AD7002
from the analog modulator, will the output data be correct. The
analog modulator, on coming out of sleep, will generate mean-
ingful data after 21 master clock cycles.
convenient or necessary. Only the digital result following the fall
of CAL will be loaded into each offset register. After CAL falls,
normal operation resumes immediately.
10.00
0.00
01...111
01...110
–10.00
–20.00
–30.00
–40.00
–50.00
–60.00
–70.00
00...001
00...000
11...111
–80.00
–90.00
11...110
–100.00
–110.00
–120.00
–130.00
–140.00
10...001
10...000
0
100 200 300 400
600 700 800
500
900 1000
–V
V
+V
FULLSCALE
FULLSCALE
REF
FREQUENCY – kHz
V
, INPUT VOLTAGE
IN
Figure 14. Digital Filter Frequency Response
Figure 13. ADC Transfer Function for I and Q Receive
Channels
The offset registers are static and retain their contents even
during sleep mode (Rx SLEEP1 and Rx SLEEP2 high). They
need only be updated if drifts in the analog dc offsets are experi-
enced or expected. However, on initial application of power to
the digital supply pins the offset registers may contain grossly
incorrect values and, therefore, calibration must be activated at
least once after power is applied even if the facility of calibration
is not regularly used.
Calibration
Included in the digital filter is a means by which receive signal
offsets may be calibrated out. Calibration can be effected
through the use of the CAL and MZERO pins.
Each channel of the digital low-pass filter section has an offset
register. The offset register can be made to contain a value
representing the dc offset of the preceding analog circuitry. In
normal operation, the value stored in the offset register is sub-
tracted from the filter output data before the data appears on
the serial output pin. By so doing, the dc offset is cancelled.
Table II. Truth Table for the MODE and RATE Pins
MODE RATE
Data Format
Output Word Rate
In each channel the offset register is cleared (twos complement
zero) when CAL is high and becomes loaded with the first digi-
tal filter result after CAL falls. This result will be a measure of
the channel dc offset if the analog channel is switched to zero
prior to CAL falling. Time must be provided for the analog
circuitry and the digital filter to settle after the analog circuitry is
switched to zero and before CAL falls. The offset register will
then be loaded with the proper representation of the dc offset.
0
0
1
1
0
1
0
1
IQ Data I/Q
IQ Data I/Q
270.8 kHz
541.7 kHz
270.8 kHz
541.7 kHz
I Data
I Data
Q Data
Q Data
The MZERO pin can be used to zero the sigma-delta modula-
tors if calibration of preceding analog circuitry is not required.
Each analog modulator has an internal analog multiplexer con-
trolled by MZERO. With MZERO low, the modulator inputs
are connected to the I Rx and Q Rx pins for normal operation.
With MZERO high, both modulator inputs are connected to the
CAL must be high for more than 608 master clock cycles
(CLK1, CLK2). If the analog channels are switched to zero
coincident with CAL rising, this time is also sufficient to satisfy
the settling time of the analog sigma-delta modulators and the
digital filters. CAL may be held high for an unlimited time if
V
REF pin, which is analog ground for the modulators. If calibra-
tion of external analog circuitry is desired, MZERO should be
kept low during the calibration cycle.
Rx SLEEP1
Rx SLEEP2
t38
t39
CAL
t40
RATE, MODE,
THREE- STATE
CONTROL
Figure 15. Calibration and Control Timing Diagram
REV. B
–11–
ADI [ ADI ]
