AD7400YRWZ

Isolated Sigma-Delta Modulator AD7400 Data Sheet FEATURES GENERAL DESCRIPTION 10 MHz clock rate Second-order modulator 16 bits no missing codes ±2 LSB INL typical at 16 bits 3.5 μV/°C maximum offset drift On-board digital isolator On-board reference Low power operation: 18 mA maximum at 5.25 V −40°C to +105°C operating range 16-lead SOIC package Safety and regulatory approvals UL recognition 5000 V rms for 1 minute per UL 1577 CSA Component Acceptance Notice #5A VDE Certificate of Conformity DIN V VDE V 0884-10 (VDE V 0884-10):2006-12 VIORM = 891 V peak The AD74001 is a second-order, sigma-delta (Σ-Δ) modulator that converts an analog input signal to a high speed, 1-bit data stream with on-chip digital isolation based on Analog Devices, Inc. iCoupler® technology. The AD7400 operates from a 5 V power supply and accepts a differential input signal of ±200 mV (±320 mV full scale). The analog input is continuously sampled by the analog modulator, eliminating the need for external sample-and-hold circuitry. The input information is contained in the output stream as a density of ones with a data rate of 10 MHz. The original information can be reconstructed with an appropriate digital filter. The serial I/O can use a 5 V or a 3 V supply (VDD2). The serial interface is digitally isolated. High speed CMOS, combined with monolithic air core transformer technology, means the on-chip isolation provides outstanding performance characteristics superior to alternatives such as optocoupler devices. The part contains an on-chip reference. The AD7400 is offered in a 16-lead SOIC and has an operating temperature range of −40°C to +105°C. APPLICATIONS AC motor controls Data acquisition systems A/D + opto-isolator replacements An external clock version, AD7401, is also available. 1 Protected by U.S. Patents 5,952,849; 6,873,065; and 7,075,329. FUNCTIONAL BLOCK DIAGRAM VDD1 VDD2 AD7400 VIN+ T/H Σ-∆ ADC UPDATE ENCODE BUF REF CONTROL LOGIC UPDATE ENCODE GND1 WATCHDOG DECODE MDAT WATCHDOG MCLKOUT DECODE GND2 04718-001 VIN– Figure 1. Rev. H Document Feedback Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 ©2006–2018 Analog Devices, Inc. All rights reserved. Technical Support www.analog.com AD7400 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Typical Performance Characteristics ..............................................9 Applications ....................................................................................... 1 Terminology .................................................................................... 12 General Description ......................................................................... 1 Theory of Operation ...................................................................... 13 Functional Block Diagram .............................................................. 1 Circuit Information .................................................................... 13 Revision History ............................................................................... 2 Analog Input ............................................................................... 13 Specifications..................................................................................... 3 Differential Inputs ...................................................................... 14 Timing Specifications .................................................................. 4 Digital Filter ................................................................................ 15 Insulation and Safety-Related Specifications ............................ 5 Applications Information .............................................................. 17 Regulatory Information ............................................................... 5 Grounding and Layout .............................................................. 17 DIN V VDE V 0884-10 (VDE V 0884-10) Insulation Characteristics .............................................................................. 6 Evaluating the AD7400 Performance ...................................... 17 Absolute Maximum Ratings ............................................................ 7 Outline Dimensions ....................................................................... 18 ESD Caution .................................................................................. 7 Ordering Guide .......................................................................... 18 Insulation Lifetime ..................................................................... 17 Pin Configuration and Function Descriptions ............................. 8 REVISION HISTORY 5/2018—Rev. G to Rev. H Changes to Minimum External Air Gap (Clearance) Parameter, Minimum External Tracking (Creepage) Parameter, Tracking Resistance (Comparative Tracking Index) Parameter, and Isolation Group Parameter, Table 3, and 5000 V rms Isolation Voltage Parameter, Table 4 .............................................................. 5 Added Note 1 and Note 2, Table 3; Renumbered Sequentially .. 5 6/2013—Rev. F to Rev. G Changes to Figure 12 and Figure 13............................................. 10 3/2012—Rev. E to Rev. F Changed IDD1 Parameter from 12 mA to 13 mA, Table 1 ............ 3 7/2011—Rev. D to Rev. E Changes to Minimum External Air Gap (Clearance) Parameter, Table 3 and Minimum External Tracking (Creepage) Parameter, Table 3 ................................................................................................ 5 Changes to Figure 5; Pin 1 Description, Table 8; and Pin 7 Description, Table 8.......................................................................... 8 4/2011—Rev. C to Rev. D Changes to Dynamic Input Current Parameter, Table 1 ..............3 1/2011—Rev. B to Rev. C Changes to Features Section ............................................................1 Changes to Input-to-Output Momentary Withstand Voltage Parameter, Table 3, UL Column, Table 4, and Note 1, Table 4...........5 Changes to Ordering Guide .................................................................... 18 9/2007—Rev. A to Rev. B Updated VDE Certification Throughout ......................................1 Changes to Table 6.............................................................................7 12/2006—Rev. 0 to Rev. A Changes to Features ..........................................................................1 Changes to Table 6.............................................................................7 Changes to Analog Input Section................................................. 13 Changes to Figure 26...................................................................... 15 1/2006—Revision 0: Initial Version Rev. H | Page 2 of 20 Data Sheet AD7400 SPECIFICATIONS VDD1 = 4.5 V to 5.25 V, VDD2 = 3 V to 5.5 V, VIN+ = −200 mV to +200 mV, and VIN− = 0 V (single-ended); TA = TMIN to TMAX, fMCLK = 10 MHz, tested with Sinc3 filter, 256 decimation rate, as defined by Verilog code, unless otherwise noted.1 Table 1. Parameter STATIC PERFORMANCE Resolution Integral Nonlinearity3 Differential Nonlinearity3 Offset Error3 Offset Drift vs. Temperature Offset Drift vs. VDD1 Gain Error3 Gain Error Drift vs. Temperature Gain Error Drift vs. VDD1 ANALOG INPUT Input Voltage Range Dynamic Input Current Input Capacitance DYNAMIC SPECIFICATIONS Signal-to-(Noise + Distortion) Ratio (SINAD)3 Signal-to-Noise Ratio (SNR) Total Harmonic Distortion (THD)3 Peak Harmonic or Spurious Noise (SFDR)3 Effective Number of Bits (ENOB)3 Isolation Transient Immunity3 LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL POWER REQUIREMENTS VDD1 VDD2 IDD14 IDD25 Y Version1, 2 Unit Test Conditions/Comments 16 ±15 ±25 ±0.9 ±0.5 ±50 3.5 1 120 ±1 23 110 Bits min LSB max LSB max LSB max mV max µV typ µV/°C max µV/°C typ µV/V typ mV max µV/°C typ µV/V typ Filter output truncated to 16 bits −40°C to +85°C; ±2 LSB typical >85°C to 105°C Guaranteed no missing codes to 16 bits ±200 ±8 ±0.5 10 mV min/mV max µA max µA typ pF typ 70 65 79 71 −88 −88 11.5 25 30 dB min dB min dB typ dB min dB typ dB typ Bits kV/µs min kV/µs typ VDD2 − 0.1 0.4 V min V max IO = −200 µA IO = +200 µA 4.5/5.25 3/5.5 13 6 4 V min/V max V min/V max mA max mA max mA max VDD1 = 5.25 V VDD2 = 5.5 V VDD2 = 3.3 V Temperature range is −40°C to +85°C. All voltages are relative to their respective ground. See the Terminology section. 4 See Figure 14. 5 See Figure 15. 1 2 3 Rev. H | Page 3 of 20 TA = 25°C −40°C to +105°C −40°C to +105°C For specified performance; full range ±320 mV VIN+ = 400 mV, VIN− = 0 V VIN+ = VIN− = 0 V VIN+ = 35 Hz, 400 mV p-p sine −40°C to +85°C >85°C to 105°C −40°C to +105°C AD7400 Data Sheet TIMING SPECIFICATIONS VDD1 = 4.5 V to 5.25 V, VDD2 = 3 V to 5.5 V, TA = TMAX to TMIN, unless otherwise noted.1 Table 2. Parameter fMCLKOUT2 t13 t2 3 t3 t4 Limit at TMIN, TMAX 10 9/11 40 10 0.4 × tMCLKOUT 0.4 × tMCLKOUT Unit MHz typ MHz min/MHz max ns max ns min ns min ns min Description Master clock output frequency Master clock output frequency Data access time after MCLK rising edge Data hold time after MCLK rising edge Master clock low time Master clock high time Sample tested during initial release to ensure compliance. Mark space ratio for clock output is 40/60 to 60/40. 3 Measured with the load circuit of Figure 2 and defined as the time required for the output to cross 0.8 V or 2.0 V. 1 2 200µA +1.6V CL 25pF 200µA 04718-002 TO OUTPUT PIN IOL IOH Figure 2. Load Circuit for Digital Output Timing Specifications t4 t1 t2 MDAT Figure 3. Data Timing Rev. H | Page 4 of 20 t3 04718-003 MCLKOUT Data Sheet AD7400 INSULATION AND SAFETY-RELATED SPECIFICATIONS Table 3. Parameter Input-to-Output Momentary Withstand Voltage Minimum External Air Gap (Clearance) Symbol VISO L(I01) Value 5000 min 7.81, 2 min Unit V rms mm Minimum External Tracking (Creepage) L(I02) 7.81, 2 min mm 0.017 min >400 II mm Minimum Internal Gap (Internal Clearance) Tracking Resistance (Comparative Tracking Index) Isolation Group 1 2 CTI V Conditions 1-minute duration Measured from input terminals to output terminals, shortest distance through air Measured from input terminals to output terminals, shortest distance path along body Insulation distance through insulation DIN IEC 112/VDE 0303 Part 1 Material group (DIN VDE 0110, 1/89, Table 1) In accordance with IEC 60950-1 guidelines for the measurement of creepage and clearance distances for a pollution degree of 2 and altitudes ≤2000 m. Consideration must be given to pad layout to ensure the minimum required distance for clearance is maintained. REGULATORY INFORMATION Table 4. UL1 Recognized Under 1577 Component Recognition Program1 5000 V rms Isolation Voltage File E214100 1 2 CSA Approved under