Datasheet AD71056 (Analog Devices) - 14

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AD71056 Digital-to-Frequency Conversion
This higher output frequency is generated by accumulating the instantaneous real power signal over a much shorter time while As previously described, the digital output of the low-pass converting it to a frequency. This shorter accumulation period filter after multiplication contains the real power information. means less averaging of the cos(2ωt) component. Consequently, However, because this LPF is not an ideal brick wall filter some of this instantaneous power signal passes through the implementation, the output signal also contains attenuated digital-to-frequency conversion. This is not a problem in the components at the line frequency and its harmonics—that is, application. Where CF is used for calibration purposes, the cos(hωt), where h = 1, 2, 3, . and so on. frequency should be averaged by the frequency counter to The magnitude response of the filter is given by remove any ripple. If CF is being used to measure energy, for example in a microprocessor-based application, the CF output H ( f ) 1 = (7) should also be averaged to calculate power. 2 f 1 + Because the F1 and F2 outputs operate at a much lower 2 4 45 . frequency, a lot more averaging of the instantaneous real power For a line frequency of 50 Hz, this gives an attenuation of signal is carried out. The result is a greatly attenuated sinusoidal the 2ω (100 Hz) component of approximately 22 dB. The content and a virtually ripple free frequency output. dominating harmonic is twice the line frequency (2ω) due
Connecting to a Microcontroller for Energy
to the instantaneous power calculation.
Measurement
Figure 25 shows the instantaneous real power signal at the The easiest way to interface the AD71056 to a microcontroller is output of the LPF that still contains a significant amount of to use the CF high frequency output with the output frequency instantaneous power information, that is, cos(2ωt). This signal scaling set to 2048 × F1, F2. This is done by setting SCF = 0 and is then passed to the digital-to-frequency converter where it is S0 = S1 = 1 (see Table 7). With full-scale ac signals on the analog integrated (accumulated) over time to produce an output inputs, the output frequency on CF is approximately 2.867 kHz. frequency. The accumulation of the signal suppresses or Figure 26 illustrates one scheme to digitize the output frequency averages out any non-dc components in the instantaneous real and carry out the necessary averaging mentioned in the Digital- power signal. The average value of a sinusoidal signal is zero. to-Frequency Conversion section. Thus, the frequency generated by the AD71056 is proportional
CF
to the average real power. Figure 25 shows the digital-to-
FREQUENCY RIPPLE
frequency conversion for steady load conditions, that is, constant voltage and current.
AVERAGE ±10% FREQUENCY F1 Y DIGITAL-TO- NC FREQUENCY UE F1 Q V TIME RE F2 F TIME MCU MULTIPLIER DIGITAL-TO- AD71056 LPF FREQUENCY CF COUNTER Y I CF NC CF LPF TO EXTRACT UE Q REAL POWER (DC TERM) RE
6
F
-02
TIME TIMER V × I
636
2
05
COS (2ω)
Figure 26. Interfacing the AD71056 to an MCU
ATTENUATED BY LPF
As shown, the frequency output CF is connected to an MCU counter or port. This counts the number of pulses in a given
0 ω 2ω FREQUENCY (RAD/s)
5 integration time that is determined by an MCU internal timer. 02
INSTANTANEOUS REAL POWER SIGNAL
6-
(FREQUENCY DOMAIN)
63 The average power proportional to the average frequency is 05 Figure 25. Real Power-to-Frequency Conversion given by Figure 25 shows that the frequency output CF varies over time, Counter Average Frequency = Average Power = (8) even under steady load conditions. This frequency variation is Time primarily due to the cos(2ωt) component in the instantaneous real power signal. The output frequency on CF can be up to 2048 times higher than the frequency on F1 and F2. Rev. A | Page 14 of 20 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS Timing Diagram ABSOLUTE MAXIMUM RATINGS ESD CAUTION TERMINOLOGY PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION POWER FACTOR CONSIDERATIONS NONSINUSOIDAL VOLTAGE AND CURRENT APPLICATIONS ANALOG INPUTS Channel V1 (Current Channel) Channel V2 (Voltage Channel) Typical Connection Diagrams POWER SUPPLY MONITOR HPF and Offset Effects Digital-to-Frequency Conversion Connecting to a Microcontroller for Energy Measurement Power Measurement Considerations INTERNAL OSCILLATOR (OSC) TRANSFER FUNCTION Frequency Outputs F1 and F2 Frequency Output CF SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION Frequency Outputs NO LOAD THRESHOLD NEGATIVE POWER INFORMATION OUTLINE DIMENSIONS ORDERING GUIDE