ADA4961ACPZN-R7 datasheet(19/24 Pages) AD

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Data Sheet

ADA4961

Rev. C | Page 19 of 24

–25

–20

–15

–10

–5

100

1000

10000

FREQUENCY (MHz)

Figure 42. Measured Frequency Response of the Wideband ADC Interface

Shown in Figure 41

Figure 41 uses a 1:2 impedance transformer to provide the 100 Ω

input impedance of the ADA4961 with a matched input. The

open collector outputs of the ADA4961 are biased through the

two 0.5 μH inductors, and the two 0.1 μF capacitors on the

outputs decouple the 5 V inductor voltage from the input

common-mode voltage of the ADA4961. The two 25 Ω resistors,

in parallel with the 100 Ω input impedance of the AD9625, provide

the 50 Ω load to the ADA4961, where the gain is load

dependent. The 2 nH inductors and 1.5 pF internal capacitance

of the AD9625 constitute the 1 GHz, 1 dB low-pass filter. The

two 5 Ω isolation resistors suppress any switching currents from

the ADC input sample-and-hold circuitry. The circuit shown in

Figure 41 provides variable gain, isolation, filtering, and source

matching for the AD9625. By using this circuit with the

ADA4961 in a gain of 15 dB (maximum gain), a full-scale SNR

(SNRFS) of 55 dB and an SFDR performance of 77 dBc are

achieved at 1 GHz, as shown in Figure 43.

–150

–135

–120

–105

–90

–75

–60

–45

–30

–15

150M

300M

450M

600M

750M

900M 1.05G

1.2G

FREQUENCY (Hz)

4 6

Figure 43. Measured Single Tone Performance of the Circuit Shown

in Figure 41 for a 1 GHz Input Signal using Maximum Gain (15 dB)

The two-tone 1 GHz IMDs of two 0.6 V p-p signals have an

SFDR of greater than 75 dBc, as shown in Figure 44.

–150

–135

–120

–105

–90

–75

–60

–45

–30

–15

150M

300M

450M

600M

750M

900M 1.05G

1.2G

FREQUENCY (Hz)

2F1 – F2

2F2 – F1

2F1 + F2

F1 + F2

2F2 + F1

F2 – F1

Figure 44. Measured Two-Tone Performance of the Circuit Shown in

Figure 41 for a 1 GHz Input Signal Using Maximum Gain (15 dB)

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