An article explains the difference between zero calibration and zero elimination in power analyzers

1. Zero calibration

Let's first explain the impact of zero-point error on normal measurement. The basic measurement circuit is shown in the figure below:

Figure 1 Basic measurement circuit diagram for zero calibration

Suppose the actual external input signal is , for the op amp conditioning circuit, the zero offset voltage will be amplified along with the signal, and the zero point error is , as shown in Figure 1, then the actual input to the amplifier circuit is , and the signal actually input to the ADC after amplification is .

Zero point error is superimposed on the actual input signal to cause measurement error. Generally, the instrument will be zero-calibrated before leaving the factory, but the zero point will drift due to the influence of the use environment and device aging. Therefore, zero point calibration can also be enabled when the customer uses it. The principle is shown in the figure below:

Figure 2 Enable calibration circuit diagram

During zero calibration, SW1 is disconnected and SW2 is closed. The signal collected by the ADC is : During normal sampling, SW2 is disconnected and SW1 is closed. The signal collected by the ADC is :

At this time, the signal collected is the actual desired signal, the zero voltage is eliminated, the accuracy is higher, but it has no effect on the actual input signal. The PA series supports zero calibration function, as shown in Figure 3, you can click the "zero calibration" button to manually calibrate the zero or turn on "auto zero calibration". When the range, filter and other settings are changed, automatic re-zeroing is required.

Figure 3 Calibration instrument interface settings

2. Zero elimination

Zero elimination is the NULL function of PA. The NULL function does not change the actual input signal, but only performs arithmetic addition and subtraction on the collected signal to achieve translation, which is similar to providing the user with an arithmetic zero point to replace the actual signal zero point. As shown in Figure 4, measure the sine signal superimposed on the Vdc DC, connect the normal signal to the instrument and click the "NULL" button, then the DC signal Vdc will be used as the new zero point value, and the sine signal centered at 0 will be presented to the user after sampling, and Vdc will be eliminated. At this time, if the DC signal connected to Vdc will show that the measured signal is 0.

Figure 4 Schematic diagram of zero elimination

The NULL function is only suitable for specific users to process some specific signals. The signal presented to the user is different from the actual signal. Turning on the NULL function in normal use will cause measurement errors. The PA series power analyzer supports the NULL function. The setting diagram is as follows. It also supports users to enter a custom NULL value, and the set offset value is subtracted from the DC component of the measurement loop.

Figure 5 Instrument NULL setting menu

In summary, the zero calibration function is used by the instrument to eliminate the zero point deviation of the internal measurement circuit and improve the measurement accuracy. The collected signal is still the original signal. The NULL function is that the instrument provides an arithmetic zero point to shift the signal as a whole. The collected signal has a fixed DC deviation from the original signal, which is suitable for specific applications.