2
AD7711A
–13–REV. D
The AD7711A provides a number of calibration options that
can be programmed via the on-chip control register. A calibra-
tion cycle can be initiated at any time by writing to this control
register. The part can perform self-calibration, using the on-chip
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
performs self-calibration and updates the calibration coeffi-
cients. Once the part is in this mode, the user does not have to
worry about issuing periodic calibration commands to the device
or ask the device to recalibrate when there is a change in the
ambient temperature or power supply voltage.
The AD7711A 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 E
2
PROM. This gives
the microprocessor much greater control over the AD7711A’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 E
2
PROM.
The AD7711A can be operated in single-supply systems pro-
vided that the analog input voltage does not go more negative
than –30 mV. For larger bipolar signals, a V
SS
of –5 V is required
by the part. For battery operation, the AD7711A also offers a
software programmable standby mode that reduces idle power
consumption to 7 mW typically.
THEORY OF OPERATION
The general block diagram of a sigma-delta ADC is shown in
Figure 4. It contains the following elements:
•
A sample-hold amplifier
•
A differential amplifier or subtracter
•
An analog low-pass filter
•
A 1-bit A/D converter (comparator)
•
A 1-bit DAC
•
A digital low-pass filter
ANALOG
LOW-PASS
FILTER
DIGITAL
FILTER
DIGITAL
DATA
+
–
S/H AMP
DAC
COMPARATOR
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).
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
1-bit ADC or comparator yields an SNR of 7.78 dB.
The AD7711A samples the input signal at a frequency of
19.5 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 performance 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
comparator. 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 deriva-
tion of the alternative name for these devices: charge-balancing
ADCs.
DAC
COMPARATOR
+FS
–FS
INTEGRATOR
DIFFERENTIAL
AMPLIFIER
V
IN
e
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 AD7711A uses a second-order sigma-delta modulator and
a digital filter that provides a rolling average of the sampled
output. 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.
