AD9221/AD9223/AD9220
The calibration procedure consists of a two step process. First,
the bipolar offset is calibrated by selecting CH2, the 2.5 V sys-
tem reference, of the analog multiplexer and preloading the
DAC, U5, with a midscale code of 1000 0000. If possible, sev-
eral readings of the A/D should be taken and averaged to deter-
mine the required digital offset adjustment code, U5. This
averaged offset code requires an extra bit of resolution since 1
LSB of U5 equates to 1/2 LSB of the AD9221/AD9223/AD9220.
The required offset correction code to U5 can then be deter-
mined. Second, the system gain is calibrated by selecting CH2,
a 1.25 V input which corresponds to –FS of the A/D. Before the
value is read, U4 should be preloaded with a code of 00 (Hex).
Several readings can also be taken and averaged to determine
the digital gain adjustment code to U2A. In this case, 1 LSB of
the A/D corresponds to 1 LSB of U4.
Referring to Figure 61, the AD9221/AD9223/AD9220 is config-
ured for single-ended operation with a 2.5 V p-p input span and
a 2.5 V common-mode voltage using an external, precision 2.5
voltage reference, U1. This configuration and input span allows
the buffer amplifier, U4, to be single supply. Also, it simplifies
the design of the low temperature drift autocalibration circuitry
which uses thin-film resistors for temperature stability and ratio-
metric accuracy. The input of the AD9221/AD9223/AD9220
can be easily configured for a wider span but it should remain
within the input/output swing capabilities of a high speed, rail-
to-rail, single-supply amplifier, U4 (e.g., AD8041).
The gain and offset calibration circuitry is based on two 8-bit,
current-output DAC08s, U3 and U5. The gain calibration
circuitry consisting of U3, and an op amp, U2A, is configured
to provide a low drift nominal 1.25 V reference to the AD9221/
AD9223/AD9220. The resistor values which set the gain cali-
bration range were selected to provide a nominal adjustment
span of ±128 LSBs with 1 LSB resolution with respect to the
A/D. Note that the bandwidth of the reference is low and, as a
result, it is not possible to change the reference voltage rapidly
in this mode.
Due to the AD9221/AD9223/AD9220’s excellent INL perfor-
mance, a two-point calibration procedure (i.e., –FS to midscale)
instead of an endpoint calibration procedure was chosen. Also,
since the bipolar offset is insensitive to any gain adjustment (due
to the differential SHA of the A/D), an iterative calibration
process is not required. The temperature stability of the circuit
is enhanced by selecting a dual precision op amp for U2 (e.g.,
OP293) and low temperature drift, thin film resistors. Note
that this application circuit was not built at the release of this
data sheet. Please consult Analog Devices for application assis-
tance or comments.
The offset calibration circuitry consists of a DAC, U5 and the
buffer amplifier, U4. The DAC is configured for a bipolar ad-
justment span of ±64 LSB with a 1/2 LSB resolution span with
respect to the AD9221/AD9223/AD9220. Note that both cur-
rent outputs of U5 were configured to provide a bipolar adjust-
ment span. Also, RC is used to decouple the output of both
DACs, U3 and U5, from their respective op amps.
0.1F
1.25k⍀
1.25V
162⍀
2.5k⍀
U2B
2.5k⍀
0.1F
1.25V
؎39mV
U2A
2.5k⍀
0.1F
2.5k⍀
1.1k⍀
U1
REF43
0.1F
10F
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH8
+5V
SENSE
VREF
R
100⍀
2.5k⍀
C
IOUT
VREF(+)
VREF(–)
OUT
U6
ADG608
U3
DAC08
AD9221/
AD9223/
AD9220
IOUT
BIT 1 – BIT 12
OTR
2.5k⍀
39⍀
VINA
U4
39⍀
2.50V
VINB
R
100⍀
R
C
100⍀
C
2.5k⍀
VREF(+)
VREF(–)
IOUT
U5
DAC08
IOUT
2.5k⍀
Figure 61. Typical Multichannel Data Acquisition System
–23–
REV. D
ADI [ ADI ]
