a FEATURES AD7776: Single Channel AD7777: 4-Channel AD7778: 8-Channel Fast 10-Bit ADC: 2.5 ms Worst Case +5 V Only Half-Scale Conversion Option Fast Interface Port Power-Down Mode APPLICATIONS HDD Servos Instrumentation LC2MOS, High Speed 1-, 4- & 8-Channel 10-Bit ADCs AD7776/AD7777/AD7778* FUNCTIONAL BLOCK DIAGRAMS CONTROL REGISTER 10 AIN 1 By setting a bit in a control register within both the four-channel version, AD7777, and the eight-channel version, AD7778, the input channels can either be independently sampled or any two channels of choice can be simultaneously sampled. For all versions the specified input signal range is of the form VBIAS ± VSWING. However, if the RTN pin is biased at, say, 2 V the analog input signal range becomes 0 V to +2 V for all input channels. This is covered in more detail under the section Changing the Analog Input Voltage Range. The voltage VBIAS is the offset of the ADC’s midpoint code from ground and is supplied either by an onboard reference available to the user (REFOUT) or by an external voltage reference applied to REFIN. The full-scale range (FSR) of the ADC is equal to 2 VSWING where VSWING is nominally equal to REFIN/2. Additionally, when placed in the half-scale conversion mode, the value of REFIN is converted. This allows the channel offset(s) to be measured. Control register loading and ADC register reading, channel select and conversion start are under the control of the µP. The twos complemented coded ADCs are easily interfaced to a standard 16-bit MPU bus via their 10-bit data port and standard microprocessor control lines. The AD7776/AD7777/AD7778 are fabricated in linear compatible CMOS (LC2MOS), an advanced, mixed technology process that combines precision bipolar circuits with low power CMOS logic. The AD7776 is available in a 24-pin SOIC package; the AD7777 is available in both 28-pin DIP and 28-pin SOIC packages; the AD7778 is available in a 44-pin PQFP package. MUX ADCREG1 10-BIT ADC T/H DB0–DB9 10 CREFIN VBIAS REFIN REFIN RTN REF VSWING GENERAL DESCRIPTION The AD7776, AD7777 and AD7778 are a family of high speed, multichannel, 10-bit ADCs primarily intended for use in R/W head positioning servos found in high density hard disk drives. They have unique input signal conditioning features that make them ideal for use in such single supply applications. VCC CLKIN CONTROL LOGIC CS AD7776 AGND DGND RD WR BUSY/INT REFOUT AGND VCC CLKIN AIN 1 CONTROL REGISTER AIN 2 MUX 1 AIN 3 T/H 1 AIN 4 10 ADCREG2 10-BIT ADC DB0–DB9 ADCREG1 10 CREFIN VBIAS REFIN MUX 2 T/H 2 VSWING REF RTN REFOUT AGND AD7777 CONTROL LOGIC REFIN DGND VCC CLKIN AIN 1 AIN2 AIN3 AIN4 AIN5 AIN6 AIN7 AIN8 AGND CONTROL REGISTER MUX 1 T/H 1 10 ADCREG2 10-BIT ADC DB0–DB9 ADCREG1 10 CREFIN VBIAS REFIN MUX 2 RTN T/H 2 VSWING REFOUT REF AGND REFIN CONTROL LOGIC *Protected by U.S. Patent No. 4,990,916. CS RD WR BUSY/INT AD7778 DGND AGND REV. 0 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 which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 World Wide Web Site: http://www.analog.com Fax: 617/326-8703 © Analog Devices, Inc., 1997 AD7776/AD7777/AD7778–SPECIFICATIONS (VCC = +5 V 6 5%; AGND = DGND = O V; CLKIN = 8 MHz; RTN = O V; CREFIN = 10 nF; all specifications TMIN to TMAX unless otherwise noted.) Parameter A Versions1 Units Conditions/Comments DC ACCURACY Resolution2 Relative Accuracy Differential Nonlinearity Bias