MCP3912 datasheet(34/82 Pages) MICROCHIP

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MCP3912

DS20005348A-page 34

 2014 Microchip Technology Inc.

5.5

ADC Output Coding

The second-order modulator, SINC3+SINC1 filter,

PGA, VREF and the analog input structure all work

together to produce the device transfer function for the

analog-to-digital conversion (see Equation 5-3).

Channel data is calculated on 24-bit (23-bit plus sign)

and coded in two's complement format, MSB first. The

output format can then be modified by the

WIDTH_DATA<1:0> settings in the STATUSCOM

Section 8.5 “STATUSCOM Register – Status and

Communication Register”

for more information).

case

positive

saturation

(CHn+ – CHn-

>VREF/1.5), the output is locked to 7FFFFF for 24-bit

mode.

case

negative

saturation

(CHn+ - CHn- < -VREF/1.5), the output code is locked

to 800000 for 24-bit mode.

Equation 5-3 is only true for DC inputs. For AC inputs,

this transfer function needs to be multiplied by the

transfer function of the SINC3+SINC1 filter (see

Equation 5-1 and Equation 5-3).

EQUATION 5-3:

For other than the default 24-bit data formats,

Equation 5-3 should be multiplied by a scaling factor

depending on the data format used (defined by

WIDTH_DATA<1:0>). The data format and associated

scaling factors are given in Figure 5-5.

FIGURE 5-5:

Output Data Formats.

The ADC resolution is a function of the OSR

(Section 5.4 “SINC3 + SINC1 Filter”). The resolution

is the same for all channels. No matter what the

resolution is, the ADC output data is always calculated

in 24-bit words, with added zeros at the end if the OSR

is not large enough to produce 24-bit resolution (left

justification).

DATA_CHn

–



REF+

REF-

–

-----------------------------------------







8,388,608

1.5



For 24-bit Mode, WIDTH_Data<1:0> = 01(Default)

WIDTH_DATA<1:0> =

32-bit with sign extension

DATA

<23>

DATA

<23:16>

DATA

<15:8>

DATA

<7:0>

WIDTH_DATA<1:0> =

32-bit with zeros padded

0x00

DATA

<23:16>

DATA

<7:0>

DATA

<15:8>

WIDTH_DATA<1:0> =

24-bit

DATA

<23:16>

DATA

<15:8>

DATA

<7:0>

WIDTH_DATA<1:0> =

16-bit

DATA

<15:8>

DATA

<23:16>

Rounded

Unformatted ADC data

DATA

<23:16>

DATA

<15:8>

DATA

<7:0>

x1/256

x256

Scaling

Factor

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