Applications information – Rainbow Electronics MAX1121 User Manual

Page 13

Applications Information

Full-Scale Range Adjustments Using the

Internal Bandgap Reference

The MAX1121 supports a full-scale adjustment range of
10% (±5%). To decrease the full-scale range, an exter-
nal resistor value ranging from 13kΩ to 1MΩ may be
added between REFADJ and AGND. A similar
approach can be taken to increase the ADCs full-scale
range. Adding a variable resistor, potentiometer, or
predetermined resistor value between REFADJ and
REFIO increases the full-scale range of the data con-
verter. Figure 6 shows the two possible configurations
and their impact on the overall full-scale range adjust-
ment of the MAX1121. Do not use resistor values of less
than 13kΩ to avoid instability of the internal gain regula-
tion loop for the bandgap reference.

Differential, AC-Coupled, PECL-Compatible

Clock Input

The preferred method of clocking the MAX1121 is differ-
entially with LVDS- or PECL-compatible input levels. To
accomplish this, a 50Ω reverse-terminated clock signal
source with low phase noise is AC-coupled into a fast dif-
ferential receiver such as the MC100LVEL16 (Figure 7).
The receiver produces the necessary PECL output levels
to drive the clock inputs of the data converter.

MAX1121

1.8V, 8-Bit, 250Msps Analog-to-Digital Converter

with LVDS Outputs for Wideband Applications

______________________________________________________________________________________

REFERENCE

BUFFER

REFIO

REFADJ

/ 2

CONTROL LINE TO

DISABLE REFERENCE

BUFFER

ADC FULL-SCALE = REFT - REFB

0.1µF

REFERENCE-

SCALING

AMPLIFIER

REFT

REFB

13kΩ
TO 1MΩ

MAX1121

Figure 6. Circuit Suggestions to Adjust the ADC’s Full-Scale
Range

MAX1121

50Ω

CLKP

CLKN

SINGLE-ENDED

INPUT TERMINAL

MC100LVEL16

510Ω

150Ω

CLK

VGND

0.1µF

0.01µF

D0P/N–D7P/N

AGND

OGND

INP

INN

Figure 7. Differential, AC-Coupled, PECL-Compatible Clock Input Configuration