ADA4898-2YRDZ-R7 datasheet(15/20 Pages) AD

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Data Sheet

ADA4898-1/ADA4898-2

APPLICATIONS INFORMATION

HIGHER FEEDBACK RESISTOR GAIN OPERATION

The ADA4898-1/ADA4898-2 schematic for the noninverting

gain configuration shown in Figure 45 is nearly a textbook

example. 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 46 shows the difference between using a 100 Ω resistor

and a 1 kΩ feedback resistor. Due to the high input capacitance in

the ADA4898-1/ADA4898-2 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 an RF of 100 Ω, 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 46 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 than 2 dB.

VIN

VOUT

10µF

+VS

–VS

10µF

0.1µF

Figure 45. 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 46. Small Signal Frequency Response for

Various Feedback Impedances

RECOMMENDED VALUES FOR VARIOUS GAINS

Table 8 provides a useful reference for determining various gains

and associated performance. 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 8. Gains and Recommended Resistor Values Associated with Them (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/ADA4898-2

Voltage Noise (nV/√Hz), RTO

Total System Noise

(nV/√Hz), RTO

Not applicable

0.9

1.29

100

1.8

3.16

100

24.9

4.5

7.07

Rev. E | Page 15 of 20

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