Datasheet

Data Sheet AD7264
Rev. B | Page 25 of 28
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 t
12
before
CS
goes low. If the
CS
pin goes low before t
12
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 t
12
and t
11
have been adhered to, the
calibration is complete after the 19
th
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.
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 AV
CC
/2
Bits
,
which is 5/16,384 or 305 μV for the AD7264. The maximum
removable offset voltage is given by
Gain
μV305
LSB500 ×±
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
06732-035
CS
SCLK
CAL
201 2 3 2119
t
2
t
6
t
8
t
7
t
12
t
11
3332212019321
Figure 36. Calibration Timing Diagram