SBAS404D October 2006 – February 2016 ADS8556 , ADS8557 , ADS8558
PRODUCTION DATA.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The ADS855x devices enable high-precision measurement of up to six analog signals simultaneously. The following sections summarize some of the typical use cases for the ADS855x devices and the main steps and components used around the analog-to-digital converter. The design example is carried out specifically using the ADS8556 for a 16-bit data acquisition system.
This section describes the measurement of electrical variables in a 3-phase power system. The accurate measurement of electrical variables in a power grid is extremely critical because this measurement helps determine the operating status and running quality of the grid. Such accurate measurements also help diagnose problems with the power network, thereby enabling prompt solutions and minimizing down time. The key electrical variables measured in 3-phase power systems are the three line voltages and the three line currents; see Figure 43. These variables enable metrology and power automation systems to determine the amplitude, frequency, and phase information to perform harmonic analysis, power factor calculation, power quality assessment, and so forth.
To begin the design process, a few parameters must be decided upon. The designer must know the following:
Figure 44 shows the topology chosen to meet the design requirements. A feedback capacitor C_{F} is included to provide a low-pass filter characteristic and attenuate signals outside the band of interest.
The potential transformers (PTs) and current transformers (CTs) used in the system depicted in Figure 43 provide the six input variables required. These transformers have a ±10-V output range. Although the PTs and CTs provide isolation from the power system, the value of R_{IN} is selected as 100 kΩ to provide an additional, high-impedance safety element to the input of the ADC. Moreover, selecting a low-frequency gain of –1 V/V (as shown in Equation 5) provides a ±10-V output that can be fed into the ADS8556 device; therefore, the value of R_{F} is selected as 100 kΩ.
The primary goal of the acquisition system depicted in Figure 43 is to measure up to 20 harmonics in a 60-Hz power network. Thus, the analog front-end must have sufficient bandwidth to detect signals up to 1260 Hz, as shown in Equation 6.
Based on the bandwidth found in Equation 6, the ADS8556 device is set to simultaneously sample all six channels at 15.36 kSPS, which provides enough samples to clearly resolve even the highest harmonic required.
The passband of the configuration shown in Figure 44 is determined by the –3-dB frequency according to Equation 7. The value of C_{F} is selected as 820 pF, which is a standard capacitance value available in 0603 size (surface-mount component) and such values, combined with that of R_{F}, result in sufficient bandwidth to accommodate the required 20 harmonics (at 60 Hz).
The value of R_{1} is selected as the parallel combination of R_{IN} and R_{F} to prevent the input bias current of the operational amplifier from generating an offset error.
The value of component C_{1} is chosen as 0.1 µF to provide a low-impedance path for noise signals that can be picked up by R_{1}; this 0.1-µF capacitance value improves the EMI robustness and noise performance of the system.
The OPA2277 device is chosen for its low input offset voltage, low drift, bipolar swing, sufficient gain-bandwidth product and low quiescent current. For additional information on the procedure to select SAR ADC input drivers, see 16-bit 400KSPS 4-Ch. Multiplexed Data Acquisition Ref Design for High Voltage Inputs, Low Distortion, TIPD151.
The charge injection damping circuit is composed of R_{2} (49.9 Ω) and C_{2} (370 pF); these components reject high-frequency noise and meet the settling requirements of the ADS8556 device input.
Figure 45 shows the reference block used in this design.
For more information on the design of charge injection damping circuits and reference driving circuits for SAR ADCs, consult the reference guide Power-optimized 16-bit 1MSPS Data Acquisition Block for Lowest Distortion and Noise Reference Design, TIDU014.
Figure 46 shows the frequency spectrum of the data acquired by the ADS8556 device for a sinusoidal, 20-V_{PP} input at 60 Hz.
The ac performance parameters are: