18-Bit, 2.5 LSB INL, 100 kSPS SAR ADC AD7678 FUNCTIONAL BLOCK DIAGRAM FEATURES 18-bit resolution with no missing codes No pipeline delay (SAR architecture) Differential input range: ±VREF (VREF up to 5 V) Throughput: 100 kSPS INL: ±2.5 LSB max (±9.5 ppm of full scale) Dynamic range: 103 dB typ (VREF = 5 V) S/(N+D): 100 dB typ @ 2 kHz (VREF = 5 V) Parallel (18-,16-, or 8-bit bus) and serial 5 V/3 V interface SPI®/QSPI™/MICROWIRE™/DSP compatible On-board reference buffer Single 5 V supply operation Power dissipation: 18 mW @ 100 kSPS 180 μW @ 1 kSPS 48-lead LQFP or 48-lead LFCSP package Pin-to-pin compatible upgrade of AD7674/AD7676/AD7679 APPLICATIONS CT scanners High dynamic data acquisition Geophone and hydrophone sensors - replacement (low power, multichannel) Instrumentation Spectrum analysis Medical instruments GENERAL DESCRIPTION The AD7678 is an 18-bit, 100 kSPS, charge redistribution SAR, fully differential analog-to-digital converter that operates on a single 5 V power supply. The part contains a high speed 18-bit sampling ADC, an internal conversion clock, an internal reference buffer, error correction circuits, and both serial and parallel system interface ports. The part is available in 48-lead LQFP or 48-lead LFCSP packages with operation specified from –40°C to +85°C. PDBUF REF REFGND AGND AVDD IN– OVDD AD7678 SERIAL PORT REFBUFIN IN+ DVDD DGND OGND 18 SWITCHED CAP DAC D[17:0] BUSY PARALLEL INTERFACE CLOCK PD RESET RD CS CONTROL LOGIC AND CALIBRATION CIRCUITRY MODE0 MODE1 CNVST 03084–0–001 Figure 1. Functional Block Diagram Table 1. PulSAR Selection Type/kSPS PseudoDifferential True Bipolar True Differential 18-Bit Multichannel/ Simultaneous 100–250 AD7651 AD7660/AD7661 AD7663 AD7675 500–570 AD7650/AD7652 AD7664/AD7666 AD7665 AD7676 AD7678 AD7679 AD7654 AD7655 800– 1000 AD7653 AD7667 AD7671 AD7677 AD7674 PRODUCT HIGHLIGHTS 1. High Resolution, Fast Throughput. The AD7678 is a 100 kSPS, charge redistribution, 18-bit SAR ADC (no latency). 2. Excellent Accuracy. The AD7678 has a maximum integral nonlinearity of 2.5 LSB with no missing 18-bit codes. 3. Serial or Parallel Interface. Versatile parallel (18-, 16-, or 8-bit bus) or 2-wire serial interface arrangement compatible with both 3 V and 5 V logic. Rev. A 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. www.analog.com Tel: 781.329.4700 Fax: 781.461.3113 ©2003–2009 Analog Devices, Inc. All rights reserved. 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AD7678 TABLE OF CONTENTS Specifications..................................................................................... 3 Digital Interface .......................................................................... 20 Timing Specifications....................................................................... 5 Parallel Interface ......................................................................... 20 Absolute Maximum Ratings............................................................ 7 Serial Interface ............................................................................ 20 ESD Caution .................................................................................. 7 Master Serial Interface ............................................................... 21 Pin Configuration and Function Descriptions ............................. 8 Slave Serial Interface .................................................................. 22 Definition of Specifications ........................................................... 11 Microprocessor Interfacing ....................................................... 24 Typical Performance Characteristics ........................................... 12 Application Hints ........................................................................... 25 Circuit Information ........................................................................ 15 Layout .......................................................................................... 25 Converter Operation .................................................................. 15 Evaluating the AD7678’s Performance .................................... 25 Typical Connection Diagram ................................................... 17 Outline Dimensions ....................................................................... 26 Power Dissipation versus Throughput .................................... 19 Ordering Guide .......................................................................... 26 Conversion Control.................................................................... 19 REVISION HISTORY 6/09—Rev. 0 to Rev. A Removed Endnote 3 from DC Accuracy; Zero Error, TMIN to TMAX Parameter; Table 2 ................................................................... 3 Changes to Endnote 3, Table 2 ........................................................ 4 Moved ESD Caution ......................................................................... 7 Changes to Figure 4 and Table 6 ..................................................... 8 Changes to Evaluating the AD7678’s Performance Section...... 25 Updated Outline Dimensions ....................................................... 26 Changes to Ordering Guide .......................................................... 26 8/03—Revision 0: Initial Version Rev. A | Page 2 of 28 AD7678 SPECIFICATIONS Table 2. –40°C to +85°C, VREF = 4.096 V, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted. Parameter RESOLUTION ANALOG INPUT Voltage Range Operating Input Voltage Analog Input CMRR Input Current Input Impedance1 THROUGHPUT SPEED Complete Cycle Throughput Rate DC ACCURACY Integral Linearity Error Differential Linearity Error No Missing Codes Transition Noise Zero Error, TMIN to TMAX Zero Error Temperature Drift Gain Error, TMIN to TMAX3 Gain Error Temperature Drift Power Supply Sensitivity AC ACCURACY Signal-to-Noise Dynamic Range Spurious-Free Dynamic Range Total Harmonic Distortion Signal-to-(Noise + Distortion) –3 dB Input Bandwidth SAMPLING DYNAMICS Aperture Delay Aperture Jitter Transient Response Overvoltage Recovery REFERENCE External Reference Voltage Range REF Voltage with Reference Buffer Reference Buffer Input Voltage Range REFBUFIN Input Current REF Current Drain Conditions Min 18 VIN+ – VIN– VIN+, VIN– to AGND fIN = 100 kHz 100 kSPS Throughput –VREF –0.1 Typ Max Unit Bits +VREF AVDD + 0.1 V V dB μA 10 100 μs kSPS +2.5 +1.75 LSB2 LSB Bits LSB LSB ppm/°C % of FSR ppm/°C LSB 65 4 0 –2.5 –1 18 VREF = 5 V 0.7 –40 –0.048 AVDD = 5 V ± 5% fIN = 2 kHz, VREF = 5 V VREF = 4.096 V fIN = 10 kHz, VREF = 4.096 V fIN = 45 kHz, VREF = 4.096 V VIN+ = VIN– = VREF/2 = 2.5 V fIN = 2 kHz fIN = 10 kHz fIN = 45 kHz fIN = 2 kHz fIN = 10 kHz fIN = 45 kHz fIN = 2 kHz fIN = 2 kHz, –60 dB Input 98 ±40 ±0.5 See Note 3 ±1.6 ±4 101 100 99.5 98 103 120 117 110 –118 –115 –110 100 41 900 dB4 dB dB dB dB dB dB dB dB dB dB dB dB kHz 2 5 ns ps rms μs μs Full-Scale Step REF REFBUFIN = 2.5 V REFBUFIN 100 kSPS Throughput Rev. A | Page 3 of 28 +0.048 8.5 8.5 3 4.05 1.8 –1 4.096 4.096 2.5 42 AVDD + 0.1 4.15 2.6 +1 V V V μA μA AD7678 Parameter DIGITAL INPUTS Logic Levels VIL VIH IIL IIH DIGITAL OUTPUTS Data Format5 Pipeline Delay6 VOL VOH POWER SUPPLIES Specified Performance AVDD DVDD OVDD Operating Current AVDD DVDD8 OVDD8 Conditions Min –0.3 2.0 –1 –1 ISINK = 1.6 mA ISOURCE = –500 μA 4.75 4.75 2.7 100 kSPS Throughput PDBUF High TMIN to TMAX 1 Max Unit +0.8 DVDD + 0.3 +1 +1 V V μA μA 0.4 V V 5.25 5.25 DVDD + 0.37 V V V OVDD – 0.6 5 5 2.6 1 40 18 180 31 PDBUF High @ 100 kSPS PDBUF High @ 1 kSPS PDBUF Low @ 100 kSPS TEMPERATURE RANGE9 Specified Performance Typ –40 26 +85 mA mA μA mW μW mW °C See the Analog Inputs section. LSB means Least Significant Bit. With the ±4.096 V input range, 1 LSB is 31.25 μV. 3 See the Definition of Specifications section. The nominal gain error is not centered at zero and is −0.029% of FSR. This specification is the deviation from this nominal value. These specifications do not include the error contribution from the external reference, but do include the error contribution from the reference buffer if used. 4 All specifications in dB are referred to a full-scale input, FS. Tested with an input signal at 0.5 dB below full scale, unless otherwise specified. 