Data Sheet

AD8027/AD8028

WIDEBAND OPERATION

analog.com

Rev. E | 22 of 27

Voltage feedback amplifiers can use a wide range of resistor values

to set their gain. Proper design of the feedback network of the

application requires consideration of the following issues:

►

Poles formed by the amplifier input capacitances with the resis-

tances seen at the amplifier input terminals

►

Effects of mismatched source impedances

►

Resistor value impact on the voltage noise of the application

►

Amplifier loading effects

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

capacitance forms a pole with the amplifier feedback network, de-

stabilizing 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 low

input voltage noise of 4.3 nV/√Hz.

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

numerous applications and configurations. The AD8027/AD8028

device shown in Figure 62 is configured as a noninverting amplifier.

Table 9 provides an easy selection table of gain, resistor values,

bandwidth, and noise performance, and Figure 63 shows the invert-

ing configuration.

Figure 62. Wideband Noninverting Gain Configuration

Figure 63. Wideband Inverting Gain Configuration

CIRCUIT CONSIDERATIONS

BALANCED INPUT IMPEDANCES

Balanced input impedances can help to improve distortion perform-

ance. 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 by 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 64 shows the effect of the imbalance and degradation

in SFDR performance for a 50 Ω source impedance, with and

without a 50 Ω balanced feedback path.

Figure 64. SFDR vs. Frequency and Various RF

Table 9. Component Values, Bandwidth, and Noise Performance (VS = ±2.5 V)

Noise Gain

(Noninverting)

RSOURCE (Ω)

RF (Ω)

RG (Ω)

−3 dB Small Signal BW (MHz)

Output Noise with Resistors (nV/√Hz)

1

50

0

Not applicable

190

4.4

2

50

499

499

95

10

10

50

499

54.9

13

45