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ADRF6821

Data Sheet

Rev. A | Page 30 of 61

ANALOG-TO-DIGITAL CONVERTER (ADC)

INTERFACING

The ADRF6821 perfectly suits in a zero IF receiver chain. The

integrated IF amplifier of the ADRF6821 provides variable and

sufficient drive capabilities for both buffered and unbuffered

ADCs. It also provides isolation between the sampling edges of

the ADC and the mixer core. As a result, an antialiasing low-pass

filter is sufficient when interfacing with an ADC.

The filter resides between the ADRF6821 and the ADC. The

low-pass filter eliminates all out of band signals that may alias

onto the actual band and degrade the performance of the

ADRF6821 and the ADC pair. Selection of the low-pass filter

center and bandwidth is application specific. Take into account

the trade-off between the amount of rejection required and the

insertion loss to choose the order of the filter. A higher order

filter also requires more layout space, which is another design

criterion.

For the purposes of ADC interfacing, consider a DPD receiver

chain, correcting for a 70 MHz bandwidth signal. Assuming a

fifth-order correction, 350 MHz from the output of the power

amplifier must be sampled. With the use of a zero-IF receiver,

I and Q bandwidths are half of the 350 MHz, that is, 175 MHz.

Next, determine the sampling rate of the ADC. To relax the

antialiasing filter requirements, use a slightly oversampled

system. Considering the bandwidth of interest for I and Q

(that is, 175 MHz), 500 MSPS is a sufficiently large sampling

rate. With a 500 MSPS sampling rate, the second Nyquist zone

lies between 250 MHz and 500 MHz. Because there are no

interferers present in a DPD chain, only the replica of the signal

of interest in the second Nyquist zone (between 325 MHz and

500 MHz) is of concern. As a result, the antialiasing filter provides

sufficient attenuation starting from 325 MHz.

The required attenuation from the filter is determined with the

dynamic range requirement. As previously mentioned, in a DPD

receiver chain, ideally only the signal of interest is present.

Therefore, the filter requirements can be relaxed further. Because of

this, consider a 40 dB rejection at the aliased portion.

Considering the pass band, the stop band, and the attenuation

at stop band, a seventh-order Chebyshev low-pass filter is

suitable with a 0.1 dB ripple in-band. Keep in mind that the

ADRF6821 is optimized with a 100 Ω load at the output.

Therefore, design the filter for an input and output impedance

of differential 100 Ω. The Chebyshev filter design is discussed

extensively and is straightforward with the use of a filter wizard,

such as ADS built-in filter design tool from Keysight. Filter

design tools provide component values that are not necessarily

commercially available. It is recommended that designers

use commercially available component models and take into

account the layout effects. Figure 56 displays the component

values for the antialiasing LPF. Table 17 provides commercially

available component values for the low-pass filter design.

56nH

68nH

56nH

25Ω

I/Q OUTPUTS

56nH

68nH

56nH

6.8pF

25Ω

100Ω

15pF

8.2pF

Figure 56. Low-Pass Filter Schematic

Table 17. Component Values for the Low-Pass Filter Design

(1 dB Corner Frequency of 150 MHz)

Parameter

Value

Type

Manufacturer

Inductors

56 nH

0402 CS

Coilcraft

68 nH

0402 CS

Coilcraft

Capacitors

8.2 pF

0402 C0G

Murata

6.8 pF

0402 C0G

Murata

15 pF

0402 C0G

Murata

Figure 57 compares the measured low-pass filter response and

the normalized gain of the ADRF6821 LPF pair. Refer to the

highest gain of the ADRF6821 (without the low-pass filter) to

acheive normalization. The in band roll-off is associated to the

finite Q and trace and pad losses.

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

200

400

600

800

1000

100

300

500

700

900

IF FREQUENCY (MHz)

LPF RESPONSE

NORMALIZED GAIN OF ADRF6821 LPF

Figure 57. Frequency Response for the 150 MHz Low-Pass Filter and

Frequency Response for ADRF6821 LPF Pair

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