Simultaneous sampling A/D converter and its application

    Two new simultaneous-sampling A/D converters, the MAX125 and MAX126, are complete 14-bit data acquisition systems that can simultaneously track and hold 4 of the 8 inputs (see Figure 1). The on-board sequencer can be programmed to select which 4 (or less) channels to digitize. Throughput rates range from 250ksps (one channel) to 76ksps (all 4 channels). The input range is ±5V (MAX125) and ±2.5V (MAX126).

    Each of the 4 track/hold (T/H) stages is switchable between A and B inputs, resulting in 8 possible input channels. Figure 2 shows the A channel in tracking mode. T switches in each T/H input minimize crosstalk between adjacent channels. 4 address pins select the channel number and operating mode, and each input circuit tolerates ±17V overvoltage. The chip also contains a voltage reference (30ppm/°C drift).

    MAX125/126 converters have a wide range of applications, including:

    ·Field-oriented control, making AC motors work like DC motors;

    ·Measurement of high-voltage, three-phase waveforms in line fault protection systems;

    ·Phase difference in Coriolis-based mass flow meters Detection;

    · Digitize the I and Q signals of the satellite tuner IC;

    · Used in radar-based collision warning systems in the automotive manufacturing industry, the intermediate frequency stage can be removed.

Field

    -oriented control Field-oriented control (FOC) makes an AC motor work like a DC motor. This is a major application of the MAX125/126 converter. The brush and commutator assembly in a DC motor ensures that the field (stator) current always maintains the correct angle to the rotor current. This is called field orientation. This condition allows the two elements of the rotor to produce the calibrated maximum torque to be separated and controlled directly, so that the field orientation can provide a fast and precise dynamic response to the motor. In order to change the motor torque, the rotor current component Iq needs to be changed. Iq determines the torque generated and keeps the field or magnetizing current component constant. The magnetizing current can be determined from Figure 3:

    I d =V d =jωL m (1)

    where Id: magnetizing current component, Vd: stator voltage, ω: applied voltage angular frequency, Lm: magnetizing inductance of the rotor.

    Therefore, keeping the Vd/ω ratio constant maintains constant torque at different speeds. In addition, changing the stator voltage Vd can also control the speed. Although the Vd voltage cannot be measured directly, Vd can be deduced if the input voltage VXR, stator current Is and stator resistance Rs of one of the three phases of the motor at different temperatures are known:

    V d =V xR -IsR S (2)

    Field Orientation control is divided into three categories: direct, indirect and sensorless.

    Direct FOC directly measures the rotor angle, measured with a sensor installed in the motor housing.

    The indirect FOC measures the speed, uses a solver to measure the speed, and then integrates the speed to determine the slip angle. In an AC asynchronous motor, the rotational field in the rotor causes the rotor to rotate in the same direction as the stator field, but at a lower angular frequency. The difference between their frequencies is called slip frequency, and the angle between them is called slip angle. The rotor angular frequency plus the slip frequency gives the required stator frequency. Therefore, frequency is a by-product of this control technique rather than a control variable.

    Sensorless FOC has attracted much attention, especially in some situations where direct signal feedback from the rotor cannot be obtained during activation, sensorless FOC is required. For example, offshore oil rigs have subsea pumps and other systems whose motors and drive electronics are far away. Sensorless FOC differs from direct and indirect FOC in that it performs all measurements and calculations at the stator end of the motor (see Figure 4). It can be seen from the vector diagrams in Figures 4 and 5 that MAX125 digitizes the two rotor phase currents ib and ic. Note that only two phase currents are required, and the third phase current ia can be derived from the assumption that the three phase currents are 120° phase difference and the instantaneous summation value is 0. The three currents are then transformed into a two-phase orthogonal system of α and β axes through Clarke transformation technology.

    For simplicity, the α-axis may be equal to the a-axis. The two orthogonal currents iα and iβ can be transformed into a time-invariant rotating orthogonal system. This system is represented by the magnetic field and torque components d and q equivalent to the rotor currents id and iq. The α/β coordinate system is rotated counterclockwise to align with the rotor vector flux axis ψr. The rotation angle θ is determined by the Park coordinate transformation rotation motor model. Park transform represents the current as a DC quantity, which greatly reduces the complexity of the system. Combining the rotor vector flux angle θ with the current obtained from the Park transform yields the actual motor field and torque. The Park transform plays a major role in the control loop due to its ability to compare reference torque with measured torque. After obtaining the desired torque and vector flux, Park inverse transformation first inversely transforms the reference torque and field current (idref and iqref) into the orthogonal stator format current (idref and iqref) and inversely transforms into the orthogonal stator coordinate system current ia ,ib and ic. All transformations are performed by DSP. The microprocessor performs control and real-time execution of input commands.

Line Fault Protection Systems

    Simultaneous sampling ADCs are also indispensable for measuring high voltage, 3-phase waveforms in line fault protection systems (see Figure 6). The measured slow-changing 50-60Hz signal can use a multi-channel ΣΔ converter. This converter has high resolution and does not require an anti-aliasing filter. Although a single ΣΔ ADC is inexpensive, several ΣΔ ADCs are typically required in this application (3 voltages and 4 currents), which increases the converter cost to about 4 times that of a single MAX 125.

Based on the Coriolis mass flow meter,

    the Coriolis principle uses low-frequency vibration to excite the surface of the tube and pick up the deformation of the tube caused by the mass flow injected through the tube. The circuit is shown in Figure 7. The excitation source is usually a vibrating coil, and the deflection is picked up by a voice coil or optical means. When comparing the excitation and pickup signals, these deflections appear as phase differences, which can be detected using simultaneous sampling methods. Although the signal frequency is quite low (typically 50 to 500 Hz), detecting small phase shifts requires a high-speed, high-resolution simultaneous sampling ADC.

High Frequency Applications

    In the high frequency domain, simultaneous sampling is used to digitize the I and Q signals from a direct downconversion satellite tuner IC. The commercial satellite receiver system contains a specially designed dual 6-bit high-speed (60-90MHz) simultaneous sampling ADC (see Figure 8). In collision warning and adaptive cruise control maneuvering systems, the mid-frequency stage can be eliminated with simultaneous sampling at the same speed. However, this approach requires a fairly expensive parallel ADC with 8-bit resolution. Similar results below 1Msps can be achieved using undersampling techniques. Onboard radar detection also requires simultaneous sampling with 10-30Msps 12-bit resolution. This functionality can be accomplished with any two of the MAX1424 series (depending on speed requirements). The MAX1424's trimmable reference input adjusts system gain and compensation.
Conclusion

    The high-speed 14-bit simultaneous sampling A/D converter introduced in this article expands the application range of accurate phase measurement of two or more waveforms, which is an economical and cost-effective method.