Offset Error Bias Offset Error Match Plus or Minus Full-Scale Error Plus or Minus Full-Scale Error Match 10 ±1 ±1 ± 12 10 ± 12 10 Bits LSB max LSB max LSB max LSB max LSB max LSB max See Terminology No Missing Codes; See Terminology See Terminology Between Channels, AD7777/AD7778 Only; See Terminology See Terminology Between Channels, AD7777/AD7778 Only; See Terminology ANALOG INPUTS Input Voltage Range All Inputs Input Current VBIAS ± VSWING +200 V min/V max µA max VIN = VBIAS ± VSWING; Any Channel REFERENCE INPUT REFIN REFIN Input Current 1.9/2.1 +200 V min/V max µA max For Specified Performance 1.9/2.1 5 ±2 ±5 V min/V max Ω typ mV max mV max Nominal REFOUT = 2.0 V 20 mA max LOGIC OUTPUTS DB0–DB9, BUSY/INT VOL, Output Low Voltage VOH, Output High Voltage Floating State Leakage Current Floating State Capacitance3 ADC Output Coding 0.4 4.0 ± 10 10 Twos Complement V max V min µA max pF max LOGIC INPUTS DB0–DB9, CS, WR, RD, CLKIN Input Low Voltage, VINL Input High Voltage, VINH Input Leakage Current Input Capacitance3 0.8 2.4 10 10 V max V min µA max pF max 4.5 tCLKIN 5.5 tCLKIN + 70 14 tCLKIN 28 tCLKIN 125/500 50 40 ns min ns max ns max ns max ns min/ns max ns min ns min Period of Input Clock CLKIN Minimum High Time for CLKIN Minimum Low Time for CLKIN +4.75/+5.25 15 1.5 V min/V max mA max mA max For Specified Performance CS = RD = +5 V, CR8 = 0 CR8 = 1. All Linear Circuitry OFF 500 µs max From Power-Down Mode REFERENCE OUTPUT REFOUT DC Output Impedance Reference Load Change Short Circuit Current3 CONVERSION TIMING Acquisition Time Single Conversion Double Conversion tCLKIN tCLKIN High tCLKIN Low POWER REQUIREMENTS VCC Range ICC, Normal Mode ICC, Power-Down Mode Power-Up Time to Operational Specifications DYNAMIC PERFORMANCE Signal to Noise and Distortion S/(N+D) Ratio Total Harmonic Distortion (THD) Intermodulation Distortion (IMD) Channel-to-Channel Isolation For Reference Load Current Change of 0 to ± 500 µA For Reference Load Current Change of 0 to ± 1 mA Reference Load Should Not Change During Conversion See Terminology ISINK = 1.6 mA ISOURCE = 200 µA See Terminology See Terminology –57 –60 –75 dB min dB min dB typ –90 dB typ VIN = 99.88 kHz Full-Scale Sine Wave with fSAMPLING = 380.95 kHz VIN = 99.88 kHz Full-Scale Sine Wave with fSAMPLING = 380.95 kHz fa = 103.2 kHz, fb = 96.5 kHz with fSAMPLING = 380.95 kHz. Both Signals Are Sine Waves at Half-Scale Amplitude VIN = 100 kHz Full-Scale Sine Wave with fSAMPLING = 380.95 kHz NOTES 1 Temperature range as follows: A = –40°C to +85°C. 2 1 LSB = (2 × VSWING)/1024 = 1.95 mV for V SWING = 1.0 V. 3 Guaranteed by design, not production tested. Specifications subject to change without notice. –2– REV. 0 AD7776/AD7777/AD7778 TIMING SPECIFICATIONS1, 2 (V CC = +5 V 6 5%; AGND = DGND = 0 V; all specifications TMIN to TMAX unless otherwise noted.) Parameter Label Limit at TMIN to TMAX Units INTERFACE TIMING CS Falling Edge to WR or RD Falling Edge WR or RD Rising Edge to CS Rising Edge WR Pulse Width CS or RD Active to Valid Data3 Bus Relinquish Time after RD4 t1 t2 t3 t4 t5 0 0 53 60 10 45 55 10 1.5 tCLKIN 2.5 tCLKIN + 70 ns min ns min ns min ns max ns min ns max ns min ns min ns min ns max 19.5 tCLKIN + 70 33.5 tCLKIN + 70 60 ns max ns max ns max Data Valid to WR Rising Edge Data Valid after WR Rising Edge WR Rising Edge to BUSY Falling Edge WR Rising Edge to BUSY Rising Edge or INT Falling Edge WR or RD Falling Edge to INT Rising Edge t6 t7 t8 t9 t10 t11 Test Conditions/Comments Timed from Whichever Occurs Last CR9 = 0 Single Conversion, CR6 = 0 Double Conversion, CR6 = 1 CR9 = 1 NOTES 1 See Figures 1 to 3. 