CSA Component Acceptance Notice 5A VDE2 Certified according to DIN V VDE V 0884-10 (VDE V 0884-10):2006-122 Basic insulation per CSA 60950-1-07 and IEC 60950-1, 780 V rms maximum working voltage Reinforced insulation per CSA 60950-1-03 and IEC 60950-1, 390 V rms maximum working voltage File 205078 Reinforced insulation per DIN V VDE V 0884-10 (VDE V 0884-10):2006-12, 891V peak File 2471900-4880-0001 In accordance with UL 1577, each AD7400 is proof tested by applying an insulation test voltage ≥ 6000 V rms for 1 second (current leakage detection limit = 15 µA). In accordance with DIN V VDE V 0884-10, each AD7400 is proof tested by applying an insulation test voltage ≥ 1671 V peak for 1 second (partial discharge detection limit = 5 pC). Rev. H | Page 5 of 20 AD7400 Data Sheet DIN V VDE V 0884-10 (VDE V 0884-10) INSULATION CHARACTERISTICS This isolator is suitable for reinforced electrical isolation only within the safety limit data. Maintenance of the safety data is ensured by means of protective circuits. Table 5. Description INSTALLATION CLASSIFICATION PER DIN VDE 0110 For Rated Mains Voltage ≤ 300 V rms For Rated Mains Voltage ≤ 450 V rms For Rated Mains Voltage ≤ 600 V rms CLIMATIC CLASSIFICATION POLLUTION DEGREE (DIN VDE 0110, Table 1) MAXIMUM WORKING INSULATION VOLTAGE INPUT-TO-OUTPUT TEST VOLTAGE, METHOD B1 VIORM × 1.875 = VPR, 100% Production Test, tm = 1 sec, Partial Discharge < 5 pC INPUT-TO-OUTPUT TEST VOLTAGE, METHOD A After Environmental Test Subgroup 1 VIORM × 1.6 = VPR, tm = 60 sec, Partial Discharge < 5 pC After Input and/or Safety Test Subgroup 2/3 VIORM × 1.2 = VPR, tm = 60 sec, Partial Discharge < 5 pC HIGHEST ALLOWABLE OVERVOLTAGE (TRANSIENT OVERVOLTAGE, tTR = 10 sec) SAFETY-LIMITING VALUES (MAXIMUM VALUE ALLOWED IN THE EVENT OF A FAILURE, ALSO SEE Figure 4) Case Temperature Side 1 Current Side 2 Current INSULATION RESISTANCE AT TS, VIO = 500 V 350 250 SIDE #2 200 150 SIDE #1 100 50 0 0 50 100 150 CASE TEMPERATURE (°C) 200 04718-026 SAFETY-LIMITING CURRENT (mA) 300 Figure 4. Thermal Derating Curve, Dependence of Safety-Limiting Values with Case Temperature per DIN V VDE V 0884-10 Rev. H | Page 6 of 20 Symbol Characteristic Unit VIORM I–IV I–II I–II 40/105/21 2 891 V peak 1671 V peak 1426 V peak 1069 V peak VTR 6000 V peak TS IS1 IS2 RS 150 265 335 >109 °C mA mA Ω VPR VPR Data Sheet AD7400 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. All voltages are relative to their respective ground. Table 6. Parameter VDD1 to GND1 VDD2 to GND2 Analog Input Voltage to GND1 Output Voltage to GND2 Input Current to Any Pin Except Supplies1 Operating Temperature Range Storage Temperature Range Junction Temperature SOIC Package θJA Thermal Impedance θJC Thermal Impedance Resistance (Input-to-Output), RI-O Capacitance (Input-to-Output), CI-O2 Pb-Free Temperature, Soldering Reflow ESD 1 2 Rating −0.3 V to +6.5 V −0.3 V to +6.5 V −0.3 V to VDD1 + 0.3 V −0.3 V to VDD2 + 0.3 V ±10 mA −40°C to +105°C −65°C to +150°C 150°C Table 7. Maximum Continuous Working Voltage1 Parameter AC Voltage, Bipolar Waveform AC Voltage, Unipolar Waveform DC Voltage 1 Max 565 Unit VPK Constraint 50-year minimum lifetime 891 VPK 891 V Maximum CSA/VDE approved working voltage Maximum CSA/VDE approved working voltage Refers to continuous voltage magnitude imposed across the isolation barrier. See the Insulation Lifetime section for more details. ESD CAUTION 89.2°C/W 55.6°C/W 1012 Ω 1.7 pF typ 260 (+0)°C 1.5 kV Transient currents of up to 100 mA do not cause SCR to latch-up. f = 1 MHz. Stresses at or above those listed under Absolute Maximum Ratings may cause permanent damage to the product. This is a stress rating only; functional operation of the product at these or any other conditions above those indicated in the operational section of this specification is not implied. Operation beyond the maximum operating conditions for extended periods may affect product reliability. Rev. H | Page 7 of 20 AD7400 Data Sheet PIN CONFIGURATION AND FUNCTION DESCRIPTIONS VDD1 1 16 GND2 VIN+ 2 15 NC NC 4 AD7400 TOP VIEW 14 VDD2 (Not to Scale) 13 MCLKOUT NC 5 12 NC NC 6 11 MDAT VDD1 /NC 7 10 NC GND1 8 9 GND2 NC = NO CONNECT 04718-004 VIN– 3 Figure 5. Pin Configuration Table 8. Pin Function Descriptions Pin No. 1 Mnemonic VDD1 2 3 4 to 6, 10, 12, 15 7 VIN+ VIN− NC VDD1/NC 8 9, 16 11 GND1 GND2 MDAT 13 MCLKOUT 14 VDD2 Description Supply Voltage. 4.5 V to 5.25 V. This is the supply voltage for the isolated side of the AD7400 and is relative to GND1. Positive Analog Input. Specified range of ±200 mV. Negative Analog Input. Normally connected to GND1. No Connect. Supply Voltage. 4.5 V to 5.25 V. This is the supply voltage for the isolated side of the AD7400 and is relative to GND1. No Connect (NC). If desired, Pin 7 may be allowed to float. It should not be tied to ground. The AD7400 will operate normally provided that the supply voltage is applied to Pin 1. Ground 1. This is the ground reference point for all circuitry on the isolated side. Ground 2. This is the ground reference point for all circuitry on the nonisolated side. Serial Data Output. The single bit modulator output is supplied to this pin as a serial data stream. The bits are clocked out on the rising edge of the MCLKOUT output and valid on the following MCLKOUT rising edge. Master Clock Logic Output. 10 MHz typical. The bit stream from the modulator is valid on the rising edge of MCLKOUT. Supply Voltage. 3 V to 5.5 V. This is the supply voltage for the nonisolated side and is relative to GND2. Rev. H | Page 8 of 20 Data Sheet AD7400 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, using a 20 kHz brick wall filter, unless otherwise noted. 100 –90 200mV p-p SINEWAVE ON VDD1 NO DECOUPLING VDD1 = VDD2 = 4.5V TO 5.25V 90 VDD1 = VDD2 = 5V –80 80 –70 70 –60 SINAD (dB) 50 40 –50 –40 –30 30 –20 04718-005 20 10 0 0 100 200 300 400 500 600 700 800 900 –10 0 0.195 1000 04718-008 PSRR (dB) 60 0.215 SUPPLY RIPPLE FREQUENCY (kHz) 0.255 0.295 0.315 Figure 9. SINAD vs. VIN –90 0.5 –80 0.4 VIN+ = –200mV TO +200mV VIN– = 0V 0.3 DNL ERROR (LSB) VDD1 = VDD2 = 4.5V –60 –50 –40 VDD1 = VDD2 = 5.25V VDD1 = VDD2 = 5V –30 0.2 0.1 0 –0.1 –0.2 –10 –0.3 04718-006 –20 0 0 500 1000 1500 2000 2500 3000 3500 –0.4 4000 04718-009 –70 0 10000 20000 30000 40000 50000 60000 CODE INPUT FREQUENCY (Hz) Figure 10. Typical DNL, ±200 mV Range (Using Sinc3 Filter, 256 Decimation Rate) Figure 7. SINAD vs. Analog Input Frequency for Various Supply Voltages 0.8 0 8192 POINT FFT fIN = 35Hz SINAD = 79.6991dB THD = –92.6722dB DECIMATION BY 256 –20 –40 VIN+ = –200mV TO +200mV VIN– = 0V 0.6 0.4 INL ERROR (LSB) –60 –80 –100 –120 0.2 0 –0.2 –140 –0.4 –180 0 2 4 6 8 10 12 14 FREQUENCY (kHz) 16 18 04718-010 –160 04718-007 (dB) 0.275 ± INPUT AMPLITUDE (V) Figure 6. PSRR vs. Supply Ripple Frequency Without Supply Decoupling (1 MHz Filter Used) SINAD (dB) 0.235 –0.6 0 20 Figure 8. Typical FFT, ±200 mV Range (Using Sinc3 Filter, 256 Decimation Rate) 10000 20000 30000 CODE 40000 50000 Figure 11. Typical INL, ±200 mV Range (Using Sinc3 Filter, 256 Decimation Rate) Rev. H | Page 9 of 20 60000 AD7400 Data Sheet 0.0036 VDD1 = VDD2 = 5V IDD2 @ +25°C 100 