5 Data format parallel or serial 18-bit. 6 Conversion results are available immediately after completed conversion. 7 The maximum should be the minimum of 5.25 V and DVDD + 0.3 V. 8 Tested in Parallel Reading mode. 9 Contact factory for extended temperature range. 2 Rev. A | Page 4 of 28 AD7678 TIMING SPECIFICATIONS Table 3. –40°C to +85°C, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted. Parameter Refer to Figure 27 and Figure 28 Convert Pulse Width Time between Conversions CNVST LOW to BUSY HIGH Delay BUSY HIGH All Modes Except Master Serial Read after Convert Aperture Delay End of Conversion to BUSY LOW Delay Conversion Time Acquisition Time RESET Pulsewidth Refer to Figure 29, Figure 30, and Figure 31 (Parallel Interface Modes) CNVST LOW to Data Valid Delay Data Valid to BUSY LOW Delay Bus Access Request to Data Valid Bus Relinquish Time Refer to Figure 33 and Figure 34 (Master Serial Interface Modes)1 CS LOW to SYNC Valid Delay CS LOW to Internal SCLK Valid Delay CS LOW to SDOUT Delay CNVST LOW to SYNC Delay SYNC Asserted to SCLK First Edge Delay2 Internal SCLK Period2 Internal SCLK HIGH2 Internal SCLK LOW2 SDOUT Valid Setup Time2 SDOUT Valid Hold Time2 SCLK Last Edge to SYNC Delay2 CS HIGH to SYNC HI-Z CS HIGH to Internal SCLK HI-Z CS HIGH to SDOUT HI-Z BUSY HIGH in Master Serial Read after Convert2 CNVST LOW to SYNC Asserted Delay SYNC Deasserted to BUSY LOW Delay Refer to Figure 35 and Figure 36 (Slave Serial Interface Modes) External SCLK Setup Time External SCLK Active Edge to SDOUT Delay SDIN Setup Time SDIN Hold Time External SCLK Period External SCLK HIGH External SCLK LOW 1 Symbol Min t1 t2 t3 t4 t5 t6 t7 t8 t9 10 10 t10 t11 t12 t13 t14 t15 t16 t17 t18 t19 t20 t21 t22 t23 t24 t25 t26 t27 t28 t29 t30 t31 t32 t33 t34 t35 t36 t37 Typ 35 1.5 2 10 1.5 8.5 10 1.5 20 45 15 5 10 10 10 525 3 25 12 7 4 2 3 40 10 10 10 See Table 4 1.5 25 5 3 5 5 25 10 10 In serial interface modes, the SYNC, SCLK, and SDOUT timings are defined with a maximum load CL of 10 pF; otherwise, the load is 60 pF maximum. In Serial Master Read during Convert mode. See Table 4 for Serial Master Read after Convert mode. 2 Rev. A | Page 5 of 28 Max Unit ns μs ns μs ns ns μs μs ns μs ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns μs ns 18 ns ns ns ns ns ns ns AD7678 Table 4. Serial Clock Timings in Master Read after Convert DIVSCLK[1] DIVSCLK[0] SYNC to SCLK First Edge Delay Minimum Internal SCLK Period Minimum Internal SCLK Period Maximum Internal SCLK HIGH Minimum Internal SCLK LOW Minimum SDOUT Valid Setup Time Minimum SDOUT Valid Hold Time Minimum SCLK Last Edge to SYNC Delay Minimum Busy High Width Maximum Symbol t18 t19 t19 t20 t21 t22 t23 t24 t28 Rev. A | Page 6 of 28 0 0 3 25 40 12 7 4 2 3 2.25 0 1 17 60 80 22 21 18 4 60 3 1 0 17 120 160 50 49 18 30 140 4.5 1 1 17 240 320 100 99 18 89 300 7.5 Unit ns ns ns ns ns ns ns ns μs AD7678 ABSOLUTE MAXIMUM RATINGS Table 5. AD7678 Absolute Maximum Ratings1 Parameter Analog Inputs IN+2, IN–2, REF, REFBUFIN, REFGND to AGND Ground Voltage Differences AGND, DGND, OGND Supply Voltages AVDD, DVDD, OVDD AVDD to DVDD, AVDD to OVDD DVDD to OVDD Digital Inputs Internal Power Dissipation3 Internal Power Dissipation4 Junction Temperature Storage Temperature Range Lead Temperature Range (Soldering 10 sec) Rating 1.6mA AVDD + 0.3 V to AGND – 0.3 V IOL TO OUTPUT PIN 1.4V CL 60pF1 ±0.3 V 500A –0.3 V to +7 V ±7 V –0.3 V to +7 V –0.3 V to DVDD + 0.3 V 700 mW 2.5 W 150°C –65°C to +150°C IOH 1 IN SERIAL INTERFACE MODES,THE SYNC, SCLK, AND SDOUT TIMINGS ARE DEFINED WITH A MAXIMUM LOAD CL OF 10pF; OTHERWISE,THE LOAD IS 60pF MAXIMUM. 03084–0–002 Figure 2. Load Circuit for Digital Interface Timing, SDOUT, SYNC, SCLK Outputs, CL = 10 pF 2V 0.8V tDELAY 300°C tDELAY 2V 0.8V 2V 0.8V 03084–0–003 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 See Analog Inputs section. 3 Specification is for device in free air: 48-Lead LQFP: θJA = 91°C/W, θJC = 30°C/W. 4 Specification is for device in free air: 48-Lead LFCSP: θJA = 26°C/W. Figure 3. Voltage Reference Levels for Timing ESD CAUTION Rev. A | Page 7 of 28 AD7678 REFGND REF IN– IN+ NC NC NC AVDD REFBUFIN NC AGND PDBUF PIN CONFIGURATION AND FUNCTION DESCRIPTIONS 48 47 46 45 44 43 42 41 40 39 38 37 AGND 1 AVDD 2 36 AGND PIN 1 IDENTIFIER 35 CNVST MODE0 3 34 PD MODE1 4 33 RESET D0/OB/2C 5 NC 6 32 CS AD7678 31 RD TOP VIEW (Not to Scale) NC 7 D1/A0 8 30 DGND 29 BUSY D2/A1 9 28 D17 D3 10 27 D16 D4/DIVSCLK[0] 11 26 D15 D5/DIVSCLK[1] 12 25 D14 NOTES 1. NC = NO CONNECT. 2. THE EXPOSED PAD IS INTERNALLY CONNECTED TO AGND. THIS CONNECTION IS NOT REQUIRED TO MEET THE ELECTRICAL PERFORMANCES; HOWEVER, FOR INCREASED RELIABILITY OF THE SOLDER JOINTS, IT IS RECOMMENDED THAT THE PAD BE SOLDERED TO THE ANALOG GROUND OF THE SYSTEM. 03084-004 D13/RDERROR D12/SYNC DGND D10/SDOUT D11/SCLK OVDD DVDD D9/RDC/SDIN OGND D6/EXT/INT D7/INVSYNC D8/INVSCLK 13 14 15 16 17 18 19 20 21 22 23 24 Figure 4. 48-Lead LQFP and 48-Lead LFCSP (ST-48 and CP-48) Pin Configuration Table 6. Pin Function Descriptions Pin No. 1, 44 2, 47 3 4 Mnemonic AGND AVDD MODE0 MODE1 Type1 P P DI DI 5 D0/OB/2C DI/O 6, 7, 40–42, 45 8 NC D1/A0 DI/O 9 D2/A1 DI/O 10 D3 DO 11, 12 D[4:5]or DIVSCLK[0:1] DI/O Description Analog Power Ground Pin. Input Analog Power Pins. Nominally 5 V. Data Output Interface Mode Selection. Data Output Interface Mode Selection: Interface MODE # MODE1 MODE0 Description 0 0 0 18-Bit Interface 1 0 1 16-Bit Interface 2 1 0 Byte Interface 3 1 1 Serial Interface When MODE = 0 (18-bit interface mode), this pin is Bit 0 of the parallel port data output bus and the data coding is straight binary. In all other modes, this pin allows choice of straight binary/binary twos complement. When OB/2C is HIGH, the digital output is straight binary; when LOW, the MSB is inverted, resulting in a twos complement output from its internal shift register. No Connect. When MODE = 0 (18-bit interface mode), this pin is Bit 1 of the parallel port data output bus. In all other modes, this input pin controls the form in which data is output, as shown in Table 7. When MODE = 0 or 1 (18-bit or 16-bit interface mode), this pin is Bit 2 of the parallel port data output bus. In all other modes, this input pin controls the form in which data is output, as shown in Table 7. In all modes except MODE = 