AMC1306x Small, High-Precision, Reinforced Isolated Delta-Sigma Modulators with High CMTI (Rev. A)
SBAS734A – March2017 – revisedJuly 2017
9 Application and Implementation
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.
9.1 Application Information
9.1.1 Digital Filter Usage
The modulator generates a bitstream 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, built with minimal effort and hardware, is a sinc3-type filter, as shown in Equation 2:
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 also done with a sinc3 filter with an oversampling ratio (OSR) of 256 and an output word width of 16 bits.
The effective number of bits (ENOB) is often used to compare the performance of ADCs and ΔΣ modulators. Figure 55 shows the ENOB of the AMC1306 with different oversampling ratios. In this document, this number is calculated from the SNR by using Equation 3:
An example code for implementing a sinc3 filter in an FPGA is discussed in the Combining the ADS1202 with an FPGA Digital Filter for Current Measurement in Motor Control Applications application note, available for download at www.ti.com.
9.2 Typical Applications
9.2.1 Frequency Inverter Application
Isolated ΔΣ modulators are being widely used in 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), electrical and hybrid electrical vehicles, and other industrial applications.
Figure 56 shows a simplified schematics of the AMC1306Mx 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.
The Manchester coded bitstream output of the AMC1306Ex minimizes the wiring efforts of the connection between the power board and the control board; see Figure 57. 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, an additional fourth AMC1306 is used to support isolated voltage sensing of the dc link. This high voltage is reduced using a high-impedance 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.
126.96.36.199 Design Requirements
Table 1. Design Requirements
|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||±250 mV (maximum)|
188.8.131.52 Detailed Design Procedure
The high-side power supply (AVDD) for the AMC1306 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 AMC1306 (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 voltage drop caused by the nominal current range must not exceed the recommended differential input voltage range: VSHUNT ≤ ±250 mV
- The voltage drop caused by the maximum allowed overcurrent must not exceed the input voltage that causes a clipping output: |VSHUNT| ≤ |VClipping|
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 AMC1306. 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.
184.108.40.206 Application Curve
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 the filter 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 58 compares the settling times of different filter orders.
The delay time of the sinc filter with a continuous signal is half of the settling time.
9.2.2 Isolated Voltage Sensing
The AMC1306 is optimized for usage in current-sensing applications using low-impedance shunts. However, the device can also be used in isolated voltage-sensing applications if the affect of the (usually higher) impedance of the resistor used in this case is considered.
220.127.116.11 Design Requirements
Figure 59 shows a simplified circuit typically used in high-voltage-sensing applications. The high impedance 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 AMC1306. This resistor and the differential input impedance of the device (the AMC1306x25 is 22 kΩ, the AMC1306x05 is 4.9 kΩ) also create a voltage divider that results in an additional gain error. With the assumption of R1, R2, and RIN 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 AMC1306.
This gain error can be easily minimized during the initial system-level gain calibration procedure.
18.104.22.168 Detailed Design Procedure
As indicated in Figure 59, 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. The initial system offset calibration does not minimize this effect because the value of the bias current depends on the actual common-mode amplitude of the input signal (as illustrated in Figure 60). Therefore, in systems with high accuracy requirements, a series resistor is recommended to be used at the negative input (AINN) of the AMC1306 with a value equal to the shunt resistor R3 (that is, R3' = R3 in Figure 59) to eliminate the effect of the bias current.
This additional series resistor (R3') influences the gain error of the circuit. The effect can be calculated using Equation 5 with R5 = R5' = 50 kΩ and R4 = R4' = 2.5 kΩ (for the AMC1306x05) or 12.5 kΩ (for the AMC1306x25).
22.214.171.124 Application Curve
Figure 60 shows the dependency of the input bias current on the common-mode voltage at the input of the AMC1306.
9.2.3 Do's and Don'ts
Do not leave the inputs of the AMC1306 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 of the analog front-end of approximately 2 V. If that voltage is above the specified input common-mode range, the front gain diminishes and the modulator outputs a bitstream resembling a zero input differential voltage.
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