EVAL-ADUC831QSZ Datasheet, PDF(23/76 Page) Analog Devices – MicroConverter®, 12-Bit ADCs and DACs with Embedded 62 kBytes Flash MCU

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EVAL-ADUC831QSZ Datasheet, PDF (23/76 Pages) Analog Devices – MicroConverter®, 12-Bit ADCs and DACs with Embedded 62 kBytes Flash MCU

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ADuC831

Keep in mind that the ADCâs transfer function is 0 to VREF, and

any signal range lost to amplifier saturation near ground will

impact dynamic range. Though the op amps in Table VI are

capable of delivering output signals very closely approaching

ground, no amplifier can deliver signals all the way to ground when

available, you might consider using it to power the front end

amplifiers. If you do, however, be sure to include the Schottky

diodes shown in Figure 10 (or at least the lower of the two

diodes) to protect the analog input from undervoltage condi-

tions. To summarize this section, use the circuit of Figure 10 to

drive the analog input pins of the ADuC831.

Voltage Reference Connections

The on-chip 2.5 V band gap voltage reference can be used as the

reference source for the ADC and DACs. To ensure the accu-

racy of the voltage reference, you must decouple the VREF pin to

ground with a 0.1 ÂµF capacitor and the CREF pin to ground with

a 0.1 ÂµF capacitor as shown in Figure 11.

ADuC831

51â

2.5V

BAND GAP

REFERENCE

VREF

0.1â®F

BUFFER

CREF

0.1â®F

BUFFER

Figure 11. Decoupling VREF and CREF

If the internal voltage reference is to be used as a reference for

external circuitry, the CREF output should be used. However, a

buffer must be used in this case to ensure that no current is

drawn from the CREF pin itself. The voltage on the CREF pin is

that of an internal node within the buffer block, and its voltage

is critical to ADC and DAC accuracy. On the ADuC812 VREF

was the recommended output for the external reference; this

can be used but it should be noted that there will be a gain error

between this reference and that of the ADC.

The ADuC831 powers up with its internal voltage reference in

the âonâ state. This is available at the VREF pin, but as noted

before there will be a gain error between this and that of the

ADC. The CREF output becomes available when the ADC is

powered up.

If an external voltage reference is preferred, it should be

connected to the VREF and CREF pins as shown in Figure 12.

Bit 6 of the ADCCON1 SFR must be set to 1 to switch in the

external reference voltage.

To ensure accurate ADC operation, the voltage applied to VREF

must be between 1 V and AVDD. In situations where analog

input signals are proportional to the power supply (such as some

strain gage applications) it may be desirable to connect the CREF

and VREF pins directly to AVDD.

Operation of the ADC or DACs with a reference voltage below

1 V, however, may incur loss of accuracy eventually resulting in

missing codes or non-monotonicity. For that reason, do not use

a reference voltage less than 1 V.

VDD

EXTERNAL

VOLTAGE

REFERENCE

VREF

0.1â®F

CREF

0.1â®F

ADuC831

51â

2.5V

BAND GAP

REFERENCE

"0" = INTERNAL

"1" = EXTERNAL

BUFFER

ADCCON1.6

Figure 12. Using an External Voltage Reference

To maintain compatibility with the ADuC812, the external

reference can also be connected to the VREF pin as shown in

Figure 13, to overdrive the internal reference. Note this intro-

duces a gain error for the ADC that has to be calibrated out,

thus the previous method is the recommended one for most

users. For this method to work, ADCCON1.6 should be config-

ured to use the internal reference. The external reference will

then overdrive this.

VDD

EXTERNAL

VOLTAGE

REFERENCE

0.1â®F

VREF

0.1â®F

CREF

ADuC831

51â

2.5V

BAND GAP

REFERENCE

BUFFER

Figure 13. Using an External Voltage Reference

REV. 0

â23â

▷