3, this output is used as Bit 3 of the parallel port data output bus. This pin is always an output, regardless of the interface mode. In all modes except MODE = 3, these pins are Bit 4 and Bit 5 of the parallel port data output bus. When MODE = 3 (serial mode), EXT/INT is LOW, and RDC/SDIN is LOW (serial master read after convert), these inputs, part of the serial port, are used to slow down, if desired, the internal serial clock that clocks the data output. In other serial modes, these pins are not used. Rev. A | Page 8 of 28 AD7678 Pin No. 13 Mnemonic D6 or EXT/INT Type1 DI/O 14 D7 or INVSYNC DI/O 15 D8 or INVSCLK DI/O 16 D9 or RDC/SDIN DI/O 17 18 OGND OVDD P P 19 20 21 DVDD DGND D10 or SDOUT P P DO 22 D11 or SCLK DI/O 23 D12 or SYNC DO 24 D13 or RDERROR DO 25–28 D[14:17] DO 29 BUSY DO 30 31 32 DGND RD CS P DI DI 33 RESET DI Description In all modes except MODE = 3, this output is used as Bit 6 of the parallel port data output bus. When MODE = 3 (serial mode), this input, part of the serial port, is used as a digital select input for choosing the internal data clock or an external data clock. With EXT/INT tied LOW, the internal clock is selected on the SCLK output. With EXT/INT set to a logic HIGH, output data is synchronized to an external clock signal connected to the SCLK input. In all modes except MODE = 3, this output is used as Bit 7 of the parallel port data output bus. When MODE = 3 (serial mode), this input, part of the serial port, is used to select the active state of the SYNC signal. When LOW, SYNC is active HIGH. When HIGH, SYNC is active LOW. In all modes except MODE = 3, this output is used as Bit 8 of the parallel port data output bus. When MODE = 3 (serial mode), this input, part of the serial port, is used to invert the SCLK signal. It is active in both master and slave modes. In all modes except MODE = 3, this output is used as Bit 9 of the parallel port data output bus. When MODE = 3 (serial mode), this input, part of the serial port, is used as either an external data input or a read mode selection input depending on the state of EXT/INT. When EXT/INT is HIGH, RDC/SDIN could be used as a data input to daisy-chain the conversion results from two or more ADCs onto a single SDOUT line. The digital data level on SDIN is output on SDOUT with a delay of 18 SCLK periods after the initiation of the read sequence. When EXT/INT is LOW, RDC/SDIN is used to select the read mode. When RDC/SDIN is HIGH, the data is output on SDOUT during conversion. When RDC/SDIN is LOW, the data can be output on SDOUT only when the conversion is complete. Input/Output Interface Digital Power Ground. Output Interface Digital Power. Nominally at the same supply as the host interface (5 V or 3 V). Should not exceed DVDD by more than 0.3 V. Digital Power. Nominally at 5 V. Digital Power Ground. In all modes except MODE = 3, this output is used as Bit 10 of the parallel port data output bus. When MODE = 3 (serial mode), this output, part of the serial port, is used as a serial data output synchronized to SCLK. Conversion results are stored in an on-chip register. The AD7678 provides the conversion result, MSB first, from its internal shift register. The data format is determined by the logic level of OB/2C. In serial mode when EXT/INT is LOW, SDOUT is valid on both edges of SCLK. In serial mode when EXT/INT is HIGH and INVSCLK is LOW, SDOUT is updated on the SCLK rising edge and is valid on the next falling edge; if INVSCLK is HIGH, SDOUT is updated on the SCLK falling edge and is valid on the next rising edge. In all modes except MODE = 3, this output is used as Bit 11 of the parallel port data output bus. When MODE = 3 (serial mode), this pin, part of the serial port, is used as a serial data clock input or output, depending upon the logic state of the EXT/INT pin. The active edge where the data SDOUT is updated depends upon the logic state of the INVSCLK pin. In all modes except MODE = 3, this output is used as Bit 12 of the parallel port data output bus. When MODE = 3 (serial mode), this output, part of the serial port, is used as a digital output frame synchronization for use with the internal data clock (EXT/INT = Logic LOW). When a read sequence is initiated and INVSYNC is LOW, SYNC is driven HIGH and remains HIGH while the SDOUT output is valid. When a read sequence is initiated and INVSYNC is HIGH, SYNC is driven LOW and remains LOW while SDOUT output is valid. In all modes except MODE = 3, this output is used as Bit 13 of the parallel port data output bus. In MODE = 3 (serial mode) and when EXT/INT is HIGH, this output, part of the serial port, is used as an incomplete read error flag. In slave mode, when a data read is started and not complete when the following conversion is complete, the current data is lost and RDERROR is pulsed high. Bit 14 to Bit 17 of the Parallel Port Data Output Bus. These pins are always outputs regardless of the interface mode. Busy Output. Transitions HIGH when a conversion is started. Remains HIGH until the conversion is complete and the data is latched into the on-chip shift register. The falling edge of BUSY could be used as a data ready clock signal. Must Be Tied to Digital Ground. Read Data. When CS and RD are both LOW, the interface parallel or serial output bus is enabled. Chip Select. When CS and RD are both LOW, the interface parallel or serial output bus is enabled. CS is also used to gate the external clock. Reset Input. When set to a logic HIGH, reset the AD7678. Current conversion, if any, is aborted. If not used, this pin could be tied to DGND. Rev. A | Page 9 of 28 AD7678 Pin No. 34 Mnemonic PD Type1 DI 35 CNVST DI 36 37 AGND REF P AI 38 39 43 46 REFGND IN– IN+ REFBUFIN AI AI AI AI 48 PDBUF DI 49 (EPAD) Exposed Pad (EPAD) Description Power-Down Input. When set to a logic HIGH, power consumption is reduced and conversions are inhibited after the current one is completed. Start Conversion. If CNVST is held HIGH when the acquisition phase (t8) is complete, the next falling edge on CNVST puts the internal sample/hold into the hold state and initiates a conversion. If CNVST is held LOW when the acquisition phase is complete, the internal sample/hold is put into the hold state and a conversion is started immediately. Must Be Tied to Analog Ground. Reference Input Voltage and Internal Reference Buffer Output. Apply an external reference on this pin if the internal reference buffer is not used. Should be decoupled effectively with or without the internal buffer. Reference Input Analog Ground. Differential Negative Analog Input. Differential Positive Analog Input. Reference Buffer Input Voltage. The internal reference buffer has a fixed gain. It outputs 4.096 V typically when 2.5 V is applied on this pin. Allows Choice of Buffering Reference. When LOW, buffer is selected. When HIGH, buffer is switched off. The exposed pad is internally connected to AGND. This connection is not required to meet the electrical performances; however, for increased reliability of the solder joints, it is recommended that the pad be soldered to the analog ground of the system. 1 AI = Analog Input; AO = Analog Output; DI = Digital Input; DI/O = Bidirectional Digital; DO = Digital Output; P = Power. Table 7. Data Bus Interface Definitions MODE MODE1 MODE0 D0/OB/2C D1/A0 D2/A1 D[3] D[4:9] D[10:11] D[12:15] D[16:17] Description 0 1 0 0 0 1 R[0] OB/2C R[1] A0:0 R[2] R[2] R[3] R[3] R[4:9] R[4:9] R[10:11] R[10:11] R[12:15] R[12:15] R[16:17] R[16:17] 18-Bit Parallel 16-Bit High Word 1 0 1 OB/2C A0:1 R[0] R[1] 2 1 0 OB/2C A0:0 A1:0 2 1 0 OB/2C A0:0 A1:1 2 1 0 OB/2C A0:1 A1:0 2 1 0 OB/2C A0:1 A1:1 All Hi-Z 3 1 1 OB/2C All Hi-Z All Zeros 16-Bit Low Word R[10:11] R[12:15] R[16:17] All Hi-Z R[2:3] R[4:7] R[8:9] All Hi-Z R[0:1] All Hi-Z All Zeros All Zeros Serial Interface R[0:17] is the 18-bit ADC value stored in its output register. Rev. A | Page 10 of 28 R[0:1] 8-Bit HIGH Byte 8-Bit MID Byte 8-Bit LOW Byte 8-Bit LOW Byte Serial Interface AD7678 DEFINITION OF SPECIFICATIONS Integral Nonlinearity Error (INL) Total Harmonic Distortion (THD) Linearity error refers to the deviation of each individual code from a line drawn from negative full scale through positive full scale. The point used as negative full scale occurs ½ LSB before the first code transition. Positive full scale is defined as a level 1½ LSB beyond the last code transition. The deviation is measured from the middle of each code to the true straight line. THD is the ratio of the rms sum of the first five harmonic components to the rms value of a full-scale input signal, and is expressed in decibels. Differential Nonlinearity Error (DNL) In an ideal ADC, code transitions are 1 LSB apart. Differential nonlinearity is the maximum deviation from this ideal value. It is often specified in terms of resolution for which no missing codes are guaranteed. Gain Error The first transition (from 000…00 to 000…01) should occur for an analog voltage ½ LSB above the nominal –full scale (–4.095991 V for the ±4.096 V range). The last transition (from 111…10 to 111…11) should occur for an analog voltage 1½ LSB below the nominal full scale (4.095977 V for the ±4.096 V range). The gain error is the deviation of the difference between the actual level of the last transition and the actual level of the first transition from the difference between the ideal levels. Zero Error The zero error is the difference between the ideal midscale input voltage (0 V) from the actual voltage producing the midscale output code. Dynamic Range Dynamic range is the ratio of the rms value of the full scale to the rms noise measured with the inputs shorted together. The value for dynamic range is expressed in decibels. Signal-to-Noise Ratio (SNR) SNR is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the Nyquist frequency, excluding harmonics and dc. The value for SNR is expressed in decibels. Signal-to-(Noise + Distortion) Ratio (S/[N+D]) S/(N+D) is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the Nyquist frequency, including harmonics but excluding dc. The value for S/(N+D) is expressed in decibels. Aperture Delay Aperture delay is a measure of the acquisition performance and is measured from the falling edge of the CNVST input to when the input signal is held for a conversion. Transient Response Transient response is the time required for the AD7678 to achieve its rated accuracy after a full-scale step function is applied to its input. Spurious-Free Dynamic Range (SFDR) SFDR is the difference, in decibels (dB), between the rms amplitude of the input signal and the peak spurious signal. Effective Number of Bits (ENOB) ENOB is a measurement of the resolution with a sine wave input, and is expressed in bits. It is related to S/(N+D) by the following formula: ENOB = (S/[N+D]dB – 1.76)/6.02 Rev. A | Page 11 of 28 AD7678 TYPICAL PERFORMANCE CHARACTERISTICS 2.5 2.0 2.0 1.5 1.5 1.0 DNL-LSB (18-Bit) INL-LSB (18-Bit) 1.0 0.5 0 –0.5 0.5 0 –1.0 –1.5 –0.5 –2.0 –2.5 0 65536 131072 CODE 196608 –1.0 262144 0 65536 131072 CODE 03084-0-005 Figure 5. Integral Nonlinearity vs. Code 196608 262144 03084-0-008 Figure 8. Differential Nonlinearity vs. Code 70000 90000 83610 VREF = 5V 60158 59966 VREF = 5V 80000 60000 70000 50000 COUNTS COUNTS 60000 40000 30000 50000 40000 30000 23000 21862 20000 20000 5919 0 0 10000 3931 42 32 0 0 0 0 0 20015 20016 20017 20018 20019 2001A2001B2001C2001D 2001E CODE IN HEX Figure 6. Histogram of 131,072 Conversions of a DC Input at the Code Transition AMPLITUDE (dB of Full Scale) SNR AND S/[N+D] (dB) fS = 100kSPS fIN = 11kHz VREF = 4.096V SNR = 99.6dB THD = –116dB SFDR = 116.2dB S/(N+D) = 99.5dB –60 1 0 2001B 2001C 2001D 2001E 2001F 20020 20021 20022 20023 CODE IN HEX Figure 9. Histogram of 131,072 Conversions of a DC Input at the Code Center 0 –40 1053 522 03084-0-009 03084-0-006 –20 0 –80 –100 –120 102 16.6 100 16.4 SNR 98 16.2 S/(N+D) 16.0 96 –140 ENOB –160 –180 0 5 10 15 20 25 30 35 FREQUENCY (kHz) 40 45 94 50 0 10 20 30 FREQUENCY (kHz) 03084-0-012 Figure 7. FFT (11 kHz Tone) 15.8 50 40 03084-0-015 Figure 10. SNR, S/(N+D), and ENOB vs. Frequency Rev. A | Page 12 of 28 ENOB (Bits) 10000 AD7678 –100 –80 THD, HARMONICS (dB) THD, HARMONICS (dB) –90 –100 –110 THD –120 THIRD HARMONIC SECOND HARMONIC –130 –110 THD THIRD HARMONIC –120 SECOND HARMONIC –130 –140 –140 –55 –150 0 20 30 FREQUENCY (kHz) 10 40 50 5 25 45 65 85 105 125 03084-0-019 Figure 14. THD and Harmonics vs. Temperature 10000 104 VREF = 4.096V 1000 103 OPERATING CURRENT (A) SNR REFERRED TO FULL SCALE (dB) –15 TEMPERATURE (C) Figure 11. THD and Harmonics vs. Frequency 102 SNR 101 S/(N+D) 100 100 10 AVDD 1 DVDD 0.1 OVDD 99 0.01 98 –60 –50 –40 –30 –20 –10 INPUT LEVEL (dB) 0.001 0 1 SNR 16.0 100 S/(N+D) ENOB 99 15.5 15.0 98 97 –55 –35 –15 5 25 45 65 TEMPERATURE (C) 85 100 1k SAMPLING RATE (SPS) 10k 100k 03084-0-020 Figure 15. Operating Current vs. Sampling Rate 16.5 105 14.5 125 03084-0-018 Figure 13. SNR, S/(N+D), and ENOB vs. Temperature 1000 POWER-DOWN OPERATING CURRENTS (nA) 101 10 03084-0-017 Figure 12. SNR and S/(N+D) vs. Input Level SNR, S/[N+D] (dB) –35 03084-0-016 800 DVDD 600 400 AVDD 200 0 –55 OVDD –35 –15 5 25 45 65 TEMPERATURE (C) 85 105 Figure 16. Power-Down Operating Currents vs. Temperature Rev. A | Page 13 of 28 125 03084-0-021 AD7678 50 50 OVDD = 2.7V @ 85°C 30 40 GAIN ERROR 20 t12 DELAY (ns) ZERO ERROR, GAIN ERROR (LSB) 40 10 0 ZERO ERROR –10 30 20 OVDD = 2.7V @ 25°C OVDD = 5V @ 85°C –20 –30 OVDD = 5V @ 25°C 10 –40 –50 –55 –35 –15 5 25 45 65 TEMPERATURE (C) 85 105 0 125 0 50 100 150 CL (pF) 03083-0-022 Figure 17. Zero Error and Gain Error vs. Temperature Figure 18. Typical Delay vs. Load Capacitance CL Rev. A | Page 14 of 28 200 03084-0-024 AD7678 CIRCUIT INFORMATION IN+ MSB 262,144C 131,072C LSB 4C 2C C SW+ SWITCHES CONTROL C BUSY REF COMP CONTROL LOGIC OUTPUT CODE REFGND 4C 262,144C 131,072C 2C C C LSB MSB SW– IN– CNVST 03084–0–025 Figure 19. ADC Simplified Schematic The AD7678 is a very fast, low power, single-supply, precise 18-bit analog-to-digital converter (ADC) using successive approximation architecture. CONVERTER OPERATION The AD7678’s linearity and dynamic range are similar or better than many - ADCs. With the advantages of its successive architecture, which ease multiplexing and reduce power with throughput, it can be advantageous in applications that normally use - ADCs. The AD7678 provides the user with an on-chip track/hold, successive approximation ADC that does not exhibit any pipeline or latency, making it ideal for multiple multiplexed channel applications. The AD7678 can be operated from a single 5 V supply and can be interfaced to either 5 V or 3 V digital logic. It is housed in a 48-lead LQFP, or a tiny 48-lead LFCSP package that offers space savings and allows for flexible configurations as either a serial or parallel interface. The AD7678 is pin-to-pin compatible with the AD7674, AD7676, and AD7679. The AD7678 is a successive approximation ADC based on a charge redistribution DAC. Figure 19 shows the simplified schematic of the ADC. The capacitive DAC consists of two identical arrays of 18 binary weighted capacitors, which are connected to the two comparator inputs. During the acquisition phase, terminals of the array tied to the comparator’s input are connected to AGND via SW+ and SW–. All independent switches are connected to the analog inputs. Thus, the capacitor arrays are used as sampling capacitors and acquire the analog signal on IN+ and IN– inputs. When the acquisition phase is complete and the CNVST input goes low, a conversion phase is initiated. When the conversion phase begins, SW+ and SW– are opened first. The two capacitor arrays are then disconnected from the inputs and connected to the REFGND input. Therefore, the differential voltage between the IN+ and IN– inputs captured at the end of the acquisition phase is applied to the comparator inputs, causing the comparator to become unbalanced. By switching each element of the capacitor array between REFGND and REF, the comparator input varies by binary weighted voltage steps (VREF/2, VREF/4...VREF/262144). The control logic toggles these switches, starting with the MSB first, to bring the comparator back into a balanced condition. After completing this process, the control logic generates the ADC output code and brings the BUSY output low. Rev. A | Page 15 of 28 AD7678 Transfer Functions Table 8. Output Codes and Ideal Input Voltages ADC CODE (Straight Binary) Except in 18-bit interface mode, the AD7678 offers straight binary and twos complement output coding when using OB/2C. See Figure 20 and Table 8 for the ideal transfer characteristic. Description FSR –1 LSB FSR – 2 LSB Midscale + 1 LSB Midscale Midscale – 1 LSB –FSR + 1 LSB –FSR 111...111 111...110 111...101 Analog Input VREF = 4.096 V 4.095962 V 4.095924 V 31.25 μV Straight Binary (Hex) 3FFFF1 3FFFE 20001 Twos Complement (Hex) 1FFFF1 1FFFE 00001 0V –31.25 μV 20000 1FFFF 00000 3FFFF -4.095962 V -4.096 V 00001 000002 20001 200002 000...010 000...001 000...000 –FS 1 –FS + 1 LSB +FS – 1 LSB –FS + 0.5 LSB 2 +FS – 1.5 LSB ANALOG INPUT This is also the code for overrange analog input (VIN+ – VIN– above VREF – VREFGND). This is also the code for underrange analog input (VIN+ – VIN– below –VREF + VREFGND). 03084-0-026 Figure 20. ADC Ideal Transfer Function DVDD ANALOG SUPPLY (5V) 20 DIGITAL SUPPLY (3.3V OR 5V) NOTE 5 + 10F ADR421 + 100nF AVDD AGND 10 F 100nF DGND DVDD 100nF OVDD 1M 50k NOTE 1 SCLK SDOUT NOTE 3 CREF REF BUSY 10F NOTE 2 C/P/DSP REFGND CNVST 50 ANALOG INPUT+ – U1 + AD8021 IN+ AD7678 CC MODE1 MODE0 OB/2C PDBUF 50 NOTE 4 ANALOG INPUT– SERIAL PORT 100nF 100nF NOTE 4 10 F OGND REFBUFIN 2.5V REF + CLOCK CS RD – U2 + AD8021 DVDD IN– RESET PD CC NOTES 1. SEE VOLTAGE REFERENCE SECTION. 2. CREF is 10F CERAMIC CAPACITOR OR LOW ESR TANTALUM. CERAMIC SIZE 1206 PANASONIC ECJ-3xB0J106 IS RECOMMENDED. SEE VOLTAGE REFERENCE SECTION. 3. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION. 4.THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION. 5. OPTION, SEE POWER SUPPLY SECTION. Figure 21. Typical Connection Diagram (Internal Reference Buffer, Serial Interface) Rev. A | Page 16 of 28 03084-0-027 AD7678 TYPICAL CONNECTION DIAGRAM Figure 21 shows a typical connection diagram for the AD7678. Different circuitry shown on this diagram is optional and is discussed later in this data sheet. Analog Inputs Figure 22 shows a simplified analog input section of the AD7678. The diodes shown in Figure 22 provide ESD protection for the inputs. Care must be taken to ensure that the analog input signal never exceeds the absolute ratings on these inputs. This will cause these diodes to become forward-biased and start conducting current. These diodes can handle a forward-biased current of 120 mA max. This condition could eventually occur when the input buffer’s U1 or U2 supplies are different from AVDD. In such a case, an input buffer with a short-circuit current limitation can be used to protect the part. AVDD During the acquisition phase for ac signals, the AD7678 behaves like a 1-pole RC filter consisting of the equivalent resistance, R+, R–, and CS. Resistors R+ and R– are typically 3 k and are lumped components made up of a serial resistor and the on resistance of the switches. CS is typically 60 pF and mainly consists of the ADC sampling capacitor. This 1-pole filter with a –3 dB cutoff frequency of 900 kHz typ reduces any undesirable aliasing effect and limits the noise coming from the inputs. Because the input impedance of the AD7678 is very high, the part can be driven directly by a low impedance source without gain error. Driver Amplifier Choice Although the AD7678 is easy to drive, the driver amplifier needs to meet the following requirements: The driver amplifier and the AD7678 analog input circuit have to be able to settle for a full-scale step of the capacitor array at an 18-bit level (0.0004%). In the amplifier’s data sheet, settling at 0.1% or 0.01% is more commonly specified. This could differ significantly from the settling time at an 18-bit level and, therefore, should be verified prior to driver selection. The tiny op amp AD8021, which combines ultralow noise and high gain-bandwidth, meets this settling time requirement. The noise generated by the driver amplifier needs to be kept as low as possible in order to preserve the SNR and transition noise performance of the AD7678. The noise coming from the driver is filtered by the AD7678 analog input circuit 1-pole low-pass filter made by R+, R–, and CS. R+ = 3k IN+ CS CS IN– R– = 3k AGND 03084-0-028 Figure 22. Simplified Analog Input This analog input structure is a true differential structure. By using these differential inputs, signals common to both inputs are rejected as shown in Figure 23, which represents typical CMRR over frequency. The SNR degradation due to the amplifier is SNRLOSS 20 log 80 25 625 f –3dB (Ne N ) 2 75 where: f–3dB is the –3 dB input bandwidth in MHz of the AD7678 (0.9 MHz). N is the noise factor of the amplifiers (1 if in buffer configuration). eN is the equivalent input noise voltage of each op amp in nV/Hz. CMRR (dB) 70 65 60 55 50 1 10 100 FREQUECY (kHz) 1000 Figure 23. Analog Input CMRR vs. Frequency For instance, for a driver with an equivalent input noise of 6 nV/Hz (e.g., AD8610) configured as a buffer, thus with a noise gain of +1, the SNR degrades by only 0.65 dB. 10000 03084-0-029 Rev. A | Page 17 of 28 The driver needs to have a THD performance suitable to that of the AD7678. AD7678 The AD8021 meets these requirements and is usually appropriate for almost all applications. The AD8021 needs a 10 pF external compensation capacitor, which should have good linearity as an NPO ceramic or mica type. The AD8022 could be used if a dual version is needed and gain of 1 is present. The AD829 is an alternative in