2 Timing specifications in bold print are 100% production tested. All other times are guaranteed by design, not production tested. All input signals are specified with tr = tf = 5 ns (10% to 90% of 5 V) and timed from a voltage level of 1.6 V. 3 t4 is measured with the load circuit of Figure 4 and defined as the time required for an output to cross 0.8 V or 2.4 V. 4 t5 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 4. The measured time is then extrapolated back to remove the effects of charging or discharging the 100 pF capacitor. This means that the time t 5 quoted above is the true bus relinquish time of the device and, as such, is independent of the external bus loading capacitance. Specifications subject to change without notice. FIRST CONVERSION FINISHED (CR6 = 0) SECOND CONVERSION FINISHED (CR6 = 1) AD7777/AD7778 ONLY t3 WR, RD t1 t2 t9 t8 CS BUSY (CR8 = 0) t11 RD t5 t4 t10 t9 INT (CR8 = 1) DB0–DB9 t10 Figure 1. Read Cycle Timing t1 Figure 3. BUSY/INT Timing IOL 1.6mA t2 CS t3 DB n WR t6 t7 +2.1V COUT 100pF DB0–DB9 IOH 200µA Figure 2. Write Cycle Timing Figure 4. Load Circuit for Bus Timing Characteristics REV. 0 –3– AD7776/AD7777/AD7778 SOIC Packages, Power Dissipation . . . . . . . . . . . . . . θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . PQFP Package, Power Dissipation . . . . . . . . . . . . . . θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . ABSOLUTE MAXIMUM RATINGS* (TA = +25°C unless otherwise noted) VCC to AGND or DGND . . . . . . . . . . . . . . . . . . –0.3 V, +7 V AGND, RTN to DGND . . . . . . . . . . . . . –0.3 V, VCC + 0.3 V CS, RD, WR, CLKIN, DB0–DB9, BUSY/INT to DGND . . . . . . . . . . . . . –0.3 V, VCC + 0.3 V Analog Input Voltage to AGND . . . . . . . –0.3 V, VCC + 0.3 V REFOUT to AGND . . . . . . . . . . . . . . . . –0.3 V, VCC + 0.3 V REFIN to AGND . . . . . . . . . . . . . . . . . . –0.3 V, VCC + 0.3 V Operating Temperature Range All Versions . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150°C DIP Package, Power Dissipation . . . . . . . . . . . . . . . . 875 mW θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 75°C/W Lead Temperature, Soldering (10 sec) . . . . . . . . . . +260°C 875 mW 75°C/W +215°C +220°C 500 mW 95°C/W +215°C +220°C *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 listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD7776/AD7777/AD7778 feature proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. WARNING! ESD SENSITIVE DEVICE PIN CONFIGURATIONS 24-Pin SOIC 40 39 38 37 NC 41 RTN 42 REFIN 43 AGND 44 CREFIN 22 RTN NC 3 DB0 23 AGND DB2 NC 24 CREFIN 2 DB1 1 DB1 NC DB0 NC 44-Pin PQFP 36 35 34 DB3 4 21 REFIN NC 1 33 AIN 8 DGND 5 20 AIN NC 2 32 AIN 7 DB4 6 DB2 3 31 AIN 6 30 AIN 5 17 VCC DB7 9 16 CLKIN DB8 10 15 WR 11 14 CS BUSY/INT 12 13 RD (MSB) DB9 4 5 AD7778 29 AIN 4 DB4 6 TOP VIEW (Not to Scale) 28 AIN 3 DB5 7 27 AIN 2 24 REFOUT 11 23 VCC DB0 1 28 CREFIN 12 13 14 15 16 17 18 19 DB1 2 27 AGND CS 10 NC RD NC BUSY/INT AGND NC AIN 1 25 (MSB) DB9 26 9 DB8 8 NC DB6 DB7 NC 28-Pin DIP & SOIC DB3 DGND NC 3 26 RTN DB2 4 25 REFIN DB3 5 24 AIN 4 DGND 6 23 AIN 3 DB4 7 22 AIN2 DB5 8 DB6 9 AD7777 19 REFOUT DB8 11 18 VCC (MSB) DB9 12 