This application example involves the measurement of electrical variables in a power system. The accurate measurement of electrical variables in a power grid is extremely critical because this measurement helps to determine the operating status and running quality of the grid. Such accurate measurements also help to diagnose potential problems with the power network so that these problems can be resolved quickly without having any significant service impact. The key electrical parameters include amplitude, frequency, and phase measurement of the voltage and current on the power lines. These parameters are important to enable metrology in the power automation system to perform harmonic analysis, power factor calculation, power quality assessment, and so forth. The ADS93x4 includes various digital features like offset calibration, gain calibration and phase adjustment that simplify the DAQ design for power automation systems.

The analog input for each channel of ADS9324 presents a constant resistive impedance of 1MΩ independent of the ADC sampling frequency and range setting. The high input impedance of the analog front end circuit allows direct connection to potential transformers (PT) and current transformers (CT). The PT and CT used in the system, as illustrated in Figure 9-1, have a ±10V output range. The ADC inputs can support up to ±10V range, hence, eliminates the need for external amplifiers or attenuation stage.

Although the PT and CT provide isolation from the power system, a series resistor must be placed on the analog input channels. The series resistor helps limit the input current to ±10mA if the input voltages exceed the device maximum rated voltage (±18V). Internal ESD clamp circuit helps against any transient voltage exceed the maximum rating. For applications that require protection against overvoltage or fast transient events beyond the specified absolute maximum ratings of the device, an external protection clamp circuit using transient voltage suppressors (TVS) and ESD diodes is recommended.

A low-pass filter is used on each analog input channel to eliminate high-frequency noise pickup and minimize aliasing. Figure 9-2 shows an example of the recommended configuration for an input RC filter. A balanced RC filter configuration matches the external source resistance on the positive path (AINnP) with an equal resistance on the negative path (AINnM). Matching the source impedance in the positive and negative path allows for better common-mode noise rejection and helps in maintaining the DC accuracy of the system by canceling any additional offset error contributed by the external series resistance.