IDD2 @ +85°C 0.0035 50 0.0034 VDD1 = VDD2 = 4.5V –50 IDD2 (A) OFFSET (µV) 0 VDD1 = VDD2 = 5V IDD2 @ –40°C 0.0033 0.0032 –100 04718-014 0.0031 VDD1 = VDD2 = 5.25V –150 TEMPERATURE (°C) 0.34 0.30 0.26 0.22 0.18 0.14 0.10 0.06 0.02 –0.02 –0.06 –0.10 –0.14 –0.18 –0.22 –0.26 –0.30 04718-011 5 15 25 35 45 55 65 75 85 95 105 –0.34 0.0030 –200 –45 –35 –25 –15 –5 VIN DC INPUT VOLTAGE (V) Figure 15. IDD2 vs. VIN at Various Temperatures Figure 12. Offset Drift vs. Temperature for Various Supply Voltages 9 VDD1 = VDD2 = 4.5V TO 5.25V 0.3 6 0.2 3 IIN (µA) GAIN (%) 0.1 VDD1 = VDD2 = 5V VDD1 = VDD2 = 5.25V 0 0 –3 VDD1 = VDD2 = 4.5V –0.1 0.30 0.25 0.20 0.15 0.10 0.05 –0.05 –0.10 –0.15 –0.20 –0.25 –0.30 04718-012 15 25 35 45 55 65 75 85 95 105 TEMPERATURE (°C) 0 5 –0.35 –9 –0.3 –45 –35 –25 –15 –5 0.35 04718-015 –6 –0.2 VIN+ DC INPUT (V) Figure 13. Gain Error Drift vs. Temperature for Various Supply Voltages Figure 16. IIN vs. VIN+ DC Input 0.0099 0 VDD1 = VDD2 = 5V 0.0098 –10 0.0097 –20 TA = +85°C TA = +25°C 0.0096 –30 CMRR (dB) 0.0094 TA = –40°C 0.0093 –40 –50 –60 0.0092 –70 0.0091 04718-016 0.30 0.26 0.22 0.18 0.14 0.10 0.06 0.02 –0.02 –0.06 –0.10 –0.14 –0.18 –0.22 –0.26 –0.30 0.0089 0.34 04718-013 –80 0.0090 –0.34 IDD1 (A) 0.0095 –90 –100 0.1 1 10 100 1000 RIPPLE FREQUENCY (kHz) VIN DC INPUT VOLTAGE (V) Figure 17. CMRR vs. Common-Mode Ripple Frequency Figure 14. IDD1 vs. VIN at Various Temperatures Rev. H | Page 10 of 20 10000 Data Sheet AD7400 1.0 11.0 BANDWIDTH = 100kHz 10.8 0.8 10.4 MCLKOUT (MHz) 9.8 VDD1 = VDD2 = 5.25V 9.6 0.2 VDD1 = VDD2 = 5V 04718-024 04718-017 9.4 VIN DC INPUT (V) 95 TEMPERATURE (°C) Figure 18. RMS Noise Voltage vs. VIN DC Input Figure 19. MCLKOUT vs. Temperature for Various Supplies Rev. H | Page 11 of 20 105 85 75 65 55 45 35 25 –5 –15 –25 –35 9.0 –45 9.2 0.30 0.25 0.20 0.15 0.10 0.05 0 –0.05 –0.10 –0.15 –0.20 –0.25 0 10.0 5 0.4 VDD1 = VDD2 = 4.5V 10.2 15 0.6 –0.30 NOISE (mV) 10.6 AD7400 Data Sheet TERMINOLOGY Differential Nonlinearity Differential nonlinearity is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Integral Nonlinearity Integral nonlinearity is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints of the transfer function are specified negative full scale, −200 mV (VIN+ − VIN−), Code 12,288 for the 16-bit level, and specified positive full scale, +200 mV (VIN+ − VIN−), Code 53,248 for the 16-bit level. Offset Error Offset error is the deviation of the midscale code (Code 32,768 for the 16-bit level) from the ideal VIN+ − VIN− (that is, 0 V). Gain Error Gain error includes both positive full-scale gain error and negative full-scale gain error. Positive full-scale gain error is the deviation of the specified positive full-scale code (53,248 for the 16-bit level) from the ideal VIN+ − VIN− (+200 mV) after the offset error is adjusted out. Negative full-scale gain error is the deviation of the specified negative full-scale code (12,288 for the 16-bit level) from the ideal VIN+ − VIN− (−200 mV) after the offset error is adjusted out. Gain error includes reference error. Signal-to-(Noise + Distortion) Ratio (SINAD) This ratio is the measured ratio of signal-to-(noise + distortion) at the output of the ADC. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal-to-(noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB Therefore, for a 12-bit converter, this is 74 dB. Effective Number of Bits (ENOB) The ENOB is defined by ENOB = (SINAD − 1.76)/6.02 Total Harmonic Distortion (THD) THD is the ratio of the rms sum of harmonics to the fundamental. For the AD7400, it is defined as THD(dB) = 20 log V2 2 + V32 + V4 2 + V5 2 + V6 2 V1 where: V1 is the rms amplitude of the fundamental. V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2, excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, it is a noise peak. Common-Mode Rejection Ratio (CMRR) CMRR is defined as the ratio of the power in the ADC output at ±200 mV frequency, f, to the power of a 200 mV p-p sine wave applied to the common-mode voltage of VIN+ and VIN− of frequency fS, expressed as CMRR (dB) = 10log(Pf/PfS) where: Pf is the power at frequency f in the ADC output. PfS is the power at frequency fS in the ADC output. Power Supply Rejection Ratio (PSRR) Variations in power supply affect the full-scale transition but not converter linearity. PSRR is the maximum change in the specified full-scale (±200 mV) transition point due to a change in power supply voltage from the nominal value (see Figure 6). Isolation Transient Immunity The isolation transient immunity specifies the rate of rise/fall of a transient pulse applied across the isolation boundary beyond which clock or data is corrupted. (It was tested using a transient pulse frequency of 100 kHz.) Rev. H | Page 12 of 20 Data Sheet AD7400 THEORY OF OPERATION CIRCUIT INFORMATION The AD7400 isolated Σ-Δ modulator converts an analog input signal into a high speed (10 MHz typical), single-bit data stream; the time average of the modulator’s single-bit data is directly proportional to the input signal. Figure 22 shows a typical application circuit where the AD7400 is used to provide isolation between the analog input, a current sensing resistor, and the digital output, which is then processed by a digital filter to provide an N-bit word. ANALOG INPUT The differential analog input of the AD7400 is implemented with a switched capacitor circuit. This circuit implements a second-order modulator stage that digitizes the input signal into a 1-bit output stream. The sample clock (MCLKOUT) provides the clock signal for the conversion process as well as the output data-framing clock. This clock source is internal on the AD7400. The analog input signal is continuously sampled by the modulator and compared to an internal voltage reference. A digital stream that accurately represents the analog input over time appears at the output of the converter (see Figure 20). A differential input of 320 mV results in a stream of, ideally, all 1s. This is the absolute full-scale range of the AD7400, while 200 mV is the specified full-scale range, as shown in Table 9. Table 9. Analog Input Range Analog Input Full-Scale Range Positive Full Scale Positive Specified Input Range Zero Negative Specified Input Range Negative Full Scale Voltage Input +640 mV +320 mV +200 mV 0 mV −200 mV −320 mV To reconstruct the original information, this output needs to be digitally filtered and decimated. A Sinc3 filter is recommended because this is one order higher than that of the AD7400 modulator. If a 256 decimation rate is used, the resulting 16-bit word rate is 39 kHz, assuming a 10 MHz internal clock frequency. Figure 21 shows the transfer function of the AD7400 relative to the 16-bit output. 