BUSY/INT 13 RD 14 21 22 NC = NO CONNECT ORDERING GUIDE TOP VIEW 21 AIN 1 (Not to Scale) 20 AGND DB7 10 20 NC 8 WR DB6 CLKIN DB5 AD7776 19 AGND TOP VIEW 7 (Not to Scale) 18 REFOUT 17 CLKIN 16 WR 15 CS Model Temperature Range No. of Channels Package Option1 AD7776AR2 AD7777AN AD7777AR2 AD7778AS2 –40°C to +85°C –40°C to +85°C –40°C to +85°C –40°C to +85°C 1 4 4 8 R-24 N-28 R-28 S-44 NOTES 1 R = SOIC, N = Plastic DIP, S = PQFP. 2 Analog Devices reserves the right to ship devices branded with a J in place of the A, e.g., AD7776JR instead of AD7776AR. Temperature range remains –40°C to +85°C. NC = NO CONNECT –4– REV. 0 AD7776/AD7777/AD7778 PIN FUNCTION DESCRIPTION Mnemonic Description VCC +5 V Power Supply. AGND Analog Ground. DGND Digital Ground. Ground reference for digital circuitry. DB0–DB9 Input/Output Data Bus. This is a bidirectional data port from which ADC output data may be read and to which control register data may be written. BUSY/INT Busy/Interrupt Output. Active low logic output indicating A/D converter status. This logic output has two modes of operation depending on whether location CR9 of the control register has been set low or high: If CR9 is set low, then the BUSY/INT output will behave as a BUSY signal. The BUSY signal will go low and stay low for the duration of a single conversion, or if simultaneous sampling has been selected, BUSY will stay low for the duration of both conversions. If CR9 is set high, then the BUSY/INT output behaves as an INTERRUPT signal. The INT signal will go low and remain low after either a single conversion is completed or after a double conversion is completed if simultaneous sampling has been selected. With CR9 high, the falling edge of WR or RD resets the INT line high. CS Chip Select Input. The device is selected when this input is low. WR Write Input (Active Low). It is used in conjunction with CS to write data to the control register. Data is latched to the registers on the rising edge of WR. Following the rising edge of WR, the analog input is acquired and a conversion is started. RD Read Input (Active Low). It is used in conjunction with CS to enable the data outputs from the ADC registers. AIN1–8 Analog Inputs 1–8. The analog input range is VBIAS ± VSWING where VBIAS and VSWING are defined by the reference voltage applied to REFIN. Input resistance between any of the analog input pins and AGND is 10 kΩ or greater. REFIN Voltage Reference Input. The AD7776/AD7777/AD7778 are specified over a voltage reference range of 1.9 V to 2.1 V with a nominal value of 2.0 V. This REFIN voltage provides the VBIAS and VSWING levels for the input channel(s). VBIAS is equal to REFIN and VSWING is nominally equal to REFIN/2. Input resistance between this REFIN pin and AGND is 10 kΩ or greater. REFOUT Voltage Reference Output. The internal voltage reference, which is nominally 2.0 V and can be used to provide the bias voltage (VBIAS) for the input channel(s), is provided at this pin. CREFIN Reference Decoupling Capacitor. A 10 nF capacitor must be connected from this pin to AGND to ensure correct operation of the high speed ADC. RTN Signal Return Path for the input channel(s). Normally RTN is connected to AGND at the package. CIRCUIT DESCRIPTION ADC Transfer Function 1FF 1FE For all versions, an input signal of the form VBIAS ± VSWING is expected. This VBIAS signal level operates as a pseudo ground to