ADL5561ACPZ-R7 datasheet(17/25 Pages) AD

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ADL5561

Data Sheet

Rev. G | Page 16 of 24

The single-ended gain can be determined using the following

formula. The values of RIN and RX for each gain configuration

are shown in Table 7.













400

(2)

Table 7. Values of RIN and RX for Single-Ended Gain

Gain (dB)

RIN (Ω)

RX (Ω)

5.6

200

R2 || 3071

11.1

100

R2 || 1791

14.1

66.7

R2 || 1321

These values are based on a 50 Ω output match.

GAIN ADJUSTMENT AND INTERFACING

The effective gain of the ADL5561 can be reduced using a number

of techniques. A matched attenuator network can reduce the

effective gain, but this requires the addition of a separate

component that can be prohibitive in size and cost. Instead, a

simple voltage divider can be implemented using the combination

of an addition series resistor at the amplifier input and the input

impedance of the ADL5561, as shown in Figure 38. A shunt

resistor is used to match to the impedance of the previous stage.

0.1µF

1/2 RSHUNT

1/2 RS

0.1

µF 1/2 RSERIES

VIP1

VIN2

VIN1

VIP2

1/2 RSERIES

1/2 RSHUNT

ADL5561

Figure 38. Gain Adjustment Using Series Resistor

Figure 38 shows a typical implementation of the divider concept

that effectively reduces the gain by adding attenuation at the

input. For frequencies less than 100 MHz, the input impedance

of the ADL5561 can be modeled as a real 133 Ω, 200 Ω, or 400 Ω

resistance (differential) for maximum, middle, and minimum

gains, respectively. Assuming that the frequency is low enough

to ignore the shunt reactance of the input and high enough so

that the reactance of moderately sized ac-coupling capacitors

can be considered negligible, the insertion loss, Il, due to the

shunt divider can be expressed as













SERIES

log

)

(

(3)

The necessary shunt component, RSHUNT, to match to the source

impedance, RS, can be expressed as

SERIES

SHUNT

−

(4)

The insertion loss and the resultant power gain for multiple shunt

resistor values are summarized in Table 8. The source resistance

and input impedance need careful attention when using Equation 3

and Equation 4. The reactance of the input impedance of the

ADL5561 and the ac-coupling capacitors must be considered

before assuming they make a negligible contribution.

Table 8. Gain Adjustment Using Series Resistors

Il (dB)

RIN (Ω)

RS (Ω)

RSERIES (Ω)

RSHUNT (Ω)

400

105

54.9

400

232

54.9

200

51.1

61.9

200

115

133

34.8

71.5

400

200

102

332

400

200

232

294

200

51.1

976

200

115

549

400

105

54.9

400

232

54.9

200

51.1

61.9

ADC INTERFACING

The ADL5561 is a high output linearity amplifier that is optimized

for ADC interfacing. There are several options available to the

designer when using the ADL5561. Figure 39 shows a simplified

wideband interface with the ADL5561 driving the AD9445. The

AD9445 is a 14-bit, 125 MSPS ADC with a buffered wideband input.

For optimum performance, the ADL5561 should be driven dif-

ferentially using an input balun. Figure 39 uses a wideband 1:1

transmission line balun followed by two 34.8 Ω resistors in parallel

with the three input impedances (which change with the gain

selection of the ADL5561) to provide a 50 Ω differential input

impedance. This provides a wideband match to a 50 Ω source.

The ADL5561 is ac-coupled from the AD9445 to avoid common-

mode dc loading. The 33 Ω series resistors help to improve the

isolation between the ADL5561 and any switching currents present

at the analog-to-digital sample-and-hold input circuitry. The

AD9445 input presents a 2 kΩ differential load impedance and

requires a 2 V p-p differential input swing to reach full scale

(VREF = 1 V).

0.1µF

34.8Ω

50Ω

0.1

µF

ETC1-1-13

VIN1

VIP1

VIP2

VIN2

34.8Ω

ADL5561 0.1µF

0.1

µF

3.3V

33Ω

VOP

VON

33Ω

AD9445

14-BIT ADC

VIN+

VIN–

Figure 39. Wideband ADC Interfacing Example Featuring the AD9445

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