Low-Power, 16-Bit Smart ADC

8

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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-

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

digitized sensor signals become -3.255 / 5 = -0.651

and +1.395 / 5 = +0.279.

The ADC output clips to ±1.0 when input values exceed

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).

-0.010

-0.006

-0.008

-0.002

0.004

0.002

0

0.004

0.008

0.006

0.010

-100

-60 -40 -20

-80

0

20 40

80

60

100

SENSOR SIGNAL INPUT OR ADC INPUT/OUTPUT RANGE (%)

Figure 2a. Analog Front-End INL (typical)

-4

-3

0

-1

-2

1

2

3

4

-100

-20

-40

-80 -60

0

20 40 60 80 100

SENSOR SIGNAL INPUT OR ADC INPUT RANGE (%)

Figure 2b. Analog Front-End Differential Nonlinearity (DNL)

(typical)

0

1.0

0.5

1.5

3.0

3.5

2.5

2.0

4.0

-100

-60 -40 -20

0

-80

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)