which all input signals must be referred. The VBIAS level is determined by the voltage applied to the REFIN pin. This can be driven by an external voltage source or, alternatively, the onboard 2 V reference, available at REFOUT, can be used. The magnitude of the input signal swing is equal to VBIAS/2 (or REFIN/2) and is set internally. With a REFIN of 2 V, the analog input signal level varies from 1 V up to 3 V i.e., 2 ± 1 V. Figure 5 shows the transfer function of the ADC and its relationship to VBIAS and VSWING. The half-scale twos complement code of the ADC, 000 Hex (00 0000 0000 Binary), occurs at an input voltage equal to VBIAS. The input full-scale range of the ADC is equal to 2 VSWING, so that the Plus Full-Scale transition (1FE to 1FF) occurs at a voltage equal to VBIAS + VSWING – 1.5 LSBs and the minus full-scale code transition (200 to 201) occurs at a voltage VBIAS – VSWING + 0.5 LSBs. REV. 0 ADC OUTPUT CODE (HEX) 000 202 201 200 VBIAS VBIAS –V SWING ANALOG INPUT, V IN VBIAS +V SWING Figure 5. ADC Transfer Function –5– AD7776/AD7777/AD7778 CR6: Determines whether operation is on a single channel or simultaneous sampling on two channels. Location CR6 is a “don’t care” for the AD7776. CONTROL REGISTER The control register is 10-bits wide and can only be written to. On power-on, all locations in the control register are automatically loaded with 0s. For the single channel AD7776, locations CR0 to CR6 of the control register are “don’t cares.” For the quad channel AD7777, locations CR2 and CR5 are “don’t cares.” Individual bit functions are described below. CR6 Function 0 Single channel operation. Channel select address is contained in locations CR0–CR2. Two channels simultaneously sampled and sequentially converted. Channel select addresses contained in locations CR0–CR2 and CR3–CR5. 1 CR0–CR2: Channel Address Locations. Determines which channel will be selected and converted for single channel operation. For simultaneous sampling operation CR0–CR2 holds the address of one of the two channels to be sampled. CR7: Determines whether the device is in the normal operating mode or in the half-scale test mode. AD7776 CR2 CR1 CR0 Function CR7 Function X* X Select AIN1 0 1 Normal Operating Mode Half-Scale Test Mode X *X = Don’t Care In the half-scale test mode REFIN is internally connected as an analog input(s). In this mode locations CR0–CR2 and CR3– CR5 are all “don’t cares” since it is REFIN which will be converted. For the AD7777 and AD7778, the contents of location CR6 still determine whether a single or a double conversion is carried out on the REFIN level. AD7777 CR2 X* X X X CR1 0 0 1 1 CR0 0 1 0 1 Function Select AIN1 Select AIN2 Select AIN3 Select AIN4 CR8: Determines whether the device is in the normal operating mode or in the powerdown mode. *X = Don’t Care AD7778 CR2 0 0 0 0 1 1 1 1 CR1 0 0 1 1 0 0 1 1 CR0 0 1 0 1 0 1 0 1 Function Select AIN1 Select AIN2 Select AIN3 Select AIN4 Select AIN5 Select AIN6 Select AIN7 Select AIN8 CR8 Function 0 1 Normal Operating Mode Powerdown Mode In the powerdown mode all linear circuitry is turned off and the REFOUT output is weakly (5 kΩ) pulled to AGND. The input impedance of the analog inputs and of the REFIN input remains the same in either normal mode or powerdown mode. See under Circuit Description—Powerdown Mode. CR9: Determines whether BUSY/INT output flag goes low and remains low during conversion(s) or else goes low and remains low after the conversion(s) is (are) complete. CR3–CR5: Channel Address Locations. Only applicable for simultaneous sampling with the AD7777 or AD7778 when CR3–CR5 holds the address of the second channel to be sampled. CR9 BUSY/INT Functionality 0 Output goes low and remains low during conversion(s). Output goes low and remains low after conversion(s) is (are) complete. 