applications where high frequency (above 100 kHz) performance is not required. In gain of 1 applications, it requires an 82 pF compensation capacitor. The AD8610 is another option when low bias current is needed in low frequency applications. Single-to-Differential Driver For applications using unipolar analog signals, a single-endedto-differential driver will allow for a differential input into the part. The schematic is shown in Figure 24. When provided an input signal of 0 to VREF, this configuration will produce a differential ±VREF with midscale at VREF/2. If the application can tolerate more noise, the AD8138 differential driver can be used. ANALOG INPUT (UNIPOLAR 0V TO 4.096V) U1 AD8021 10pF 590 IN+ 590 1.82k AD7678 IN– REF U2 AD8021 8.25k 100nF 10pF To use the internal reference buffer, PDBUF should be LOW. A 2.5 V reference voltage applied on the REFBUFIN input will result in a 4.096 V reference on the REF pin. In both cases, the voltage reference input REF has a dynamic input impedance and therefore requires an efficient decoupling between REF and REFGND inputs. The decoupling consists of a low ESR 47 μF tantalum capacitor connected to the REF and REFGND inputs with minimum parasitic inductance. Care should also be taken with the reference temperature coefficient of the voltage reference, which directly affects the full-scale accuracy if this parameter matters. For instance, a ±4 ppm/°C temperature coefficient of the reference changes the full scale by ±1 LSB/°C. Power Supply The AD7678 uses three sets of power supply pins: an analog 5 V supply (AVDD), a digital 5 V core supply (DVDD), and a digital output interface supply (OVDD). The OVDD supply defines the output logic level and allows direct interface with any logic working between 2.7 V and DVDD + 0.3 V. To reduce the number of supplies needed, the digital core (DVDD) can be supplied through a simple RC filter from the analog supply, as shown in Figure 21. The AD7678 is independent of power supply sequencing once OVDD does not exceed DVDD by more than 0.3 V, and is therefore free from supply voltage induced latch-up. Additionally, it is very insensitive to power supply variations over a wide frequency range, as shown in Figure 25. REFBUFIN 10F 65 2.5V 03084-0-030 60 Voltage Reference The AD7678 allows the use of an external voltage reference with or without the internal reference buffer. Using the internal reference buffer is recommended when sharing a common reference voltage between multiple ADCs is desired. However, the advantages of using the external reference voltage directly are The SNR and dynamic range improvement (about 1.7 dB) resulting from the use of a reference voltage very close to the supply (5 V) instead of a typical 4.096 V reference when the internal buffer is used. The power saving when the internal reference buffer is powered down (PDBUF HIGH). Rev. A | Page 18 of 28 PSRR (dB) Figure 24. Single-Ended-to-Differential Driver Circuit (Internal Reference Buffer Used) 55 50 45 40 1 10 100 FREQUECY (kHz) 1000 Figure 25. PSRR vs. Frequency 10000 03084-0-031 AD7678 POWER DISSIPATION VERSUS THROUGHPUT t2 The AD7678 automatically reduces its power consumption at the end of each conversion phase. During the acquisition phase, the operating currents are very low, which allows for a significant power savings when the conversion rate is reduced, as shown in Figure 26. This feature makes the AD7678 ideal for very low power battery applications. It should be noted that the digital interface remains active even during the acquisition phase. To reduce the operating digital supply currents even further, the digital inputs need to be driven close to the power rails (DVDD and DGND), and OVDD should not exceed DVDD by more than 0.3 V. 100000 t1 CNVST BUSY t4 t3 t6 t5 MODE ACQUIRE CONVERT ACQUIRE t7 t8 CONVERT 03084-0-033 Figure 27. Basic Conversion Timing Although CNVST is a digital signal, it should be designed with special care with fast, clean edges and levels with minimum overshoot and undershoot or ringing. POWER DISSIPATION (mW) 10000 For other applications, conversions can be automatically initiated. If CNVST is held low when BUSY is low, the AD7678 controls the acquisition phase and then automatically initiates a new conversion. By keeping CNVST low, the AD7678 keeps the conversion process running by itself. It should be noted that the analog input has to be settled when BUSY goes low. Also, at power-up, CNVST should be brought low once to initiate the conversion process. In this mode, the AD7678 could sometimes run slightly faster than the guaranteed limits of 100 kSPS. 1000 100 10 1 PDBUF HIGH 0.1 1 10 100 1k SAMPLING RATE (SPS) 10k 100k 03084-0-032 t9 Figure 26. Power Dissipation vs. Sample Rate RESET CONVERSION CONTROL Figure 27 shows the detailed timing diagrams of the conversion process. The AD7678 is controlled by the CNVST signal, which initiates conversion. Once initiated, it cannot be restarted or aborted, even by the power-down input PD, until the conversion is complete. The CNVST signal operates independently of the CS and RD signals. BUSY DATA BUS t8 CNVST 03084-0-034 Figure 28. RESET Timing Rev. A | Page 19 of 28 AD7678 DIGITAL INTERFACE CS The AD7678 has a versatile digital interface; it can be interfaced with the host system by using either a serial or parallel interface. The serial interface is multiplexed on the parallel data bus. The AD7678 digital interface also accommodates both 3 V and 5 V logic by simply connecting the AD7678’s OVDD supply pin to the host system interface digital supply. Finally, by using the OB/2C input pin in any mode except 18-bit interface mode, both twos complement and straight binary coding can be used. The two signals, CS and RD, control the interface. When at least one of these signals is high, the interface outputs are in high impedance. Usually, CS allows the selection of each AD7678 in multicircuit applications, and is held low in a single AD7678 design. RD is generally used to enable the conversion result on the data bus. RD BUSY DATA BUS CURRENT CONVERSION t12 t13 03084-0-036 Figure 30. Slave Parallel Data Timing for Reading (Read after Convert) CS = 0 t1 CNVST, RD BUSY t4 CS = RD = 0 t3 t1 CNVST t12 t13 t4 BUSY t3 DATA BUS PREVIOUS CONVERSION DATA BUS t10 03084-0-037 Figure 31. Slave Parallel Data Timing for Reading (Read during Convert) t11 PREVIOUS CONVERSION DATA NEW DATA 03084-0-035 CS Figure 29. Master Parallel Data Timing for Reading (Continuous Read) RD PARALLEL INTERFACE The AD7678 is configured to use the parallel interface with an 18-bit, a 16-bit, or an 8-bit bus width, according to Table 7. The data can be read either after each conversion, which is during the next acquisition phase, or during the following conversion, as shown in Figure 30 and Figure 31, respectively. When the data is read during the conversion, however, it is recommended that it is read only during the first half of the conversion phase. This avoids any potential feedthrough between voltage transients on the digital interface and the most critical analog conversion circuitry. Refer to Table 7 for a detailed description of the different options available. A0, A1 PINS D[15:8] HI-Z HIGH BYTE t12 PINS D[7:0] HI-Z LOW BYTE LOW BYTE t12 HIGH BYTE HI-Z t13 HI-Z 03084-0-038 Figure 32. 