65535 MODULATOR OUTPUT +FS ANALOG INPUT 53248 SPECIFIED RANGE ADC CODE Figure 20. Analog Input vs. Modulator Output 12288 A differential signal of 0 V results (ideally) in a stream of 1s and 0s at the MDAT output pin. This output is high 50% of the time and low 50% of the time. A differential input of 200 mV produces a stream of 1s and 0s that are high 81.25% of the time. A differential input of −200 mV produces a stream of 1s and 0s that are high 18.75% of the time. 0 –320mV –200mV +200mV +320mV 04718-020 ANALOG INPUT 04718-019 –FS ANALOG INPUT ANALOG INPUT Figure 21. Filtered and Decimated 16-Bit Transfer Characteristic NONISOLATED 5V/3V ISOLATED 5V + INPUT CURRENT VDD1 AD7400 VIN+ Σ-Δ MOD/ ENCODER VDD2 VDD SINC3 FILTER CS DECODER MDAT MDAT MCLKOUT MCLK SCLK VIN– SDAT RSHUNT GND1 ENCODER GND2 Figure 22. Typical Application Circuit Rev. H | Page 13 of 20 GND 04718-018 DECODER AD7400 Data Sheet DIFFERENTIAL INPUTS The analog input to the modulator is a switched capacitor design. The analog signal is converted into charge by highly linear sampling capacitors. A simplified equivalent circuit diagram of the analog input is shown in Figure 23. A signal source driving the analog input must be able to provide the charge onto the sampling capacitors every half MCLKOUT cycle and settle to the required accuracy within the next half cycle. The recommended circuit configuration for driving the differential inputs to achieve best performance is shown in Figure 24. A capacitor between the two input pins sources or sinks charge to allow most of the charge that is needed by one input to be effectively supplied by the other input. The series resistor again isolates any op amp from the current spikes created during the sampling process. Recommended values for the resistors and capacitor are 22 Ω and 47 pF, respectively. φA MCLKOUT 2pF φA 2pF φB φA φB φA φB Figure 23. Analog Input Equivalent Circuit R VIN+ Because the AD7400 samples the differential voltage across its analog inputs, low noise performance is attained with an input circuit that provides low common-mode noise at each input. The amplifiers used to drive the analog inputs play a critical role in attaining the high performance available from the AD7400. Rev. H | Page 14 of 20 C R VIN– AD7400 04718-028 VIN– 1kΩ φB 04718-027 VIN+ 1kΩ When a capacitive load is switched onto the output of an op amp, the amplitude momentarily drops. The op amp tries to correct the situation and, in the process, hits its slew rate limit. This nonlinear response, which can cause excessive ringing, can lead to distortion. To remedy the situation, a low-pass RC filter can be connected between the amplifier and the input to the AD7400. The external capacitor at each input aids in supplying the current spikes created during the sampling process, and the resistor isolates the op amp from the transient nature of the load. Figure 24. Differential Input RC Network Data Sheet AD7400 DIGITAL FILTER /*`Data is read on negative clk edge*/ module DEC256SINC24B(mdata1, mclk1, reset, DATA); input mclk1; input reset; input mdata1; filtered*/ /*used to clk filter*/ /*used to reset filter*/ /*ip data to be output [15:0] DATA; /*filtered op*/ integer location; integer info_file; MCLKOUT ACC1+ IP_DATA1 Z ACC2+ Z + ACC3 Z + 04718-021 A Sinc3 filter is recommended for use with the AD7400. This filter can be implemented on an FPGA or a DSP. The following Verilog code provides an example of a Sinc3 filter implementation on a Xilinx® Spartan-II 2.5 V FPGA. This code can possibly be compiled for another FPGA, such as an Altera® device. Note that the data is read on the negative clock edge in this case, although it can be read on the positive edge if preferred. Figure 28 shows the effect of using different decimation rates with various filter types. + Figure 25. Accumulator Z = one sample delay MCLKOUT = modulators conversion bit rate */ always @ (negedge mclk1 or posedge reset) if (reset) begin /*initialize acc registers on reset*/ acc1