EVAL-CED1Z View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
EVAL-CED1Z
Analog Devices
'EVAL-CED1Z' PDF : 29 Pages View PDF
CALIBRATION
INTERNAL OFFSET CALIBRATION
The AD7264 allows the user to calibrate the offset of the device
using the CAL pin. This is achieved by setting the CAL pin to a
high logic level, which initiates a calibration on the next CS
falling edge. The calibration requires one full conversion cycle,
which contains a CS falling edge followed by 19 SCLK cycles.
The CAL pin can remain high for more than one conversion, if
desired, and the AD7264 continues to calibrate.
The CAL pin should be driven high only when the CS pin is
high or after 19 SCLK cycles have elapsed when CS is low, that
is, between conversions. The CAL pin must be driven high t12
before CS goes low. If the CS pin goes low before t12 elapses, the
calibration result will be inaccurate for the current conversion;
if the CAL pin remains high, the subsequent calibration conver-
sion is correct. If the CAL pin is set to a logic high state during a
conversion, that conversion result is corrupted.
If the CAL pin has been held high for a minimum of one
conversion and when t12 and t11 have been adhered to, the
calibration is complete after the 19th SCLK cycle and the CAL
pin can be driven to a logic low state. The next CS falling edge
after the CAL pin has been driven to a low logic state initiates
a conversion of the differential analog input signal for both
ADC A and ADC B.
Alternatively, the control register can be used to initiate an
offset calibration. This is done by setting the CAL bit in the
control register to 1. The calibration is then initiated on the next
CS falling edge, but the current conversion is corrupted. The
ADCs on the AD7264 must remain fully powered up to
complete the internal calibration.
AD7264
The AD7264 registers store the offset value, which can easily be
accessed by the user (see the Reading from a Register section).
When the device is calibrating, the differential analog inputs
for each respective ADC are shorted together internally and a
conversion is performed. A digital code representing the offset is
stored internally in the offset registers, and subsequent conver-
sion results have this measured offset removed.
When the AD7264 is calibrated, the calibration results stored in
the internal device registers are relevant only for the particular
PGA gain selected at the time of calibration. If the PGA gain is
changed, the AD7264 must be recalibrated. If the device is not
recalibrated when the PGA gain is changed, the offset for the
previous gain setting continues to be removed from the digital
output code, which may lead to inaccuracies.
The offset range that can be calibrated for is ±500 LSB at a gain
of 1. The maximum offset voltage that can be calibrated for is
reduced as the gain of the PGA is increased.
Table 12 details the maximum offset voltage that can be
removed by the AD7264 without compromising the available
digital output code range. The least significant bit size is
AVCC/2Bits, which is 5/16,384 or 305 μV for the AD7264. The
maximum removable offset voltage is given by
± 500 LSB × 305 μV
Gain
Table 12. Offset Voltage Range
Gain
Maximum Removable Offset Voltage
1
±152.5 mV
2
±76.25 mV
3
±50.83 mV
32
±4.765 mV
CAL
CS
SCLK
t11
t12
t8
t2
1
2
t6
3
19
20
21
32
33
t7
1
2
3
Figure 36. Calibration Timing Diagram
19
20
21
Rev. A | Page 25 of 32
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