CN-0227

Circuit Note

Rev. A | Page 2 of 5

An input signal of 6.0 dBm produces a full-scale 2 V p-p

differential signal at the ADC input.

The antialiasing filter is a third-order Butterworth filter designed

with a standard filter design program. A Butterworth filter was

chosen because of its flat response within the pass band. A third

order filter yields an ac noise bandwidth ratio of 1.05 and can be

designed with the aid of several free filter programs such as

Nuhertz Technologies Filter Free, or the Quite Universal Circuit

Simulator (Qucs) Free Simulation.

To achieve best performance, load the ADL5562 with a net

differential load of 200 Ω. The 15 Ω series resistors isolate the

filter capacitance from the amplifier output, and the 243 Ω resistors

in parallel with the downstream impedance yield a net load

impedance of 203 Ω when added to the 30 Ω series resistance.

The 20 Ω resistors in series with the ADC inputs isolate internal

switching transients from the filter and the amplifier. The 511 Ω

resistor in parallel with the ADC serves to reduce the input

impedance of the ADC for more predictable performance.

The third-order Butterworth filter was designed with a source

impedance of 38.6 Ω, a load impedance of 269 Ω, and a 3 dB

bandwidth of 180 MHz. The calculated values from the program

are shown in Figure 1. The values chosen for the filter passive

components were the closest standard values to those generated

by the program.

19.3Ω

19.3Ω

269Ω

12pF

35.79pF

23.83nH

23.83nH

Figure 2. Design for Third-Order Differential Butterworth Filter with

The internal 3.5 pF capacitance of the ADC was subtracted

from the value of the second shunt capacitor to yield a value of

32.29 pF. In the circuit, this capacitor was realized using two

62 pF capacitors connected to ground as shown in Figure 1.

This provides the same filtering effect and offers some ac

common-mode rejection.

The measured performance of the system is summarized in

Table 1, where the 3 dB bandwidth is 152 MHz. The total insertion

loss of the network is approximately 2 dB. The bandwidth response

is shown in Figure 3; the SNR and SFDR performance are

shown in Figure 4.

Table 1. Measured Performance of the Circuit

Performance Specs at 2 V p-p FS

Final Results

Cutoff Frequency (−3 dB)

152 MHz

Pass-Band Flatness (6 MHz to 125 MHz)

1 dB

SNRFS at 120 MHz

72.6 dBFS

SFDR at 120 MHz

82.2 dBc

H2/H3 at 120 MHz

86.6 dBc/82.2 dBc

Overall Gain at 10 MHz

3.9 dB

Input Drive at 10 MHz

6.0 dBm

–20

–18

–16

–14

–12

–10

–8

–6

–4

–2

0

1

10

100

1000

ANALOG INPUT FREQUENCY (MHz)

−3dB AT 152MHz

Figure 3. Pass Band Flatness Performance vs. Frequency

60

65

70

75

80

85

90

95

100

0

20

40

60

80

100

120

140

160

180

ANALOG INPUT FREQUENCY (MHz)

SFDR (dBc)

SNRFS (dBFS)

Figure 4. SNR/SFDR Performance vs. Frequency