ADA4899-1YCPZ-R2 datasheet(14/20 Pages) AD

14 / 20 page

ADA4899-1

Rev. B | Page 14 of 20

APPLICATIONS

UNITY-GAIN OPERATION

The ADA4899-1 schematic for unity-gain configuration is

nearly a textbook example (see Figure 46). The only exception is

the small 24.9 Ω series resistor at the noninverting input. The

series resistor is only required in unity-gain configurations;

higher gains negate the need for the resistor. In Table 4, it can be

seen that the overall noise contribution of the amplifier and the

24.9 Ω resistor is equivalent to the noise of a single 87 Ω resistor.

Figure 47 shows the small signal frequency response for the

unity-gain amplifier shown in Figure 46.

VOUT

+VS

–VS

0.1µF

24.9Ω

VIN

0.1µF

Figure 46. Unity-Gain Schematic

10000

1100

1000

–12

–9

–6

–3

G = +1

RL = 100Ω

FREQUENCY (MHz)

25mV p-p

50mV p-p

200mV p-p

100mV p-p

Figure 47. Small Signal Frequency Response for Various Output Voltages

RECOMMENDED VALUES FOR VARIOUS GAINS

Table 4 provides a handy reference for determining various

gains and associated performance. For noise gains greater than

one, the Series Resistor RS is not required. Resistors RF and RG

are kept low to minimize their contribution to the overall noise

performance of the amplifier.

Table 4. Conditions: VS = ±5 V, TA = 25°C, RL = 1 kΩ

Gain

RF (Ω)

RG (Ω)

RS (Ω)

−3 dB SS BW (MHz)

(25 mV p-p)

Slew Rate (V/μs)

(2 V Step)

ADA4899-1 Voltage

Noise (nV/√Hz)

Total Voltage

Noise (nV/√Hz)

24.9

605

274

1.2

−1

100

294

265

2.7

100

277

253

2.7

200

49.9

227

6.5

+10

453

49.9

161

13.3

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