Datasheet

Data Sheet AD8375
Rev. A | 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
(dBFS)
0 5.25 10.50 15.75 21.00 26.25 31.50 36.75 42.00 47.25 52.50
FREQUENCY (MHz)
06724-040
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
(dBFs)
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
06724-041
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.