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.

12-Bit, 80Msps, 3.3V IF-Sampling ADC

______________________________________________________________________________________

21

MAX1209

1

2

3

6

5

4

N.C.

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

MAX1209

1

2

3

6

5

4

N.C.

N.C.

T2

MINICIRCUITS

ADT1-1WT

1

2

3

6

5

4

N.C.

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

RESISTORS TO LIMIT THE BANDWIDTH.

Figure 11. Transformer-Coupled Input Drive for Input Frequencies Beyond Nyquist

MAX1209

5.6pF

5.6pF

2.2

μF

INP

COM

INN

24.9

Ω

24.9

Ω

100

Ω

100

Ω

0.1

μF

MAX4108

Figure 12. Single-Ended, AC-Coupled Input Drive