Amplifier compensates piezoelectric-rate gyros

Analog Devices LT1368 LT1495

Low-power, low-cost ceramic piezoelectric rate gyros are available, but they lack the low-temperature-coefficient characteristics of their quartz counterparts. However, you can use a servo amplifier to remove the dc level shift due to temperature-related drift effects (Figure 1). The original application of these small, rugged gyros was for damping video-camcorder jitter, but these devices suit many other applications, including camera-platform stabilization, radio-controlled-airplane- and helicopter-rate-damping-control assistance, remotely piloted vehicles, and computer-mouse motion sensors.

By setting the time constant of the servo loop in Figure 1, the circuit responds at an appropriate low frequency for the system under control. The time constant you use for the servo loop depends on the dynamics of this system, but 10 to 50 times the lowest frequency of interest is a good starting point. The dc-level drift occurs at a slow rate and hence appears to be a valid response. However, the rate-gyro amplifier also passes some frequencies above the set frequency. (The corner frequency of the highpass filter depends on what the circuit is controlling.)

Servo amplifier IC1A removes the dc level shift due to temperature-related drift effects of a piezoelectric-rate gyro.
Figure 1. Servo amplifier IC1A removes the dc level shift due to temperature-related drift effects of a piezoelectric-rate gyro.

Amplifier IC1A and its servo-driver stage, IC1B, comprise a low-power op amp. The servo controlling the dc level ac-couples, or highpass-filters, the gyro's output by forcing the output of IC1A to swing around V/2. The lowest frequency that the circuit passes is a function of the C1R1 time constant. The maximum value of R1 depends on the minimum input bias current of the op amp, which is 250 pA for the LT1495. If space permits, C1 should be a good low-leakage type, such as polystyrene, Teflon, or polypropylene. A Mylar capacitor compromises the integrator's performance.

The gyro and its amplifier can be some distance from the main processing circuit, and cable capacitance can cause many op amps to oscillate. However, the LT1368, IC2B, at the output can drive cable capacitance without stability problems. The LT1368, IC2A, also serves to drive a filter capacitor to provide a low-noise V/2 reference point for the LT1495 servo driver.

Materials on the topic

  1. Datasheet Analog Devices LT1368
  2. Datasheet Analog Devices LT1495


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