Rainbow Electronics MAX1208 - Manual (Page 21)

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 10 is good
for frequencies up to Nyquist (f
CLK
/ 2).
The circuit of Figure 11 converts a single-ended input
signal to fully differential just as Figure 10. However,
Figure 11 utilizes an additional transformer to improve
the common-mode rejection, allowing high-frequency
signals beyond the Nyquist frequency. The two sets of
termination resistors provide an equivalent 75
Ω termi-
nation to the signal source. The second set of termina-
tion 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 frequencies.
These 0
Ω resistors can be replaced with low-value
resistors to limit the input bandwidth.
Single-Ended AC-Coupled Input Signal
Figure 12 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.
MAX1208
12-Bit, 80Msps, 3.3V ADC
______________________________________________________________________________________ 21
MAX1208
1
2
3
6
5
4
N.C. N.C.
T2
MINICIRCUITS
ADT1-1WT
1
2
3
6
5
4
N.C.
V
IN
0.1 µF
T1
MINICIRCUITS
ADT1-1WT
0 Ω*
0 Ω*
5.6pF
5.6pF
2.2 µF
INP
COM
INN
110 Ω
0.1%
110 Ω
0.1%
75 Ω
0.5%
75 Ω
0.5%
*0 Ω RESISTORS CAN BE REPLACED WITH LOW-VALUE
RESISTORS TO LIMIT THE INPUT BANDWIDTH.
Figure 11. Transformer-Coupled Input Drive for Input Frequencies Beyond Nyquist
MAX1208
1
2
3
6
5
4
N.C.
V
IN
0.1 µF
T1
MINICIRCUITS
TT1-6 OR T1-1T
24.9 Ω
24.9 Ω
12pF
12pF
2.2 µF
INP
COM
INN
Figure 10. Transformer-Coupled Input Drive for Input
Frequencies Up to Nyquist
MAX1208
5.6pF
5.6pF
2.2 µF
INP
COM
INN
24.9 Ω
24.9 Ω
100 Ω
100 Ω
0.1 µF
MAX4108
V
IN
Figure 12. Single-Ended, AC-Coupled Input Drive