AD9617 datasheet(8/10 Pages) AD

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AD9617

–8–

REV. B

AC GAIN CHARACTERISTICS

Closed-loop bandwidth at high frequencies is determined pri-

marily by the roll-off of T(s). But circuit layout is critical to

minimize external parasitics which can degrade performance by

causing premature peaking and/or reduced bandwidth.

The inverting and noninverting dynamic characteristics are similar.

When driving the noninverting input, the inverting input capaci-

tance (CI) will cause the noninverting closed-loop bandwidth to

be higher than the inverting bandwidth for gains less than two

(2). In the remaining cases, inverting and noninverting responses

are nearly identical.

For best overall dynamic performance, the value of the feedback

resistor (RF) should be 400 ohms. Although bandwidth reduces

as closed-loop gain increases, the change is relatively small due

to low equivalent series input impedance, ZS. (See typical

performance charts.) The simplified equations governing the

device’s dynamic performance are shown below.

Closed-Loop Gain vs. Frequency:

(noninverting operation)

V I

≈

s 1













(6)

where:

= RF

× C

C = 0.9 ns (RF = 400 Ω)

Slew Rate

≈

∆V

RF KCC

× e−τ/RFKCC

(7)

where: K = 1 +

Increasing Bandwidth at Low Gains

By reducing RF, wider bandwidth and faster pulse response can

be attained beyond the specified values, although increased

overshoot, settling time and possible ac peaking may result. As a

rule of thumb, overshoot and bandwidth will increase by 1%

and 8%, respectively, for a 5% reduction in RF at gains of ± 10.

Lower gains will increase these sensitivities.

Equations 6 and 7 are simplified and do not accurately model

the second order (open loop) frequency response term which is

the primary contributor to overshoot, peaking and nonlinear

bandwidth expansion. (See Open Loop Bode Plots.) The user

should exercise caution when selecting RF values much lower

than 400

Ω. Note that a feedback resistor must be used in all

situations, including those in which the amplifier is used in a

noninverting unity gain configuration.

Increasing Bandwidth at High Gains

Closed loop bandwidth can be extended at high closed loop gain

by reducing RF. Bandwidth reduction is a result of the feedback

current being split between RS and RI. As the gain increases (for

a given RF), more feedback current is shunted through RI, which

reduces closed loop bandwidth (see Equation 6). To maintain

specified BW, the following equations can be used to approxi-

mate RF and RI for any gain from ± l to ± 15.

RF = 424 ± 8 G

(8)

(+ for inverting and – for noninverting)

RI ≈

424

− 8 G

−1

(noninverting)

(9)

RI ≈

424

+ 8 G

−1

(inverting)

(10)

G = Closed Loop Gain.

Bandwidth Reduction

The closed loop bandwidth can be reduced by increasing RF.

Equations 6 and 7 can be used to determine the closed loop

bandwidth for any value RF. Do not connect a feedback capaci-

tor across RF, as this will degrade dynamic performance and

possibly induce oscillation.

DC Precision and Noise

Output offset voltage results from both input bias currents and

input offset voltage. These input errors are multiplied by the

noise gain term (1 + RF/RI) and algebraically summed at the

output as shown below.

VO = V IO × 1 +









± IBn × R

N ×









± IBi × R

(11)

Since the inputs are asymmetrical, IBi and IBn do not correlate.

Canceling their output effects by making RN = RF RI will not

reduce output offset errors, as it would for voltage feedback

amplifiers. Typically, IBn is 5

µA and V

IO is +0.5 mV (I sigma =

0.3 mV), which means that the dc output error can be reduced

by making RN ≈ 100 Ω. Note that the offset drift will not change

significantly because the IBn TC is relatively small. (See specifi-

cation table.)

IBi

IBn

VOUT

Figure 15. Output Offset Voltage

–55 C

–5

–10

25 C

125 C

0.5

1.0

–0.5

–1.0

IBi

VIO

IBn

Figure 16. DC Accuracy

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