ADA4961ACPZN-R7 datasheet(20/24 Pages) AD

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ADA4961

Data Sheet

Rev. C | Page 20 of 24

LOW-PASS ANTIALIAS FILTERING FOR THE ADC

INTERFACE

The high frequency distortion performance of the ADA4961

can be enhanced by adding a low-pass filter to the output (see

Figure 46 and Figure 47. A two-pole low-pass filter is used in

the ADC Driving section to illustrate the distortion improvement

capabilities and integrated noise reduction. Figure 49 shows a

simplified diagram of a two-pole low-pass (LP) filter. The

inductor capacitance (LC) values are 2 nH and 2 pF, respectively.

This filter gives an overall −3 dB BW of 2 GHz when connected

to the ADA4961. Ideally, the BW is 3.5 GHz without any

parasitics. The parasitic, C, (about 1 pF) across the 2 nH

inductor (not shown) reduces the BW to about 2.1 GHz.

Take care to ensure that the physical length of the filter is less

than 1/10 the wavelength of the 3 dB corner frequency. At

2 GHz, it is 75 mm. The Series L (along with the internal bond

wire inductance) and C parasitic parallel create a parallel

resonance that causes a reduction in overall BW. Other values

and filter types can be used depending on the end user

requirements, but care is needed to ensure that the Circuit Q

does not exceed 1. The values of 2 nH and 2 pF show the

relative improvement in distortion (single tone and IMD3) vs.

no filter at frequencies out to 1.5 GHz. At frequencies above

about 600 MHz, the HD3s begin to attenuate as is expected due

to the LP roll-off of the L (2 nH) and Shunt C (2 pF). In addition,

the inband IMD3s also improve. This improvement is due to

the peaking that results at the amplifier output due to its

internal parasitics interacting with the 2 nH inductor and its

Shunt C parasitic. This peaking reduces the input signal to the

amplifier (not shown), thus reducing inband third-order terms.

–5

–10

–15

–25

10M

100M

FREQUENCY (MHz)

10G

–20

NO FILTER

FILTER

Figure 45. Maximum Gain vs. Frequency, with and Without LC Filter

–100

–95

–90

–85

–80

–75

–70

–60

–65

–55

–50

200

400

600

800 1000 1200 1400

2000

1600 1800

FREQUENCY (MHz)

WITH FILTER

NO FILTER

Figure 46. IMD vs. Frequency, with and Without LC Filter

–100

–95

–90

–85

–80

–75

–70

–60

–65

–55

–50

200

400

600

800 1000 1200 1400

2000

1600 1800

FREQUENCY (MHz)

WITH FILTER

NO FILTER

Figure 47. HD2 vs. Frequency, with and Without LC Filter

–100

–95

–90

–85

–80

–75

–70

–60

–65

–55

–50

200

400

600

800 1000 1200 1400

2000

1600 1800

FREQUENCY (MHz)

WITH FILTER

NO FILTER

Figure 48. HD3 vs. Frequency, with and Without LC Filter

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