AD744JCHIPS datasheet(12/15 Pages) AD

12 / 15 page

REV.

–12–

AD744

In either case, the capacitance CX causes the system to go from

a one-pole to a two-pole response; this additional pole increases

settling time by introducing peaking or ringing in the op amp’s

output. If the value of CX can be estimated with reasonable accu-

racy, Equation 2 can be used to choose the correct value for

a small capacitor, CL, which will optimize amplifier response. If

the value of CX is not known, CL should be a variable capacitor.

As an aid to the designer, the optimum value of CL for one spe-

cific amplifier connection can be determined from the graph of

Figure 41. This graph has been produced for the case where the

AD744 is connected as in Figures 39 and 40 with a practical

minimum value for CSTRAY of 2 pF and a total CX value of 7.5 pF.

The approximate value of CL can be determined for almost any

application by solving Equation 2. For example, the AD565/

AD744 circuit of Figure 34 constrains all the variables of Equa-

tion 2 (GN = 3.25, R = 10 k

Ω, FO = 13 MHz, and CX = 32.5 pF)

Therefore, under these conditions, CL= 10.5 pF.

VALUE OF RESISTOR –

100

10k

100k

IN THIS REGION

CLEAD = 0pF

GN = 1 TO

GN = 1.5

GN = 2

GN = 3

GN = 1

Figure 41. Practical Values of CL vs. Resistance of R

for Various Amplifier Noise Gains

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