A more advanced class of instrument amplifier is depicted the diagram above. This class of amplifiers are known as "Automatic Null" amplifiers where the amplifier is also capable of adding or subtracting large offset voltages on command, or at the time that power is applied to the amplifier.

This apparently simple function will allow the use of sensors having much larger zero uncertainties of as much as ±100% of the full-scale output of the sensor and will allow the full use of the measurement system acceptable input voltage range without the loss in input voltage range that would be associated with simple amplification.

The block diagram shown illustrates how the auto-null amplifier functions by comparing the amplifier output, at the moment of power application, to the desired zero-output level (±5 mV) and then directing the digital counter and digital-to-analog converter to generate a voltage output that causes the output to drop towards the ±5 mV acceptable window.

The output comparator circuit then disables the counter at the moment that the desired zero-output level is achieved. By using a 10-bit counter and where the output voltage full scale is ±5 V, a zero level of ±5 mV (= 5V/210) can be easily accommodated, even when utilizing sensors showing zero uncertainties of 100% of the full-scale output of the sensor.

As a caveat, it is important that the physical input to the sensor must be known at the time that the amplifier is commanded to null the sensor output.

An example of the use of this class of amplifier is on submarine launched ballistic missiles (Trident D5) where it was discovered that even the metallic-foil structural strain-gage channels would drift and/or creep over time to levels that severely limited the ability of the PCM system to transmit the flight strain data.

In this instance, piezoresistive strain-gaged accelerometers were employed, while being able to tolerate massive input shock loading, can also show ±100% of fullscale or more zero uncertainty over time.

The use of the auto-null amplifier was particularly appropriate where the acceleration of the missile prior to launch is known to be close to 1 g. The sensor is nulled at this time, effectively eliminating the long-term zero uncertainty from the PCM channel entirely, therefore preserving the full input voltage acceptance range of the PCM channel for use in transmitting flight data.

When used on the reentry vehicles, two opportunities exist to null the sensor, once prior to launch and the second when the vehicle is exoatmospheric and in the ballistic (O-g) flight phase. Auto-nulling the sensor prior to re-entry of the vehicle provides an opportunity to eliminate powered warm-up shift errors and or thermal errors that may have manifested themselves during the boost phase of flight to allow accurate reentry accelerations to be measured.

This extract is taken from
'The Art of Practical & Precise Strain Based Measurement' by Jim Pierson.

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