Applications information – Rainbow Electronics MAX1495 User Manual

Page 17

MAX1491/MAX1493/MAX1495

3.5- and 4.5-Digit, Single-Chip

ADCs with LCD Drivers

______________________________________________________________________________________

If ghosting is present on the LCD, the RMS OFF voltage
is too high. Choose an LCD with a higher RMS OFF
voltage or decrease DV

Decimal Point Control

The MAX1491/MAX1493/MAX1495 allow for full deci-
mal-point control and feature leading-zero suppression.
Use DPON, DPSET1, and DPSET2 to set the value of
the decimal point. Tables 2 and 3 show the truth tables
of the DPON, DPSET1, and DPSET2 that determine
which decimal point is used.

Reference

The MAX1491/MAX1493/MAX1495 reference sets the
full-scale range of the ADC transfer function. With a
nominal 2.048V reference, the ADC full-scale range is
±2V with RANGE equal to GND. With RANGE equal to
DV

, the full-scale range is ±200mV. A decreased ref-

erence voltage decreases full-scale range (see the
Transfer Functions section).

The MAX1491/MAX1493/MAX1495 accept either an
external reference or an internal reference. The INTREF
input selects the reference mode.

For internal reference operation, connect INTREF to
DV

, connect REF- to GND, and bypass REF+ to

GND with a 4.7µF capacitor. The internal reference pro-
vides a nominal 2.048V source between REF+ and
GND. The internal reference temperature coefficient is
typically 40ppm/°C.

Connect INTREF to GND to use the external reference.
The external reference inputs, REF+ and REF-, are fully
differential. For a valid external reference input, V

REF+

must be greater than V

REF-

. Bypass REF+ and REF-

with a 0.1µF or greater capacitor to GND in external ref-
erence mode.

Figure 13 shows the MAX1493/MAX1495 operating with
an external differential reference. In this mode, REF- is
connected to the top of the strain gauge and REF+ is
connected to the midpoint of the resistor-divider on the
supply.

Applications Information

Power-On

At power-on, the digital filter and modulator circuits
reset. The MAX1493/MAX1495 allow 6s for the refer-
ence to stabilize before performing enhanced offset
calibration. During these 6s, the MAX1493/MAX1495
display 1.2V to 1.5V when a stable reference is detect-
ed. If a valid reference is not found, the MAX1493/
MAX1495 time out after 6s and begin enhanced offset
calibration. Enhanced offset calibration typically lasts
2s. The MAX1493/MAX1495 begin converting after
enhanced offset calibration.

Offset Calibration

The MAX1491/MAX1493/MAX1495 offer on-chip offset
calibration. The MAX1491/MAX1493/MAX1495 calibrate
offset during every conversion cycle. The MAX1495

Table 2. Decimal-Point Control Table (MAX1493/MAX1495)

DPON

DPSET1

DPSET2

DISPLAY OUTPUT

ZERO INPUT READING

1 8 8 8 8

1 8 8 8.8

0.0

1 8 8.8 8

0.00

1 8.8 8 8

0.000

1.8 8 8 8

0.0000

Table 3. Decimal-Point Control Table (MAX1491)

DPSET1

DPSET2

DISPLAY OUTPUT

ZERO INPUT READING

1 8 8.8

0.0

1 8.8 8

0.00

1.8 8 8

0.000

1 8 8 8

000