8-Bit and 16-Bit Parallel Interface SERIAL INTERFACE The AD7678 is configured to use the serial interface when MODE0 and MODE1 are held high. The AD7678 outputs 18 bits of data, MSB first, on the SDOUT pin. This data is synchronized with the 18 clock pulses provided on the SCLK pin. The output data is valid on both the rising and falling edge of the data clock. Rev. A | Page 20 of 28 AD7678 MASTER SERIAL INTERFACE In Read during Conversion mode, the serial clock and data toggle at appropriate instants, minimizing potential feedthrough between digital activity and critical conversion decisions. Internal Clock The AD7678 is configured to generate and provide the serial data clock SCLK when the EXT/INT pin is held low. The AD7678 also generates a SYNC signal to indicate to the host when the serial data is valid. The serial clock SCLK and the SYNC signal can be inverted if desired. Depending on the RDC/SDIN input, the data can be read after each conversion or during the following conversion. Figure 33 and Figure 34 show the detailed timing diagrams of these two modes. In Read after Conversion mode, it should be noted that unlike in other modes, the BUSY signal returns low after the 18 data bits are pulsed out and not at the end of the conversion phase, which results in a longer BUSY width. To accommodate slow digital hosts, the serial clock can be slowed down by using DIVSCLK. Usually, because the AD7678 is used with a fast throughput, the mode master read during conversion is the most recommended serial mode. RDC/SDIN = 0 EXT/INT = 0 CS, RD INVSCLK = INVSYNC = 0 t3 CNVST t28 BUSY t30 t29 t25 SYNC t18 t19 t14 t20 t24 t21 t26 1 SCLK 2 3 16 17 18 t15 t27 X SDOUT t16 t22 D17 D16 D2 D1 D0 t23 03084-0-039 Figure 33. Master Serial Data Timing for Reading (Read after Convert) Rev. A | Page 21 of 28 AD7678 RDC/SDIN = 1 EXT/INT = 0 CS, RD INVSCLK = INVSYNC = 0 t1 CNVST t3 BUSY t17 t25 SYNC t14 t19 t20 t21 t15 SCLK 1 t24 2 3 16 17 t18 X SDOUT t16 t27 D17 t22 t26 18 D16 D2 D1 D0 t23 03084-0-040 Figure 34. Master Serial Data Timing for Reading (Read Previous Conversion during Convert) SLAVE SERIAL INTERFACE External Clock The AD7678 is configured to accept an externally supplied serial data clock on the SCLK pin when the EXT/INT pin is held high. In this mode, several methods can be used to read the data. The external serial clock is gated by CS. When CS and RD are both low, the data can be read after each conversion or during the following conversion. The external clock can be either a continuous or a discontinuous clock. A discontinuous clock can be either normally high or normally low when inactive. Figure 35 and Figure 36 show the detailed timing diagrams of these methods. While the AD7678 is performing a bit decision, it is important that voltage transients not occur on digital input/output pins or degradation of the conversion result could occur. This is particularly important during the second half of the conversion phase because the AD7678 provides error correction circuitry that can correct for an improper bit decision made during the first half of the conversion phase. For this reason, it is recommended that when an external clock is being provided, it is a discontinuous clock that only toggles when BUSY is low or, more importantly, that it does not transition during the latter half of BUSY high. External Discontinuous Clock Data Read after Conversion This mode is the most recommended of the serial slave modes. Figure 35 shows the detailed timing diagrams of this method. After a conversion is complete, indicated by BUSY returning low, the result of this conversion can be read while both CS and RD are low. Data is shifted out MSB first with 18 clock pulses, and is valid on both the rising and falling edge of the clock. Among the advantages of this method, the conversion performance is not degraded because there are no voltage transients on the digital interface during the conversion process. Also, data can be read at speeds up to 40 MHz, accommodating both slow digital host interface and the fastest serial reading. Finally, in this mode only, the AD7678 provides a daisy-chain feature using the RDC/SDIN input pin to cascade multiple converters together. This feature is useful for reducing component count and wiring connections when desired (for instance, in isolated multiconverter applications). An example of the concatenation of two devices is shown in Figure 37. Simultaneous sampling is possible by using a common CNVST signal. It should be noted that the RDC/SDIN input is latched on the edge of SCLK opposite the one used to shift out data on SDOUT. Thus, the MSB of the upstream converter follows the LSB of the downstream converter on the next SCLK cycle. Rev. A | Page 22 of 28 AD7678 INVSCLK = 0 EXT/INT = 1 RD = 0 CS BUSY t36 SCLK t35 t37 1 2 t31 3 16 17 18 19 20 t32 SDOUT X D17 t16 D16 D15 D1 D0 X17 X16 X16 X15 X1 X0 Y17 Y16 t34 SDIN X17 t33 03084-0-041 Figure 35. Slave Serial Data Timing for Reading (Read after Convert) EXT/INT = 1 INVSCLK = 0 RD = 0 CS CNVST BUSY t3 t35 t36 SCLK t37 1 2 t31 SDOUT 3 16 17 18 t32 X D17 D16 D15 D1 D0 t16 03084-0-042 Figure 36. Slave Serial Data Timing for Reading (Read Previous Conversion during Convert) Rev. A | Page 23 of 28 AD7678 BUSY OUT BUSY BUSY AD7678 AD7678 #2 (UPSTREAM) #1 (DOWNSTREAM) RDC/SDIN SDOUT CNVST RDC/SDIN MICROPROCESSOR INTERFACING DATA OUT SDOUT CNVST CS CS SCLK SCLK SCLK IN CS IN CNVST IN The AD7678 is ideally suited for traditional dc measurement applications supporting a microprocessor, and for ac signal processing applications interfacing to a digital signal processor. The AD7678 is designed to interface either with a parallel 8-bit or 16-bit wide interface, or with a general-purpose serial port or I/O ports on a microcontroller. A variety of external buffers can be used with the AD7678 to prevent digital noise from coupling into the ADC. The following section illustrates the use of the AD7678 with an SPI equipped DSP, the ADSP-219x. 03084-0-043 SPI Interface (ADSP-219x) Figure 37. Two AD7678s in a Daisy-Chain Configuration External Clock Data Read during Conversion Figure 36 shows the detailed timing diagrams of this method. During a conversion, while both CS and RD are low, the result of the previous conversion can be read. The data is shifted out MSB first with 18 clock pulses, and is valid on both the rising and falling edge of the clock. The 18 bits have to be read before the current conversion is complete. If that is not done, RDERROR is pulsed high and can be used to interrupt the host interface to prevent incomplete data reading. There is no daisychain feature in this mode, and the RDC/SDIN input should always be tied either high or low. To reduce performance degradation due to digital activity, a fast discontinuous clock is recommended to ensure that all bits are read during the first half of the conversion phase. It is also possible to begin to read the data after conversion and continue to read the last bits even after a new conversion has been initiated. Figure 38 shows an interface diagram between the AD7678 and the SPI equipped ADSP-219x. To accommodate the slower speed of the DSP, the AD7678 acts as a slave device, and data must be read after conversion. This