ad22304 Analog Devices, Inc., ad22304 Datasheet - Page 10

• Manufacturer Part Number: ad22304
• Description: ?80?/s Single Chip Yaw Rate Gyro With Signal Conditioning
• Manufacturer: Analog Devices, Inc.

AD22304
precise. Lower frequency is preferable, but its variability usu-
ally requires it to be about 10 times greater (in order to preserve
phase integrity) than the well-controlled output pole. In gen-
eral, both –3 dB filter frequencies should be set as low as pos-
sible to reduce the amplitude of these high frequency artifacts
as well as to reduce the overall system noise.
INCREASING MEASUREMENT RANGE
The full-scale measurement range of the AD22304 can be in-
creased by placing an external resistor between the
RATEOUT (1B, 2A) and SUMJ (1C, 2C) pins, which would
parallel the internal R
180 k . For example, a 330 k external resistor will give
approximately 8.1 mV/°/sec sensitivity and a commensurate
~50% increase in the full-scale range. This is effective for up to
a 4× increase in the full-scale range (minimum value of the
parallel resistor allowed is 45 k ). Beyond this amount of ex-
ternal sensitivity reduction, the internal circuitry headroom
requirements prevent further increase in the linear full-scale
output range. The drawbacks of modifying the full-scale range
are the
additional output null drift (as much as 2°/sec over tempera-
ture) and the readjustment of the initial null bias (see the Null
Adjustment section).
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyros to
improve their overall accuracy. The AD22304 has a tempera-
ture-proportional voltage output that provides input to such a
calibration method. The voltage at TEMP (3F, 3G) is nominally
2.5 V at 27°C and has a PTAT (proportional to absolute tem-
perature) characteristic of 8.4 mV/°C. Note that the TEMP
output circuitry is limited to 50 µA source current.
Using a 3-point calibration technique, it is possible to calibrate
the AD22304’s null and sensitivity drift to an overall accuracy
of nearly 300°/hour. An overall accuracy of 70°/hour or better
is possible using more points. Limiting the bandwidth of the
device reduces the flat-band noise during the calibration process,
improving the measurement accuracy at each calibration point.
USING THE AD22304 WITH A
SUPPLY-RATIOMETRIC ADC
The AD22304’s RATEOUT signal is nonratiometric, i.e., nei-
ther the null voltage nor the rate sensitivity is proportional to
the supply. Instead they are nominally constant for dc supply
changes within the 4.75 V to 5.25 V operating range. If the
AD22304 is used with a supply-ratiometric ADC, the
AD22304’s 2.5 V output can be converted and used to make
corrections in software for the supply variations.
NULL ADJUSTMENT
Null adjustment is possible by injecting a suitable current to
SUMJ (1C, 2C). Adding a suitable resistor to either ground or
the positive supply is a simple way of achieving this. The
nominal 2.5 V null is for a symmetrical swing range at RATE-
OUT (1B, 2A). However, a nonsymmetric output swing may be
suitable in some applications. Note that if a resistor is con-
nected to the positive supply, supply disturbances may reflect
OUT
resistor that is factory-trimmed to
Rev. C | Page 10 of 12
some null instability. Digital supply noise should be avoided
particularly in this case (see the Supply and Common Consid-
erations section).
The resistor value to use is approximately
V
get null value. If the initial value is below the desired value, the
resistor should terminate on common or ground. If it is above
the desired value, the resistor should terminate on the 5 V sup-
ply. Values typically are in the 1 M to 5 M range.
If an external resistor is used across RATEOUT and SUMJ, the
parallel equivalent value is substituted into the above equation.
Note that the resistor value is an estimate since it assumes V
= 5.0 V and V
SELF-TEST FUNCTION
The AD22304 includes a self-test feature that actuates each of
the sensing structures and associated electronics in the same
manner as if subjected to angular rate. It is activated by stan-
dard logic high levels applied to inputs ST1 (5F, 5G), ST2 (4F,
4G), or both. ST1 causes the voltage at RATEOUT to change
about
–0.66 V, and ST2 causes an opposite change of +0.66 V. The
self-test response follows the viscosity temperature dependence
of the package atmosphere, approximately 0.25%/°C.
Activating both ST1 and ST2 simultaneously is not damaging.
Since ST1 and ST2 are not necessarily closely matched, actuat-
ing both simultaneously may result in an apparent null bias
shift.
CONTINUOUS SELF-TEST
The one-chip integration of the AD22304 gives it higher reli-
ability than is obtainable with any other high volume manufac-
turing method. Also, it is manufactured under a mature BIMOS
process that has field-proven reliability. As an additional failure
detection measure, power-on self-test can be performed. How-
ever, some applications may warrant continuous self-test while
sensing rate. Application notes outlining continuous self-test
techniques are also available on the Analog Devices website.
ACCELERATION SENSITIVITY
The sign convention used is that lateral acceleration is positive
in the direction from Pin Column A to Pin Column G of the
package. That is, a device has positive sensitivity if its voltage
output increases when the row of Pins 2A–6A are tipped under
the row of Pins 2G–6G in the earth’s gravity.
There are two effects of concern, shifts in the static null and
induced null noise. Scale factor is not significantly affected
until the acceleration reaches several hundred m/s
Vibration rectification for frequencies up to 20 kHz is on the
order of 0.00002(°/s)/(m/s
frequency, and has been verified up to 400 m/s
NULL0
R
NULL
is the unadjusted zero-rate output, and V
SUMJ
(
2
.
5
= 2.5 V.
u
180
,
000
2
)
2
, is not significantly dependent on
)/(
V
NULL0
–
V
NULL1
2
NULL1
rms.
)
2
.
is the tar-
CC