MAX1459 Datasheet, PDF (17/24 Pages) Maxim Integrated Products – 2-Wire, 4-20mA Smart Signal Conditioner

2-Wire, 4–20mA
Smart Signal Conditioner
+5V
C2
0.1µF
SENSOR
BDRIVE
INP
INM
ISRC
Σ
V_DD
V_DD
MAX1459
PGA
SELECT
OUTPUT
R_NBIAS
= 402k
1%
NBIAS
OUT
AMP-
AMP+
AMPOUT
C1
0.1µF
R_ISRC
R_FTC
CS
WE
SCLK
DIO
R_FTC
R_ISRC
128-BIT
V_SS
EEPROM
DIGITAL
INTERFACE
FSOTC
A=1
TEMP1
TEMP2
TEMPIN
V_SS
Figure 13. Basic Ratiometric Output Configuration
First, calculate the ideal IRO correction voltage using
the following formula, and select the nearest setting
from Table 1:
[ ] ^{IROideal = - O(T1) x V}BDRIVE^(T1)
= - ₍0.012V/V₎ x 2.5V
= - 30mV
where IROideal is the exact voltage required to perfect-
ly null the sensor, O(T1) is the sensor offset voltage in
V/V at +25°C, and V_BDRIVE(T1) is the nominal sensor
excitation voltage at +25°C. In this example, 30mV
must be subtracted from the amplifier front end to null
the sensor perfectly. From Table 1, select an IRO value
of 3 to set the IRO DAC to 27mV, which is nearest the
ideal value. To subtract this value, set the IRO sign bit
to 0. The residual output-referred offset error will be
corrected later with the offset DAC.
Determining OFFTC COEF Initial Value
Generally, OFFTC COEF can initially be set to 0 since
the offset TC error will be compensated in a later step.
However, sensors with large offset TC errors may
require an initial coarse offset TC adjustment to prevent
the PGA from saturating during the compensation pro-
cedure as temperature is increased. An initial coarse
offset TC adjustment is required for sensors with an off-
set TC greater than about 10% of the FSO. If an initial
coarse offset TC adjustment is required, use the follow-
ing equation:
OFFTC COEF =
4096 x ∆V_OUT(T)
∆V_BDRIVE(T) x 2.3
4096 x (OTC x FSO) x ∆T
≅
TCS x V_BDRIVE x 2.3 x ∆T
4096 x (-1000ppm/°C x 4V)
≅
≅ 1357
-2100ppm/°C x 2.5V x 2.3
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