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 modulator generates a bit stream that is processed by a digital filter to obtain a digital word similar to a conversion result of a conventional analog-to-digital converter (ADC). A very simple filter, shown in Equation 2, built with minimal effort and hardware, is a sinc3-type filter:
This filter provides the best output performance at the lowest hardware size (count of digital gates) for a second-order modulator. All the characterization in this document is done with a sinc3 filter with an oversampling ratio (OSR) of 256 and an output word size of 16 bits.
The effective number of bits (ENOB) is often used to compare the performance of ADCs and ΔΣ modulators. shows the ENOB of the AMC1303 with different oversampling ratios. In this document, Equation 3 calculates this number from the SNR:
An example code for implementing a sinc3 filter in an FPGA is discussed in application note Combining ADS1202 with FPGA Digital Filter for Current Measurement in Motor Control Applications, available for download at www.ti.com.
Isolated ΔΣ modulators are widely used in new-generation frequency inverter designs because of their high ac and dc performance. Frequency inverters are critical parts of industrial motor drives, photovoltaic inverters (string and central inverters), uninterruptible power supplies (UPS), and other industrial applications.
Figure 51 shows a simplified schematic of the AMC1303Mx in a typical frequency inverter application as used in industrial motor drives with shunt resistors (RSHUNT) used for current sensing. Depending on the system design, either all three or only two motor phase currents are sensed.
Figure 52 shows how the Manchester coded bitstream output of the AMC1303Ex minimizes the wiring efforts of the connection between the power and the control board. This bitstream output also allows the clock to be generated locally on the power board without the having to adjust the propagation delay time of each DOUT connection to fulfill the setup and hold time requirements of the microcontroller.
In both examples shown previously, an additional fourth AMC1303 is used to support isolated voltage sensing of the dc link. This high voltage is reduced using a resistive divider and is sensed by the device across a smaller resistor. The value of this resistor can degrade the performance of the measurement, as described in the Isolated Voltage Sensing section.
|High-side supply voltage||3.3 V or 5 V|
|Low-side supply voltage||3.3 V or 5 V|
|Voltage drop across the shunt for a linear response||AMC1303x25x: ±250 mV (maximum)|
|AMC1303x05x: ±50 mV (maximum)|
The high-side power supply (AVDD) for the AMC1303 device is derived from the power supply of the upper gate driver. Further details are provided in the Power Supply Recommendations section.
The floating ground reference (AGND) is derived from one of the ends of the shunt resistor that is connected to the negative input of the AMC1303 (AINN). If a four-pin shunt is used, the inputs of the device are connected to the inner leads and AGND is connected to one of the outer shunt leads.
Use Ohm's Law to calculate the voltage drop across the shunt resistor (VSHUNT) for the desired measured current: VSHUNT = I × RSHUNT.
Consider the following two restrictions to choose the proper value of the shunt resistor RSHUNT:
The typically recommended RC filter in front of a ΔΣ modulator to improve signal-to-noise performance of the signal path is not required for the AMC1303. By design, the input bandwidth of the analog front-end of the device is limited as specified in the Electrical Characteristics table.
For modulator output bitstream filtering, a device from TI's TMS320F2807x family of low-cost microcontrollers (MCUs) or TMS320F2837x family of dual-core MCUs is recommended. These families support up to eight channels of dedicated hardwired filter structures that significantly simplify system level design by offering two filtering paths per channel: one providing high accuracy results for the control loop and one fast response path for overcurrent detection.
In motor control applications, a very fast response time for overcurrent detection is required. The time for fully settling the filter in case of a step-signal at the input of the modulator depends on its order; that is, a sinc3 filter requires three data updates for full settling (with fDATA = fCLK / OSR). Therefore, for overcurrent protection, filter types other than sinc3 can be a better choice; an alternative is the sinc2 filter. Figure 53 and Figure 54 compare the settling times of different filter orders.
The delay time of a sinc filter with a continuous signal is half of its settling time.
The AMC1303 is optimized for usage in current-sensing applications using low-resistance shunts. However, the device can also be used in isolated voltage-sensing applications if the effect of the (usually higher) value of the resistor used in this case is considered. For best performance, TI recommends using the ±250-mV versions of the device (AMC1303x25xx) for this use case.
Figure 55 shows a simplified circuit typically used in high-voltage-sensing applications. The high value resistors (R1 and R2) are used as voltage dividers and dominate the current value definition. The resistance of the sensing resistor R3 is chosen to meet the input voltage range of the AMC1303. This resistor and the differential input resistance of the AMC1303x25x is 22 kΩ also create a voltage divider that results in an additional gain error. With the assumption of R1, R2, and RIND having a considerably higher value than R3, the resulting total gain error can be estimated using Equation 4, with EG being the gain error of the AMC1303.
This gain error can be minimized during the initial system-level gain calibration procedure.
|High-side supply voltage||3.3 V or 5 V|
|Low-side supply voltage||3.3 V or 5 V|
|Voltage drop across the resistor R3 for a linear response||AMC1303x25x: ±250 mV (maximum)|
As indicated in Figure 55, the output of the integrated differential amplifier is internally biased to a common-mode voltage of 1.9 V. This voltage results in a bias current IIB through the resistive network R4 and R5 (or R4' and R5') used for setting the gain of the amplifier. The value range of this current is specified in the Electrical Characteristics table. This bias current generates additional offset error that depends on the value of the resistor R3. Because the value of this bias current depends on the actual common-mode amplitude of the input signal (as illustrated in Figure 56), the initial system offset calibration does not minimize its effect. Therefore, in systems with high accuracy requirements, TI recommends using a series resistor at the negative input (AINN) of the AMC1303 with a value equal to the shunt resistor R3 (that is, R3' = R3 in Figure 55) to eliminate the effect of the bias current.
This additional series resistor (R3') influences the gain error of the circuit. The effect is calculated using Equation 5 with R5 = R5' = 50 kΩ and R4 = R4' = 12.5 kΩ for the AMC1303x25x.
Figure 56 shows the dependency of the input bias current on the common-mode voltage at the input of the AMC1303x25x.
Do not leave the inputs of the AMC1303 unconnected (floating) when the device is powered up. If both modulator inputs are left floating, the input bias current drives these inputs to the output common-mode voltage of the differential amplifier of approximately 1.9 V. If that voltage is above the specified input common-mode range, the gain of the differential amplifier diminishes and the modulator outputs a bitstream resembling a zero differential input voltage.