ADL5569

Data Sheet

Rev. C | Page 18 of 23

EFFECT OF LOAD CAPACITANCE

Load capacitance, including stray capacitance from PCB traces,

affects the bandwidth and flatness of the ADL5569 frequency

response, resulting in excessive peaking. It is recommended to

add 5 Ω external series resistors to each output to isolate the

load capacitance from the outputs and to effectively reduce

peaking. Respective frequency responses resulting from the

addition of a 0.5 pF to a 2.0 pF differential load capacitance

are shown in Figure 46.

25

20

15

10

5

0

–10

–5

10M

10G

FREQUENCY (Hz)

100M

1G

1.0pF

0.5pF

0pF

2.0pF

GSPS ADC INTERFACING

A wideband data acquisition system using the ADL5569 together

with the AD9689, a dual 14-bit ADC, was developed and tested as

shown in Figure 49 for ac-coupled applications. The amplifier

output common-mode voltage is set to AVDD/2 with the voltage

divider formed by the two 10 kΩ resistors on the VCOM pin.

However, the VCOM pin can be left open (with a decoupling

capacitor as described in Table 4), because there is an internal

divider to pull the node to VCC/2 in such cases. The resistor

capacitor (RC) filter after the amplifier works with the pole formed

by the ADC input capacitance to attenuate the broadband noise

and out of band harmonics generated by the amplifier. The RC

filter also blocks sharp switching pulses, that is, charge injection

from the ADC internal sampling circuitry, from reaching the

amplifier outputs and creating nonlinear effects. An additional

band-pass filter, specific to the system rejection requirements,

is also needed in front of the ADL5569 amplifier to prevent

unwanted signals from compressing the amplifier as well as from

reaching the ADC. This filter is usually added during bench testing.

See the AN-835 Application Note, Understanding High Speed ADC

Testing and Evaluation, for additional testing information and

setups.

The signal-to-noise ratio (SNR) and SFDR with respect to full

scale (FS) of the ADC system, is shown in Figure 47 and Figure 48.

40

45

50

55

60

65

70

0

500

1000

1500

2000

2500

3000

FREQUENCY (MHz)

AC-COUPLED

DC-COUPLED

NOTE: ALL TESTING WAS DONE AT –3dBFS.

Figure 47. ADC System SNR Referred to Full Scale (SNRFS) vs. Frequency

30

40

50

60

70

80

90

0

500

1000

1500

2000

2500

3000

FREQUENCY (MHz)

AC-COUPLED

DC-COUPLED

NOTE: ALL TESTING WAS DONE AT –3dBFS.

Figure 48. ADC System SFDR vs. Frequency