511-2,

an isolation resistor (RISO) may be used in series with the output

to significantly reduce the overshoot and ringing to stabilize the

amplifier.

Table 6. Capacitive Load Drive at Various RISO

Capacitive Load

100 pF

1000 pF

RISO

0 Ω

24.9 Ω 49.9 Ω 0 Ω

24.9 Ω 49.9 Ω

Positive Overshoot

Percent

42%

38%

35%

68%

32%

18%

Negative Overshoot

Percent

43%

38%

30%

71%

29%

19%

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