ADXRS401

Rev. 0 | Page 10 of 12

USE WITH A SUPPLY-RATIOMETRIC ADC

The ADXRS401’s RATEOUT signal is nonratiometric (that is,

neither the null voltage nor the rate sensitivity is proportional to

the supply). Rather, they are nominally constant for dc supply

changes within the 4.75 V to 5.25 V operating range. If the

ADXRS401 is used with a supply-ratiometric ADC, the

ADXRS401’s 2.5 V output can be converted and used to make

corrections in software for the supply variations.

NULL ADJUST

Null adjustment is possible by injecting a suitable current to

SUMJ (1C, 2C). Simply add a suitable resistor to either the

ground or the positive supply. The nominal 2.5 V null is for a

symmetrical swing range at RATEOUT (1B, 2A). In some

applications, a nonsymmetrical output swing may be suitable.

If a resistor is connected to the positive supply, supply

disturbances may reflect some null instability. Avoid digital

supply noise, particularly in this case (see the Supply and

Common Considerations section).

The resistor value to use is approximately:

)

V

–

V

180,000)/(

(2.5

R

NULL1

NULL0

NULL

×

=

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

supply. Values typically are in the 1 MΩ to 5 MΩ 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

SELF-TEST FUNCTION

The ADXRS401 includes a self-test feature that stimulates each

of the sensing structures and associated electronics in the same

manner, as if subjected to angular rate. It is activated by

standard logic high levels applied to inputs ST1 (5F, 5G), ST2

(4F, 4G), or both. ST1 causes the voltage at RATEOUT to

change about −0.800 V, and ST2 causes an opposite +0.800 V.

Activating both ST1 and ST2 simultaneously is not damaging.

Because ST1 and ST2 are not necessarily closely matched,

actuating both simultaneously may result in an apparent null

bias shift.

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 to 6A are tipped

under the row 2G to 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

acceleration reaches several hundred meters per second

squared.

Vibration rectification for frequencies up to 20 kHz is of the

the lid. It is not significantly dependent on frequency, and has

Linear vibration spectral density near the 14 kHz sensor

resonance translates into output noise. In order to have a

significant effect, the vibration must be within the angular rate

bandwidth (typically ±40 Hz of the resonance), so it takes

considerable high frequency vibration to have any effect.

Away from the 14 kHz resonance, the effect is not discernible,

except for vibration frequencies within the angular rate pass

band. The in-band effect can be seen in Figure 17. This is the

result of the static g-sensitivity. The specimen used for Figure 17

had a g-sensitivity of 0.15 °/s/g and its total in-band noise

degraded from 3 mV rms to 5 mV rms for the specified

vibration. The effect of broadband vibration up is shown in

Figure 18 and Figure 19.

The output noise of the part falls away in accordance with the

output low-pass filter and does not contain any spikes greater

than 1% of the low frequency noise. A typical noise spectrum is

shown in Figure 16.

–60

–70

–80

–90

–100

–110

–120

–130

0

10

100

1k

10k

100k

FREQUENCY (Hz)

Figure 16. Noise Spectral Density at RATEOUT – BW = 4Hz