MCP3901A2T-ISLASHSS datasheet(29/60 Pages) MICROCHIP

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DS22192A-page 29

MCP3901

5.7

Voltage Reference

5.7.1

INTERNAL VOLTAGE REFERENCE

The MCP3901 contains an internal voltage reference

source specially designed to minimize drift over

temperature. In order to enable the internal voltage

reference, the VREFEXT bit in the configuration

VREF supplies reference voltage to both channels. The

typical value of this voltage reference is 2.37V ±2%.

The internal reference has a very low typical

temperature coefficient of ±12 ppm/°C, allowing the

output codes to have minimal variation with respect to

temperature since they are proportional to (1/VREF).

The noise of the internal voltage reference is low

enough not to significantly degrade the SNR of the

ADC if compared to a precision external low-noise

voltage reference.

The output pin for the internal voltage reference is

REFIN+/OUT.

When the internal voltage reference is enabled,

REFIN- pin should always be connected to AGND.

For

optimal

ADC

accuracy,

appropriate

bypass

capacitors should be placed between REFIN+/OUT

and AGND. De-coupling at the sampling frequency,

around 1 MHz, is important for any noise around this

frequency will be aliased back into the conversion data.

0.1 µF ceramic and 10 µF tantalum capacitors are

recommended.

These bypass capacitors are not mandatory for correct

ADC operation, but removing these capacitors may

degrade accuracy of the ADC. The bypass capacitors

also help for applications where the voltage reference

output is connected to other circuits. In this case,

additional buffering may be needed as the output drive

capability of this output is low.

5.7.2

DIFFERENTIAL EXTERNAL

VOLTAGE INPUTS

When the VREFEXT bit is high, the two reference pins

(REFIN+/OUT, REFIN-) become a differential voltage

reference input. The voltage at the REFIN+/OUT is

noted VREF+ and the voltage at the REFIN- pin is noted

VREF-. The differential voltage input value is given by

the following equation:

VREF=VREF+ - VREF-

The specified VREF range is from 2.2V to 2.6V. The

REFIN- pin voltage (VREF-) should be limited to +/-0.3V.

Typically, for single-ended reference applications, the

REFIN- pin should be directly connected to AGND.

TABLE 5-6:

OSR = 64 OUTPUT CODE EXAMPLES

ADC Output code (MSB First)

Hexadecimal

Decimal

20-bit resolution

0111

1111

1 111

11 11

0 0 0 0

0x7FFFF0

+ 524, 287

0111

1111

1 111

11 10

0 0 0 0

0x7FFFE0

+ 524, 286

0000

0 000

00 00

0 0 0 0

0x000000

1111

1 111

11 11

0 0 0 0

0xFFFFF0

-1

1000

0000

0 000

00 01

0 0 0 0

0x800010

- 524,287

1000

0000

0 000

00 00

0 0 0 0

0x800000

- 524, 288

TABLE 5-7:

OSR = 32 OUTPUT CODE EXAMPLES

ADC Output code (MSB First)

Hexadecimal

Decimal

17-bit resolution

0111

1111

11 11

1111

1 0 0 0

0 0 0 0

0x7FFF80

+ 65, 535

0111

1111

11 11

1111

0 0 0 0

0x7FFF00

+ 65, 534

0000

00 00

0000

0 0 0 0

0x000000

1111

11 11

1111

1 0 0 0

0 0 0 0

0xFFFF80

-1

1000

0000

00 00

0000

1 0 0 0

0 0 0 0

0x800080

- 65,535

1000

0000

00 00

0000

0 0 0 0

0x800000

- 65, 536

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