MAX1460 Datasheet, PDF (8/20 Pages) Maxim Integrated Products – Low-Power, 16-Bit Smart ADC

Low-Power, 16-Bit Smart ADC
0.010
0.008
0.006
0.004
0.002
0
-0.002
0.004
-0.006
-0.008
-0.010
-100 -80 -60 -40 -20 0 20 40 60 80 100
SENSOR SIGNAL INPUT OR ADC INPUT/OUTPUT RANGE (%)
4
3
2
1
0
-1
-2
-3
-4
-100 -80 -60 -40 -20 0 20 40 60 80 100
SENSOR SIGNAL INPUT OR ADC INPUT RANGE (%)
Figure 2a. Analog Front-End INL (typical)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
-100 -80 -60 -40 -20 0 20 40 60 80 100
SENSOR SIGNAL INPUT OR ADC INPUT/OUTPUT RANGE (%)
Figure 2c. Analog Front-End Noise Standard Deviation of the
Samples (typical)
analog front-end settings when the sensor’s character-
istics are unknown. Use the tabulated values (Tables 1
and 2) if the peak sensor excursions are known. See
the Test System Interface section for details on writing
these analog front-end bits.
The PGA gain and the CO are very stable, but are not
accurate. Manufacturing variances on the gain and off-
set of the MAX1460 analog front-end superposition the
residual sensor errors, and are later removed during
final calibration.
For example, suppose the sensor’s sensitivity is
+10mV/V with an offset of -12mV/V. Let the supply volt-
Figure 2b. Analog Front-End Differential Nonlinearity (DNL)
(typical)
age be +5V. The full scale (-FS) output of the sensor is
then +5V(-12mV/V) = -60mV; +FS is then +5V (-12mV/V
+ 10mV/V) = -10mV. Following through the flowchart,
the PGA gain setting is +3 (gain = 93V/V) and the CO
correction setting is +1 (+25mV RTI) - (Referred-to
Input). The coarsely corrected -FS input to the ADC is
(-60mV + 25mV)93 = -3.255V. The +FS input to the
ADC is (-10mV + 25mV)93 = +1.395V. The input range
of the ADC is ±V_DD. Thus the maximum and minimum
digitized sensor signals become -3.255 / 5 = -0.651
and +1.395 / 5 = +0.279.
Notice that the bridge multiplies the signal by V_DD and
the ADC divides the signal by V_DD. Thus, the system is
ratiometric and not dependent on the DC value of V_DD.
The ADC output clips to ±1.0 when input values exceed
±V_DD. The best signal-to-noise ratio (SNR) is achieved
when the ADC input is within ±85% of V_DD (Figure 2).
The MAX1460 includes an internal temperature-sensing
bridge allowing the MAX1460 temperature to be used
as a proxy for the sensor temperature. For this reason,
the MAX1460 must be mounted in thermal proximity to
the sensor. The output of the temperature-sensing
bridge is also corrected by a 3-bit coarse-offset DAC
and processed by the ADC. The selection of the
Temperature Sensor Offset (TSO) bits in the configura-
tion register should be made so that the digitized tem-
perature signal is as close to 0.0 as possible at
midscale temperature. This is done to maximize the
dynamic range of the thermal-calibration coefficients
(Table 3).
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