Maxim Integrated MAX12527 - Manual (Page 22)

MAX12527
Applications Information
Using Transformer Coupling
In general, the MAX12527 provides better SFDR and
THD with fully differential input signals than single-
ended input drive, especially for input frequencies
above 125MHz. In differential input mode, even-order
harmonics are lower as both inputs are balanced, and
each of the ADC inputs only requires half the signal
swing compared to single-ended input mode.
An RF transformer (Figure 9) provides an excellent
solution to convert a single-ended input source signal
to a fully differential signal, required by the MAX12527
for optimum performance. Connecting the center tap of
the transformer to COM provides a V
DD / 2 DC level
shift to the input. Although a 1:1 transformer is shown, a
step-up transformer can be selected to reduce the
drive requirements. A reduced signal swing from the
input driver, such as an op amp, can also improve the
overall distortion. The configuration of Figure 9 is good
for frequencies up to Nyquist (f
CLK / 2).
The circuit of Figure 10 converts a single-ended input
signal to fully differential just as Figure 9. However,
Figure 10 utilizes an additional transformer to improve
the common-mode rejection allowing high-frequency
signals beyond the Nyquist frequency. A set of 75
Ω
and 113 Ω termination resistors provide an equivalent
50 Ω termination to the signal source. The second set of
termination resistors connects to COM_ providing the
correct input common-mode voltage. Two 0
Ω resistors
in series with the analog inputs allow high IF input fre-
quencies. These 0
Ω resistors can be replaced with low-
value resistors to limit the input bandwidth.
Single-Ended AC-Coupled Input Signal
Figure 11 shows an AC-coupled, single-ended input
application. The MAX4108 provides high speed, high
bandwidth, low noise, and low distortion to maintain the
input signal integrity.
Dual, 65Msps, 12-Bit, IF/Baseband ADC
22 ______________________________________________________________________________________
MAX12527 1
5
3
6
2
4
N.C.
V IN
0.1 µF
T1
MINICIRCUITS
TT1-6
OR
T1-1T
24.9 Ω
24.9 Ω
5.6pF
5.6pF
0.1 µF
IN_P
COM_
IN_N
Figure 9. Transformer-Coupled Input Drive for Input Frequencies
Up to Nyquist
1
5
3
6
2
4
N.C.
V IN
0.1 µF
T1
MINICIRCUITS
ADT1-1WT
5.6pF
5.6pF
IN_P
COM_
IN_N
*0 Ω RESISTORS CAN BE REPLACED WITH
LOW-VALUE RESISTORS TO LIMIT THE INPUT BANDWIDTH.
N.C.
1
5
3
6
2
4
N.C.
T2
MINICIRCUITS
ADT1-1WT
N.C.
75 Ω
1%
75 Ω
1%
113 Ω
0.5%
113 Ω
0.5%
0.1 µF
0 Ω*
0 Ω*
MAX12527
Figure 10. Transformer-Coupled Input Drive for Input Frequencies beyond Nyquist
MAX12527
MAX4108
0.1 µF
0.1 µF
0 Ω
5.6pF
IN_P
COM_
IN_N
100 Ω
100 Ω
V IN
24.9 Ω
24.9 Ω
5.6pF
Figure 11. Single-Ended, AC-Coupled Input Drive