ADA4898-1YRDZ-R7 datasheet(14/16 Pages) AD

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ADA4898-1

Rev. 0 | Page 14 of 16

APPLICATIONS INFORMATION

HIGHER FEEDBACK GAIN OPERATION

The ADA4898-1 schematic for the noninverting gain

configuration is nearly a textbook example (see Figure 42).

The only exception is the feedback capacitor in parallel with

the feedback resistor, RF, but this capacitor is recommended

only when using a large RF value (>300 Ω). Figure 43 shows the

difference between using a 100 Ω resistor and a 1 kΩ resistor.

Due to the high input capacitance in the ADA4898-1 when

using a higher feedback resistor, more peaking appears in the

closed-loop gain. Using the lower feedback resistor resolves this

issue; however, when running at higher supplies (±15 V) with

100 Ω RF, the system draws a lot of extra current into the

feedback network. To avoid this problem, a higher feedback

resistor can be used with a feedback capacitor in parallel. Figure 43

shows the effect of placing a feedback capacitor in parallel with

a larger RF. In this gain of 2 configuration, RF = RG = 1 kΩ and

CF = 2.7 pF. When using CF, the peaking drops from 6 dB to less

then 2 dB.

VIN

VOUT

0.1µF

10µF

+VS

–VS

10µF

Figure 42. Noninverting Gain Schematic

–15

–12

–9

–6

–3

FREQUENCY (Hz)

100k

10M

100M

RF = 1kΩ, CF = 2.7pF

RF = 1kΩ

RF = 100Ω

G = +2

RL = 1kΩ

VS = ±15V

Figure 43. Small Signal Frequency Response for

Various Feedback Impedances

RECOMMENDED VALUES FOR VARIOUS GAINS

Table 6 provides a useful reference for determining various gains

and associated performance. Resistor RF is set to 100 Ω for gains

greater than 1. A low feedback RF resistor value reduces peaking

and minimizes the contribution to the overall noise performance

of the amplifier.

Table 6. Various Gains and Recommended Resistor Values Associated (Conditions: VS = ±5 V, TA = 25°C, RL = 1 kΩ, RT = 49.9 Ω)

Gain

RF (Ω)

RG (Ω)

−3 dB SS BW (MHz),

VOUT = 100 mV p-p

Slew Rate (V/μs),

VOUT = 2 V Step

ADA4898-1 Voltage

Noise (nV/√Hz), RTO

Total System Noise

(nV/√Hz), RTO

0.9

1.29

100

1.8

3.16

100

24.9

4.5

7.07

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