AD7712 datasheet(13/28 Pages) AD

13 / 28 page

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REV. E

AD7712

Oversampling is fundamental to the operation of sigma-delta

ADCs. Using the quantization noise formula for an ADC:

SNR = (6.02

× number of bits + 1.76) dB,

a 1-bit ADC or comparator yields an SNR of 7.78 dB.

The AD7712 samples the input signal at a frequency of 39 kHz or

greater (see Table III). As a result, the quantization noise is

spread over a much wider frequency than that of the band of

interest. The noise in the band of interest is reduced still further

by analog filtering in the modulator loop, which shapes the

quantization noise spectrum to move most of the noise energy to

frequencies outside the bandwidth of interest. The noise perfor-

mance is thus improved from this 1-bit level to the performance

outlined in Tables I and II and in Figure 2.

The output of the comparator provides the digital input for the

1-bit DAC, so that the system functions as a negative feedback

loop that tries to minimize the difference signal. The digital data

that represents the analog input voltage is contained in the duty

cycle of the pulse train appearing at the output of the compara-

tor. It can be retrieved as a parallel binary data word using a

digital filter.

Sigma-delta ADCs are generally described by the order of the

analog low-pass filter. A simple example of a first order sigma-

delta ADC is shown in Figure 5. This contains only a first order

low-pass filter or integrator. It also illustrates the derivation of

the alternative name for these devices: Charge Balancing ADCs.

VIN

DIFFERENTIAL

AMPLIFIER

COMPARATOR

–FS

DAC

+FS

Figure 5. Basic Charge-Balancing ADC

It consists of a differential amplifier (whose output is the differ-

ence between the analog input and the output of a 1-bit DAC),

an integrator and a comparator. The term charge balancing,

comes from the fact that this system is a negative feedback loop

that tries to keep the net charge on the integrator capacitor at

zero by balancing charge injected by the input voltage with

charge injected by the 1-bit DAC. When the analog input is

zero, the only contribution to the integrator output comes from

the 1-bit DAC. For the net charge on the integrator capacitor to

be zero, the DAC output must spend half its time at +FS and

half its time at –FS. Assuming ideal components, the duty cycle

of the comparator will be 50%.

When a positive analog input is applied, the output of the 1-bit

DAC must spend a larger proportion of the time at +FS, so the

duty cycle of the comparator increases. When a negative input

voltage is applied, the duty cycle decreases.

The AD7712 uses a second-order sigma-delta modulator and a

digital filter that provides a rolling average of the sampled out-

put. After power-up, or if there is a step change in the input

voltage, there is a settling time that must elapse before valid

data is obtained.

The AD7712 provides a number of calibration options which

can be programmed via the on-chip control register. A calibra-

tion cycle may be initiated at any time by writing to this control

calibration microcontroller and SRAM to store calibration

parameters. Other system components may also be included in

the calibration loop to remove offset and gain errors in the input

channel using the system calibration mode. Another option is a

background calibration mode where the part continuously per-

forms self-calibration and updates the calibration coefficients.

Once the part is in this mode, the user does not have to worry

about issuing periodic calibration commands to the device or

asking the device to recalibrate when there is a change in the

ambient temperature or power supply voltage.

The AD7712 gives the user access to the on-chip calibration

registers allowing the microprocessor to read the device’s cali-

bration coefficients and also to write its own calibration coeffi-

cients to the part from prestored values in E2PROM. This gives

the microprocessor much greater control over the AD7712’s

calibration procedure. It also means that the user can verify that

the device has performed its calibration correctly by comparing the

coefficients after calibration with prestored values in E2PROM.

The AD7712 can be operated in single supply systems provided

that the analog input voltage on the AIN1 input does not go

more negative than –30 mV. For larger bipolar signals on the

AIN1 input, a VSS of –5 V is required by the part. For battery

operation or low power systems, the AD7712 offers a standby

mode (controlled by the

STANDBY pin) that reduces idle

power consumption to typically 100

µW.

THEORY OF OPERATION

The general block diagram of a sigma-delta ADC is shown in

Figure 4. It contains the following elements:

1. A sample-hold amplifier.

2. A differential amplifier or subtracter.

3. An analog low-pass filter.

4. A 1-bit A/D converter (comparator).

5. A 1-bit DAC.

6. A digital low-pass filter.

ANALOG

LOW-PASS

FILTER

COMPARATOR

DIGITAL DATA

DAC

S/H AMP

DIGITAL

FILTER

Figure 4. General Sigma-Delta ADC

In operation, the analog signal sample is fed to the subtracter,

along with the output of the 1-bit DAC. The filtered difference

signal is fed to the comparator, whose output samples the differ-

ence signal at a frequency many times that of the analog signal

sampling frequency (oversampling).

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