Data Sheet

ADA4961

Rev. C | Page 19 of 24

–25

–20

–15

–10

–5

0

5

10

10

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

0

0

150M

300M

450M

600M

750M

900M 1.05G

1.2G

FREQUENCY (Hz)

5

2

3

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

0

0

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)