The Prosig P8000 Auto-Zero function is a powerful tool and it is well worth spending some time understanding it.
Consider, for example, that a sensor is connected that reads temperature as a voltage.
0 degrees = 0v
and perhaps
100 degrees = 10v.
Therefore sensitivity = 10 degrees/volt
If the sensor is connected to a P8000, the initial reading will be, say, 2.2 volts, assuming ambient temperature of 22 degrees.
It is then possible to perform the test and analyse how, for example, the temperature changes when a heater or air conditioner is turned on.
However, sometimes it is necessary to find the variation only. In this case the temperature varied up or down from the ambient temperature. But if the test requires only the change in temperature due to the heater or air conditioner then the signals will not be as desired. In our example, they will have an offset equal to the ambient temperature.
What we can do is set the Auto-Zero mode to External. Then when the P8000 is armed (or when auto zero button is manually pressed) the P8000 reads the sensor. In our example, it would ready 2.2volts (22 degrees). The P8000 then uses its internal DAC (Digital to Analogue Converter) to offset this voltages automatically. So, the internal DAC would apply -2.2 volts on to the signal. The reading on the real-time display would now have changed to 0 volts or, in engineering units, 0 degrees.
If we then performed a test that involved switching on a heater that raised the temperature by 10 degrees, the recorded signal will show 0 degrees slowly rising to 10 degrees over time.
With the Auto-Zero set to Internal the signal would be seen rising from the ambient temperature of 22 degrees rising to 32 degrees over time.
With the Auto-Zero set to External the signal would be seen rising from a temperature of 0 degrees rising to 10 degrees over time.
However there are several Auto-Zero types, as shown in Figure 1.
The options are
- Internal = Reset internal offset generation to zero (Undo the external auto zero)
- External = Try to remove sensor offset to zero, by adding or removing voltage from/to signal
- Hold = Keep Current Value (From previous Internal or External setting)
Prosig would recommend leaving the Auto-Zero set to on Hold for most or all customers.
In summary, the Auto-Zero function can be used to remove a DC voltage offset.
- External – Auto-Zero will remove off set from sensor signal.
- Internal – Auto-Zero will reset P8000 channel to remove any internal offset.
- Hold – Auto-Zero does nothing.
The External Auto-Zero function is limited. Only up to 50% of voltage input range can be removed. At a gain of 1 the voltage range is +/-10 volts. Therefore, the maximum offset that the Auto-Zero can remove is +/-5 volts. At a gain of 2 the voltage range is +/-5 volts. Therefore, the maximum offset the Auto-Zero function can remove is +/-2.5 volts. And so on for each gain level.
Additionally, the Auto-Zero has no function for IEPE sensors, they are not affected by DC levels.
James Wren
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