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参数资料
| 型号： | EVAL-AD7722CBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 11/24页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD7722CBZ |
| 标准包装： | 1 |
| ADC 的数量： | 1 |
| 位数： | 16 |
| 采样率（每秒）： | 220k |
| 数据接口： | 串行，并联 |
| 输入范围： | ±VREF/2 |
| 在以下条件下的电源（标准）： | 375mW @ 220kSPS |
| 工作温度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7722 |
| 已供物品： | 板，CD |
REV. B
AD7722
–19–
Offset and Gain Calibration
A calibration of offset and gain errors can be performed in both
serial and parallel modes by initiating a calibration cycle. During
this cycle, offset and gain registers in the filter are loaded with
values representing the dc offset of the analog modulator and a
modulator gain correction factor. The correction factors are
determined by an on-chip microcontroller measuring the conver-
sion results for three different input conditions: minus full scale
(–FS), plus full scale (+FS), and midscale. In normal operation,
the offset register is subtracted from the digital filter output and
the result is multiplied by the gain correction factor to obtain an
offset and gain corrected final result.
The calibration cycle is controlled by internal logic, and the user
need only initiate the cycle. A calibration is initiated when the
rising edge of CLKIN senses a high level on the CAL input.
There is an uncertainty of up to 64 CLKIN cycles before the
calibration cycle actually begins because the current conversion
must complete before calibration commences. The calibration
values loaded into the registers only apply for the particular
analog input mode (bipolar/unipolar) selected when initiating
the calibration cycle. On changing to a different analog input
mode, a new calibration must be performed.
During the calibration cycle, in unipolar mode, the offset of the
analog modulator is evaluated; the differential inputs to the
modulator are shorted internally to AGND. Once calibration
begins, DVAL goes low and
DRDY goes high, indicating there
is invalid data in the output register. After 8192 CLKIN cycles,
when the modulator and digital filter settle, the average of eight
output results (512 CLKIN cycles) is calculated and stored in
the offset register. In unipolar mode, this result also represents
minus full scale, required to calculate the gain correction factor.
The gain correction factor can then be determined by internally
switching the inputs to +FS (VREF2). The positive input of the
modulator is switched to the reference voltage and the negative
input to AGND. Again, when the modulator and digital filter
settle, the average of the eight output results is used to calculate
the gain correction factor. DVAL goes high whenever a calcula-
tion is performed on the average of eight conversion results
(512 CLKIN cycles) and then returns low. See Figure 8.
In bipolar mode, an additional measurement is required since
zero scale is not the same as –FS. Therefore, calibration in
bipolar mode requires an additional (512 + 8192) CLKIN
cycles. Zero scale is similarly determined by shorting both
analog inputs to AGND. Then the inputs are internally
reconfigured to apply +FS and –FS (+VREF2/2 and –VREF2/2)
to determine the gain correction factor.
After the calibration registers have been loaded with new values,
the inputs of the modulator are switched back to the input pins.
However, correct data is available at the interface only after the
modulator and filter have settled to the new input values.
Should the part see a rising edge on the SYNC or RESET pin
during a calibration cycle, the calibration cycle is discontinued,
and a synchronization operation or reset will be performed.
The calibration registers are static. They need to be updated
only if unacceptable drifts in analog offsets or gain are expected.
After power-up, a RESET is not mandatory since power-on reset
circuitry clears the offset and gain registers. Care must be taken
to ensure that the CAL pin is held low during power-up. Before
initiating a calibration routine, ensure that the supplies and
reference input have settled, and that the voltage on the analog
input pins is between the supply voltages.
DATA INTERFACING
The AD7722 offers a choice of serial or parallel data interface
options to meet the requirements of a variety of system configu-
rations. In parallel mode, multiple AD7722s can be easily
configured to share a common data bus. Serial mode is ideal
when it is required to minimize the number of data interface
lines connected to a host processor. In either case, careful
attention to the system configuration is required to realize the
high dynamic range available with the AD7722. Consult the
recommendations in the Power Supply Grounding and Layout
section. The following recommendations for parallel interfacing
also apply for the system design in serial mode.
Parallel Interface
When using the AD7722, place a buffer/latch adjacent to the
converter to isolate the converter’s data lines from any noise
that may be on the data bus. Even though the AD7722 has
three-state outputs, use of an isolation latch represents good
design practice. This arrangement will inject a small amount of
digital noise on the AD7722 ground plane; these currents
should be quite small and can be minimized by ensuring that
the converter input/output does not drive a large fanout (they
normally can’t by design). Minimizing the fanout on the
AD7722’s digital port will also keep the converter logic transi-
tions relatively free from ringing and thereby minimize any
potential coupling into the analog port of the converter.
The simplified diagram (Figure 23) shows how the parallel
interface of the AD7722 can be configured to interface with the
system data bus of a microprocessor or a modern microcontroller,
such as the MC68HC16 or 8xC251.
AD7722
ADDR
DECODE
DB0–DB15
DRDY
CS
RD
16
74xx16374
OR
74xx16244
OE
D0–D15
RD
INTERRUPT
ADDR
DSP/C
Figure 23. Parallel Interface Connection
With
CS and RD tied permanently low, the data output bits are
always active. When the
DRDY output goes high for two CLKIN
cycles, the rising edge of
DRDY is used to latch the conversion
data before a new conversion result is loaded into the output
data register. The falling edge of
DRDY then sends an appro-
priate interrupt signal for interface control. Alternatively if buffers
are used instead of latches, the falling edge of
DRDY provides
the necessary interrupt when a new output word is available
from the AD7722.
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