Rainbow Electronics MAX1213 - Manual (Page 14)

MAX1213
1.8V, 12-Bit, 170Msps ADC for
Broadband Applications
14 ______________________________________________________________________________________
Applications Information
FSR Adjustments Using the Internal
Bandgap Reference
The MAX1213 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
signal range. 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
MAX1213. 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 MAX1213.
Table 1. MAX1213 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 CM + 0.371V < V CM - 0.371V 1 (0)
1111 1111 1111
(exceeds +FS, OR set) 0111 1111 1111
(exceeds +FS, OR set)
V CM + 0.371V V CM - 0.371V 0 (1) 1111 1111 1111 (+FS) 0111 1111 1111 (+FS)
V CM V CM 0 (1)
1000 0000 0000 or
0111 1111 1111 (FS/2) 0000 0000 0000 or
1111 1111 1111 (FS/2)
V CM - 0.371V V CM + 0.371V 0 (1) 0000 0000 0000 (-FS) 1000 0000 0000 (-FS)
< V CM + 0.371V > V CM - 0.371V 1 (0)
00 0000 0000
(exceeds -FS, OR set) 10 0000 0000
(exceeds -FS, OR set)
V ON
OGND
OV CC
V OP
2.2kΩ 2.2kΩ
Figure 5. Simplified LVDS Output Architecture
MAX1213
REFERENCE
BUFFER
ADC FULL SCALE = REFT-REFB
REFT: TOP OF REFERENCE LADDER.
REFB: BOTTOM OF REFERENCE LADDER.
1V
AV CC
AV CC /2
G
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
FS
(V)
875 750 500 625 250 375 125
1.17
1.19
1.21
1.23
1.25
1.27
1.29
1.31
1.33
1.35
1.15
0 1000
RESISTOR VALUE APPLIED BETWEEN
REFADJ AND REFIO INCREASES V
FS
RESISTOR VALUE APPLIED BETWEEN
REFADJ AND AGND DECREASES V
FS
Figure 6b: FS Adjustment Range vs. FS Adjustment Resistor