MCP3901 datasheet(29/60 Pages) MICROCHIP

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DS22192C-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, the

REFIN- pin should always be connected to AGND.

For

optimal

ADC

accuracy,

appropriate

bypass

capacitors should be placed between REFIN+/OUT

and AGND. Decoupling 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 the accuracy of the ADC. The bypass capaci-

tors also help the 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 pin is

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

noted as VREF-. The differential voltage input value is

given by the following equation:

EQUATION 5-4:

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

01 11

1 111

1 1 1 1

0 0 0 0

0x7FFFF0

+ 524, 287

01 11

1 111

1 1 1 1

1 1 1 0

0 0 0 0

0x7FFFE0

+ 524, 286

00 00

0 000

0 0 0 0

0x000000

11 11

1 111

1 1 1 1

0 0 0 0

0xFFFFF0

-1

10 00

0 000

0 0 0 0

0 0 0 1

0 0 0 0

0x800010

- 524,287

10 00

0 000

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

1 0 0 0

0 0 0 0

0x7FFF80

+ 65, 535

0111

1111

0 0 0 0

0x7FFF00

+ 65, 534

0000

0 0 0 0

0x000000

1111

1 0 0 0

0 0 0 0

0xFFFF80

-1

1000

0000

1 0 0 0

0 0 0 0

0x800080

- 65,535

1000

0000

0 0 0 0

0x800000

- 65, 536

VREF = VREF+ – VREF-

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