AD8027ART-R2 datasheet(20/25 Pages) AD

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AD8027/AD8028

Rev. C | Page 19 of 24

WIDEBAND OPERATION

Voltage feedback amplifiers can use a wide range of resistor

values to set their gain. Proper design of the application’s

feedback network requires consideration of the following issues:

•

Poles formed by the amplifier’s input capacitances with the

resistances seen at the amplifier’s input terminals

•

Effects of mismatched source impedances

•

Resistor value impact on the application’s voltage noise

•

Amplifier loading effects

The AD8027/AD8028 have an input capacitance of 2 pF. This

input capacitance forms a pole with the amplifier’s feedback

network, destabilizing the loop. For this reason, it is generally

desirable to keep the source resistances below 500 Ω, unless

some capacitance is included in the feedback network. Likewise,

keeping the source resistances low also takes advantage of the

AD8027/AD8028’s low input referred voltage noise of

4.3 nV/√Hz.

With a wide bandwidth of over 190 MHz, the AD8027/AD8028

have numerous applications and configurations. The AD8027/

AD8028 part shown in Figure 57 is configured as a noninvert-

ing amplifier. An easy selection table of gain, resistor values,

bandwidth, slew rate, and noise performance is presented in

Table 5, and the inverting configuration is shown in Figure 58.

–V

μF

0.1

μF

0.1

μF

VOUT

SELECT

VIN

R1 = RF||RG

AD8027/

AD8028

–

03327-A-056

Figure 57. Wideband Noninverting Gain Configuration

Table 5. Component Values, Bandwidth, and Noise

Performance (VS = ±2.5 V)

Noise Gain

(Noninverting)

RSOURCE

(Ω)

–3 dB

SS BW

(MHz)

Output

Noise with

Resistors

(nV/√Hz)

N/A

190

4.4

499

54.9

–V

μF

0.1

μF

0.1

μF

VOUT

R1 = RF||RG

VIN

SELECT

AD8027/

AD8028

–

03327-A-057

Figure 58. Wideband Inverting Gain Configuration

CIRCUIT CONSIDERATIONS

Balanced Input Impedances

Balanced input impedances can help to improve distortion

performance. When the amplifier transitions from PNP pair to

NPN pair operation, a change in both the magnitude and

direction of the input bias current occurs. When multiplied

times imbalanced input impedances, a change in offset can

result. The key to minimizing this distortion is to keep the input

impedances balanced on both inputs. Figure 59 shows the effect

of the imbalance and degradation in distortion performance for

a 50 Ω source impedance, with and without a 50 Ω balanced

feedback path.

FREQUENCY (MHz)

0.1

–100

–90

–80

–70

–60

–50

–40

–30

–20

G = +1

VOUT = 2V p-p

RL = 1kΩ

VS = +3V

RF = 24.9Ω

RF = 49.9Ω

RF = 0Ω

03327-A-058

Figure 59. SFDR vs. Frequency and Various RF

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