ADuC7023BCBZ62I-R7 datasheet(34/96 Pages) AD

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ADuC7023

Data Sheet

| Page 34 of 96

The C1 capacitors in Figure 27 are typically 4 pF and can be

primarily attributed to pin capacitance. The resistors are

lumped components made up of the on resistance of the

switches. The value of these resistors is typically about 100 Ω.

The C2 capacitors are the ADC sampling capacitors and

typically have a capacitance of 16 pF.

AVDD

R1 C2

AVDD

R1 C2

Figure 27. Equivalent Analog Input Circuit Conversion Phase: Switches Open,

Track Phase: Switches Closed

For ac applications, removing high frequency components from

the analog input signal is recommended by using an RC low-

pass filter on the relevant analog input pins. In applications

where harmonic distortion and signal-to-noise ratio are critical,

the analog input should be driven from a low impedance

source. Large source impedances significantly affect the ac

performance of the ADC. This can necessitate the use of an

input buffer amplifier. The choice of the op amp is a function of

the particular application. Figure 28 and Figure 29 give an

example of an ADC front end.

ADuC7023

ADC0

10Ω

0.01µF

Figure 28. Buffering Single-Ended Differential Input

ADuC7023

ADC0

VREF

ADC1

Figure 29. Buffering Differential Inputs

When no amplifier is used to drive the analog input, limit the

source impedance to values lower than 1 kΩ. The maximum

source impedance depends on the amount of total harmonic

distortion (THD) that can be tolerated. The THD increases as

the source impedance increases and the performance degrades.

DRIVING THE ANALOG INPUTS

Internal or external references can be used for the ADC. When

operating in differential mode, there are restrictions on the

common-mode input signal (VCM), which is dependent upon

the reference value and supply voltage used to ensure that the

signal remains within the supply rails. Table 27 gives some

calculated VCM minimum and VCM maximum values.

Table 27. VCM Ranges

AVDD

VREF

VCM Min

VCM Max

Signal Peak-to-Peak

3.3 V

2.5 V

1.25 V

2.05 V

2.5 V

2.048 V

1.024 V

2.276 V

2.048 V

1.25 V

0.75 V

2.55 V

1.25 V

3.0 V

2.5 V

1.25 V

1.75 V

2.5 V

2.048 V

1.024 V

1.976 V

2.048 V

1.25 V

0.75 V

2.25 V

1.25 V

CALIBRATION

By default, the factory-set values written to the ADC offset

(ADCOF) and gain coefficient registers (ADCGN) yield

optimum performance in terms of endpoint errors and linearity

for standalone operation of the part (see the Specifications

section). If system calibration is required, it is possible to

modify the default offset and gain coefficients to improve

endpoint errors, but note that any modification to the factory-

set ADCOF and ADCGN values can degrade ADC linearity

performance.

For system offset error correction, the ADC channel input stage

must be tied to AGND. A continuous software ADC conversion

loop must be implemented by modifying the value in ADCOF until

the ADC result (ADCDAT) reads Code 0 to Code 1. If the

ADCDAT value is greater than 1, ADCOF should be decremented

until ADCDAT reads Code 0 to Code 1. Offset error correction

is done digitally and has a resolution of 0.25 LSB and a range of

±3.125% of VREF.

For system gain error correction, the ADC channel input

stage must be tied to VREF. A continuous software ADC

conversion loop must be implemented to modify the value

in ADCGN until the ADCDAT reads Code 4094 to Code 4095.

If the ADCDAT value is less than 4094, ADCGN should be

incremented until ADCDAT reads Code 4094 to Code 4095.

Similar to the offset calibration, the gain calibration resolution

is 0.25 LSB with a range of ±3% of VREF.

TEMPERATURE SENSOR

The ADuC7023 provides a voltage output from an on-chip

band gap reference that is proportional to absolute temperature.

This voltage output can also be routed through the front-end

ADC multiplexer (effectively an additional ADC channel

input), facilitating an internal temperature sensor channel,

measuring die temperature.

An ADC temperature sensor conversion differs from a standard

ADC voltage. The ADC performance specifications do not

apply to the temperature sensor.

Chopping of the internal amplifier should be enabled using the

TSCON register. To enable this mode, the user must set Bit 0 of

TSCON. The user must also take two consecutive ADC readings

and average them in this mode.

Rev. E

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