SBAA666 February   2025 AMC0106M05 , AMC0106M25

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Design Challenges
  6. 3Design Approach
    1. 3.1 AMC0106Mxx Functionally Isolated Modulators
    2. 3.2 Circuit Design and Layout
    3. 3.3 Sinc3 Filter Design
  7. 4Test and Validation
    1. 4.1 Test Setup
    2. 4.2 Digital Interface
    3. 4.3 DC Accuracy, Noise, and Effective Number of Bits
    4. 4.4 PWM Rejection
      1. 4.4.1 DC Phase Current Measurement Over One PWM Cycle
      2. 4.4.2 AC Phase Current Measurement at 100kHz PWM
    5. 4.5 Bootstrap Supply Validation and AVDD Ripple Rejection Tests
      1. 4.5.1 LMG2100R044 Bootstrap Supply With Low Voltage-Ripple
      2. 4.5.2 Discrete Bootstrap Supply With High Voltage-Ripple
  8. 5Summary
  9. 6References

4.3 DC Accuracy, Noise, and Effective Number of Bits

A series of tests was performed to quantify measurement noise and the corresponding effective number of bits (ENOB) at different oversampling ratios and load currents. The ENOB was calculated from 1200 consecutive samples measured at the center of the 10kHz PWM over 1200 PWM periods. The PWM vector was kept constant during the test. The constant PWM vector produces constant phase voltage and DC current. The ENOB was calculated as follows:

Equation 1. ENOB = (20 x log10(50A / noiseRMS) – 1.76) / 6.02

where noiseRMS equals the standard deviation of the 1200 measurements for a given OSR. Figure 4-3 shows the histograms for OSR 32, 64, 128, and 256 measured at a 3A DC current. The resulting ENOB as a function of OSR is shown in Figure 4-4.

Phase Current Histograms for Different OSRs at 3A Phase Current Figure 4-3 Phase Current Histograms for Different OSRs at 3A Phase Current
Phase Current ENOB vs. OSR Figure 4-4 Phase Current ENOB vs. OSR

At OSR=32 the design already provides a resolution of 11.1 bit. There is no measurable difference between zero and 3A or 6A of current. As the OSR increases, the impact of system noise at larger phase currents becomes visible. For example, the ENOB at OSR 128 increases to 13.7 bit for smaller currents while slightly degrading to 13.5 bit at 6A. Table 4-1 summarizes the ENOB, noise, and latency measured at 3A with different OSRs.

Table 4-1 AMC0106M05 Measured Noise, ENOB and Latency vs OSR at 20MHz Modulator Clock
OSR 32 64 96 128 192 256
Noise(mARMS) 18.4 5.93 4.14 3.46 2.94 2.51
ENOB (bit) 11.1 12.8 13.3 13.5 13.8 14
Latency

(µs)

 2.4 4.8 7.2 9.6 14.4 19.2

A sinusoidal phase current measurement is shown in the next chapter using a high 100kHz PWM frequency to demonstrate high measurement accuracy even during PWM switching.