MCP3564 datasheet(34/108 Pages) MICROCHIP

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MCP3561/2/4

DS20006181C-page 34

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EQUATION 5-1:

TEMPERATURE SENSOR TRANSFER FUNCTION

5.1.3

ADC OFFSET CANCELLATION

ALGORITHM

The input multiplexer and the ADC include an offset

cancellation algorithm that cancels the offset contribution

of the ADC. This offset cancellation algorithm is

controlled by the AZ_MUX bit in the CONFIG2 register.

When AZ_MUX = 0 (default), the offset cancellation

algorithm is disabled and the conversions are not

affected by this setting. When AZ_MUX = 1, the

algorithm is enabled. When the offset cancellation

algorithm is enabled, ADC takes two conversions, one

with the differential input as VIN+/VIN-, one with VIN+/VIN-

inverted. Equation 5-2 calculates the ADC output code.

When AZ_MUX = 1, the Conversion Time, TCONV, is

multiplied by two, compared to the default case where

AZ_MUX = 0.

EQUATION 5-2:

AZ_MUX CONVERSION RESULT

This technique allows the cancellation of the ADC

offset error and the achievement of ultra-low offset

without any digital calibration. The resulting offset is the

residue of the difference between the two conversions,

which is on the order of magnitude of the noise floor.

This offset is effectively canceled at every conversion,

so the residual offset error temperature drift is

extremely low.

For One-Shot mode, the conversion time is simply

multiplied by two. Enabling the AZ_MUX bit is not

compatible with the Continuous Conversion mode

(because it effectively multiplexes the inputs in

between each conversion). If AZ_MUX = 1 and

CONV_MODE = 11 (Continuous Conversion mode),

the device will reset the digital filter in between each

conversion, and will therefore, have an output data rate

of 1/(2 * TCONV). The Continuous mode is replaced by

a series of One-Shot mode conversions with no delay

in between each conversion (see Section 5.13 “Con-

version Modes” and Figure 5-5 for more details about

the Conversion modes).

First-order (linear) fitting: Gain = 1, VREF =3.3V

TEMP (

C)

0.00133 ADCDATA (LSb)

267.146

–



VIN mV



0.2964 TEMP (

C) 79.32



Third-order fitting: Gain = 1, VREF =3.3V

TEMP (

C 

3.904

–

ADCDATA (LSb)



3.814 10

–



ADCDATA (LSb)



0.0002 ADCDATA (LSb)

163.978

–



VIN(mV)

4.727 10

–



TEMP (

C 



2.51288 10

–

 EMP (C



–

0.31294 TEMP (

C  79.547



ADC Output Code (AZ_MUX = 1)

ADC Output at + VIN



ADC Output at -VIN



–

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