Data Sheet

AD8375

Rev. B | Page 15 of 24

For optimum performance, the AD8375 should be driven

differentially using an input balun or impedance transformer.

Figure 38 uses a wideband 1:1 transmission line balun followed

by two 37.5 Ω resistors in parallel with the 150 Ω input imped-

ance of the AD8375 to provide a 50 Ω differential terminated

input impedance. This provides a wideband match to a 50 Ω

source. The open-collector outputs of the AD8375 are biased

through the two 1 μH inductors and are ac-coupled to the two

82 Ω load resistors. The 82 Ω load resistors in parallel with the

series-terminated ADC impedance yields the target 150 Ω

differential load impedance, which is recommended to provide

the specified gain accuracy of the device. The load resistors are

ac-coupled from the AD9445 to avoid common-mode dc

loading. The 33 Ω series resistors help to improve the isolation

between the AD8375 and any switching currents present at the

analog-to-digital sample and hold input circuitry.

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

–140

–150

0

5.25 10.50 15.75 21.00 26.25 31.50 36.75 42.00 47.25 52.50

FREQUENCY (MHz)

SNR = 64.93dBc

SFDR = 86.37dBc

NOISE FLOOR = –108.1dB

FUND = –1.053dBFs

SECOND = –86.18dBc

THIRD = –86.22dBc

1

2

3

4

5

6

+

Figure 39. Measured Single-Tone Performance of the

Circuit in Figure 38 for a 100 MHz Input Signal

The circuit depicted in Figure 38 provides variable gain,

isolation and source matching for the AD9445. Using this

circuit with the AD8375 in a gain of 20 dB (maximum gain) an

SFDR performance of 86 dBc is achieved at 100 MHz, as

indicated in Figure 39.

The addition of the series inductors L (series) in Figure 38

extends the bandwidth of the system and provides response

flatness. Using 100 nH inductors as L (series), the wideband

system response of Figure 40 is obtained. The wideband

frequency response is an advantage in broadband applications

such as predistortion receiver designs and instrumentation

applications. However, by designing for a wide analog input

frequency range, the cascaded SNR performance is somewhat

degraded due to high frequency noise aliasing into the wanted

Nyquist zone.

0

–1

–2

–3

–4

–5

–6

–7

–8

–9

–10

20

48

76

104

132

160

188

216

244

272

300

FREQUENCY (MHz)

FIRST POINT = –2.93dBFs

END POINT = –9.66dBFs

MID POINT = –2.33dBFs

MIN = –9.66dBFs

MAX = –1.91dBFs

Figure 40. Measured Frequency Response of Wideband ADC Interface

Depicted in Figure 38

An alternative narrow-band approach is presented in Figure 41.

By designing a narrow band-pass antialiasing filter between the

AD8375 and the target ADC, the output noise of the AD8375

outside of the intended Nyquist zone can be attenuated, helping

to preserve the available SNR of the ADC. In general, the SNR

improves several dB when including a reasonable order antialias-

ing filter. In this example, a low loss 1:3 input transformer is used

to match the AD8375’s 150 Ω balanced input to a 50 Ω unbal-

anced source, resulting in minimum insertion loss at the input.