Using Transformer Coupling

An RF transformer (Figure 6) provides an excellent solu-

tion to convert a single-ended source signal to a fully dif-

ferential signal, required by the MAX1190 for optimum

performance. Connecting the center tap of the trans-

input. Although a 1:1 transformer is shown, a step-up

transformer can be selected to reduce the drive require-

ments. A reduced signal swing from the input driver, such

as an op amp, can also improve the overall distortion.

In general, the MAX1190 provides better SFDR and

THD with fully differential input signals than single-

ended drive, especially for very high input frequencies.

In differential input mode, even-order harmonics are

lower as both inputs (INA+, INA- and/or INB+, INB-) are

balanced, and each of the ADC inputs only requires

half the signal swing compared to single-ended mode.

Single-Ended AC-Coupled Input Signal

Figure 7 shows an AC-coupled, single-ended applica-

tion. Amplifiers like the MAX4108 provide high speed,

high bandwidth, low noise, and low distortion to main-

tain the integrity of the input signal.

Buffered External Reference Drives

Multiple ADCs

Multiple-converter systems based on the MAX1190 are

well suited for use with a common reference voltage.

The REFIN pin of those converters can be connected

directly to an external reference source.

A precision bandgap reference like the MAX6062 gen-

erates an external DC level of 2.048V (Figure 8), and

exhibits a noise voltage density of 150nV/

√Hz. Its output

passes through a 1-pole lowpass filter (with 10Hz cutoff

frequency) to the MAX4250, which buffers the reference

before its output is applied to a second 10Hz lowpass

filter. The MAX4250 provides a low offset voltage (for

Dual 10-Bit, 120Msps, 3.3V, Low-Power ADC

with Internal Reference and Parallel Outputs

______________________________________________________________________________________

15

MAX1190

T1

N.C.

6

1

5

2

4

3

22pF

22pF

0.1

µF

0.1

µF

2.2

µF

25

Ω

25

Ω

MINICIRCUITS

TT1–6-KK81

T1

N.C.

6

1

5

2

4

3

22pF

22pF

0.1

µF

0.1

µF

2.2

µF

25

Ω

25

Ω

MINICIRCUITS

TT1-6-KK81

INA-

INA+

INB-

INB+

COM

Figure 6. Transformer-Coupled Input Drive