mode also allows the daisychain feature. The convert command could be initiated in response to an internal timer interrupt. The 18-bit output data are read with 3-byte SPI access. The reading process could be initiated in response to the end-of-conversion signal (BUSY going low) using an interrupt line of the DSP. The serial interface (SPI) on the ADSP-219x is configured for master mode (MSTR) = 1, Clock Polarity bit (CPOL) = 0, Clock Phase bit (CPHA) = 1, and SPI interrupt enable (TIMOD) = 00, by writing to the SPI control register (SPICLTx). It should be noted that to meet all timing requirements, the SPI clock should be limited to 17 Mbits/s, which allow it to read an ADC result in about 1.1 μs. DVDD ADSP-219x* AD7678* SER/PAR EXT/INT BUSY CS RD INVSCLK SDOUT SCLK CNVST PFx SPIxSEL (PFx) MISOx SCKx PFx or TFSx * ADDITIONAL PINS OMITTED FOR CLARITY 03084-0-044 Figure 38. Interfacing the AD7678 to an SPI Interface Rev. A | Page 24 of 28 AD7678 APPLICATION HINTS interface digital supply OVDD and the remaining digital circuitry. When DVDD is powered from the system supply, it is useful to insert a bead to further reduce high frequency spikes. LAYOUT The AD7678 has very good immunity to noise on the power supplies. However, care should still be taken with regard to grounding layout. The printed circuit board that houses the AD7678 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes that can be easily separated. Digital and analog ground planes should be joined in only one place, preferably underneath the AD7678, or at least as close to the AD7678 as possible. If the AD7678 is in a system where multiple devices require analog-to-digital ground connections, the connection should still be made at one point only, a star ground point that should be established as close to the AD7678 as possible. The user should avoid running digital lines under the device, because these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7678 to avoid noise coupling. Fast switching signals like CNVST or clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and should never run near analog signal paths. Crossover of digital and analog signals should be avoided. Traces on different but close layers of the board should run at right angles to each other. This will reduce the effect of feedthrough through the board. The power supply lines to the AD7678 should use as large a trace as possible to provide low impedance paths and reduce the effect of glitches on the power supply lines. Good decoupling is also important to lower the supply’s impedance presented to the AD7678 and to reduce the magnitude of the supply spikes. Decoupling ceramic capacitors, typically 100 nF, should be placed close to and ideally right up against each power supply pin (AVDD, DVDD, and OVDD) and their corresponding ground pins. Additionally, low ESR 10 μF capacitors should be located near the ADC to further reduce low frequency ripple. The DVDD supply of the AD7678 can be a separate supply or can come from the analog supply, AVDD, or the digital interface supply, OVDD. When the system digital supply is noisy or when fast switching digital signals are present, and if no separate supply is available, the user should connect the DVDD digital supply to the analog supply AVDD through an RC filter (see Figure 21), and connect the system supply to the The AD7678 has four different ground pins: REFGND, AGND, DGND, and OGND. REFGND senses the reference voltage and should be a low impedance return to the reference because it carries pulsed currents. AGND is the ground to which most internal ADC analog signals are referenced. This ground must be connected with the least resistance to the analog ground plane. DGND must be tied to the analog or digital ground plane depending on the configuration. OGND is connected to the digital system ground. The layout of the decoupling of the reference voltage is important. The decoupling capacitor should be close to the ADC and should be connected with short and large traces to minimize parasitic inductances. EVALUATING THE AD7678’S PERFORMANCE The evaluation board for the AD7678 allows a quick means to measure both dc (histograms and time domain) and ac (time and frequency domain) performances of the converter. The EVAL-AD7678CBZ is an evaluation board package that includes a fully assembled and tested evaluation board, documentation, and software. The accompanying software requires the use of a capture board that must be ordered seperately from the evaluation board (see the Ordering Guide for information). The evaluation board can also be used in a standalone configuration and does not use the software when in this mode. Refer to the EVAL-AD76XXEDZ and EVAL-AD76XXCBZ data sheets available from www.analog.com for evaluation board details. Two types of data capture boards can be used with the EVALAD7678CBZ: USB based (EVAL-CED1Z recommended) Parallel port based (EVAL-CONTROL BRD3Z not recommended because many newer PCs do not include parallel ports any longer) The recommended board layout for the AD7678 is outlined in the evaluation board data sheet. Rev. A | Page 25 of 28 AD7678 OUTLINE DIMENSIONS 0.75 0.60 0.45 9.20 9.00 SQ 8.80 1.60 MAX 37 48 36 1 PIN 1 7.20 7.00 SQ 6.80 TOP VIEW 1.45 1.40 1.35 0.15 0.05 (PINS DOWN) 0.20 0.09 7° 3.5° 0° 0.08 COPLANARITY SEATING PLANE 25 12 13 24 0.27 0.22 0.17 VIEW A 0.50 BSC LEAD PITCH 051706-A VIEW A ROTATED 90° CCW COMPLIANT TO JEDEC STANDARDS MS-026-BBC Figure 39. 48-Lead Low Profile Quad Flat Package [LQFP] (ST-48) Dimensions shown in millimeters 7.00 BSC SQ 0.60 MAX 48 EXPOSED PAD 6.75 BSC SQ 25 24 13 12 0.25 MIN 5.50 REF 0.80 MAX 0.65 TYP 0.50 BSC 5.25 5.10 SQ 4.95 (BOTTOM VIEW) 0.50 0.40 0.30 SEATING PLANE 1 0.05 MAX 0.02 NOM COPLANARITY 0.08 0.20 REF FOR PROPER CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SECTION OF THIS DATA SHEET. COMPLIANT TO JEDEC STANDARDS MO-220-VKKD-2 080108-A TOP VIEW 12° MAX PIN 1 INDICATOR 37 36 PIN 1 INDICATOR 1.00 0.85 0.80 0.30 0.23 0.18 0.60 MAX Figure 40. 48-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 7 mm × 7 mm Body, Very Thin Quad (CP-48-1) Dimensions shown in millimeters ORDERING GUIDE Model AD7678ASTZ1 AD7678ASTZRL1 AD7678ACPZ1 AD7678ACPZRL1 EVAL-AD7678CBZ2 EVAL-CONTROL BRD2Z1, 3 EVAL-CONTROL BRD3Z1, 3 EVAL-CED1Z1, 3 Temperature Range –40°C to +85°C –40°C to +85°C –40°C to +85°C –40°C to +85°C Package Description 48-Lead Low Profile Quad Flat Package [LQFP] 48-Lead Low Profile Quad Flat Package [LQFP] 48-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 48-Lead Lead Frame Chip Scale Package [LFCSP_VQ] Evaluation Board Package Option ST-48 ST-48 CP-48-1 CP-48-1 Parallel Port Capture Board, 32k RAM Parallel Port Capture Board, 128k RAM USB Data Capture Board 1 Z = RoHS Compliant Part. This board can be used as a standalone evaluation board or in conjunction with the a capture board for evaluation/demonstration purposes. These capture board allow the PC to control and communicate with all Analog Devices evaluation boards ending in ED for EVAL-CED1Z and CB for EVAL-CONTROL BRDxZ (x = 2, 3). 2 3 Rev. A | Page 26 of 28 AD7678 NOTES Rev. A | Page 27 of 28 AD7678 NOTES ©2003–2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies. D03084-0-6/09(A) Rev. A | Page 28 of 28