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MAX1215

1.8V, 12-Bit, 250Msps ADC for

Broadband Applications

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Applications Information

FSR Adjustments Using the Internal

Bandgap Reference

The MAX1215 supports a full-scale adjustment range of

10% (±5%). To decrease the full-scale signal range, an

external resistor value ranging from 13kΩ to 1MΩ may

be added between REFADJ and AGND. A similar

approach can be taken to increase the ADC’s full-scale

range (FSR). Adding a variable resistor, potentiometer,

or predetermined resistor value between REFADJ and

REFIO increases the FSR of the data converter. Figure

6a shows the two possible configurations and their

impact on the overall full-scale range adjustment of the

MAX1215. Do not use resistor values of less than 13kΩ

to avoid instability of the internal gain regulation loop

for the bandgap reference. See Figure 6b for the

results of the adjustment range for a selection of resis-

tors used to trim the full-scale range of the MAX1215.

Table 1. MAX1215 Digital Output Coding

INP ANALOG

INPUT VOLTAGE

LEVEL

INN ANALOG

INPUT VOLTAGE

LEVEL

OUT-OF-RANGE

ORP (ORN)

BINARY DIGITAL OUTPUT

CODE (D11P/N–D0P/N)

TWO’S COMPLEMENT DIGITAL

OUTPUT CODE (D11P/N–D0P/N)

> V

+ V

/ 4

< V

- V

/ 4 1 (0)

1111 1111 1111

(exceeds +FS, OR set)

0111 1111 1111

(exceeds +FS, OR set)

+ V

/ 4 V

- V

/ 4 0 (1) 1111 1111 1111 (+FS) 0111 1111 1111 (+FS)

0 (1)

1000 0000 0000 or

0111 1111 1111 (FS/2)

0000 0000 0000 or

1111 1111 1111 (FS/2)

- V

/ 4 V

+ V

/ 4 0 (1) 0000 0000 0000 (-FS) 1000 0000 0000 (-FS)

< V

+ V

/ 4

> V

- V

/ 4 1 (0)

00 0000 0000

(exceeds -FS, OR set)

10 0000 0000

(exceeds -FS, OR set)

OGND

2.2kΩ

Figure 5. Simplified LVDS Output Architecture

MAX1215

REFERENCE

BUFFER

ADC FULL SCALE = REFT - REFB

REFT: TOP OF REFERENCE LADDER.

REFB: BOTTOM OF REFERENCE LADDER.

CONTROL LINE

TO DISABLE

REFERENCE BUFFER

REFERENCE-

SCALING

AMPLIFIER

REFIO

REFADJ

13kΩ TO

1MΩ

0.1µF

REFT

REFB

13kΩ TO

1MΩ

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

Range

FS VOLTAGE vs. FS ADJUST RESISTOR

MAX1213 fig06b

FS ADJUST RESISTOR (Ω)

(V)

875750500 625250 375125

1.39

1.41

1.43

1.45

1.47

1.49

1.51

1.53

1.55

1.57

1.37

01000

RESISTOR VALUE APPLIED BETWEEN

REFADJ AND REFIO INCREASES V

RESISTOR VALUE APPLIED BETWEEN

REFADJ AND AGND DECREASES V

Figure 6b. FS Adjustment Range vs. FS Adjustment Resistor

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 19 20

Broadband Applications 1

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