1 AD7777 CR5 X* X X X CR4 0 0 1 1 CR3 0 1 0 1 Function Select AIN1 Select AIN2 Select AIN3 Select AIN4 CR3 0 1 0 1 0 1 0 1 Function Select AIN1 Select AIN2 Select AIN3 Select AIN4 Select AIN5 Select AIN6 Select AIN7 Select AIN8 ADC Conversion Start Timing Figure 6 shows the operating waveforms for the start of a conversion cycle. On the rising edge of WR, the conversion cycle starts with the acquisition and tracking of the selected ADC channel, AIN1–8. The analog input voltage is held 40 ns (typically) after the first rising edge of CLKIN following four complete CLKIN cycles. If tD in Figure 6 is greater than 12 ns, the falling edge of CLKIN as shown will be seen as the first falling clock edge. If tD is less than 12 ns, the first falling clock edge to be recognized will not occur until one cycle later. *X = Don’t Care AD7778 CR5 0 0 0 0 1 1 1 1 CR4 0 0 1 1 0 0 1 1 Following the “hold” on the analog input(s), two complete CLKIN cycles are allowed for settling purposes before the MSB decision is made. The actual decision point occurs approximately 40 ns after the rising edge of CLKIN as shown in Figure 6. A further two CLKIN cycles are allowed for the second MSB decision. The succeeding bit decisions are made approximately 40 ns after each rising edge of CLKIN until the conversion is complete. At the end of conversion, if a single conversion has been requested (CR6 = 0), the BUSY/INT line changes –6– REV. 0 AD7776/AD7777/AD7778 state (as programmed by CR9), and the SAR contents are transferred to the first register ADCREG1. The SAR is then reset in readiness for a new conversion. If simultaneous sampling has been requested (CR6 = 1), no change occurs in the status of the BUSY/INT output and the ADC automatically starts the second conversion. At the end of this conversion the BUSY/INT line changes state (as programmed by CR9) and the SAR contents are transferred to the second register, ADCREG2. WR tD * In order to achieve the lowest possible power consumption in the powerdown mode special attention must be paid to the state of the digital and analog inputs and outputs: CLKIN 40ns TYP 40ns TYP VIN CHANNEL ACQUISITION 'HOLD' DB9 (MSB) * TIMING SHOWN FOR tD GREATER THAN 12ns Figure 6. ADC Conversion Start Timing Track-and-Hold The track-and-hold (T/H) amplifiers on the analog input(s) of the AD7776/AD7777/AD7778 allow the ADC to accurately convert an input sine wave of 2 V peak-peak amplitude up to a frequency of 189 kHz, the Nyquist frequency of the ADC when operated at its maximum throughput rate of 378 kHz. This maximum rate of conversion includes conversion time and time between conversions. Because the input bandwidth of the trackand-hold is much greater than 189 kHz, the input signal should be band limited to avoid folding unwanted signals into the band of interest. Powerdown The AD7776/AD7777/AD7778 can be placed in a powerdown mode simply by writing a logic high to location CR8 of the control register. The following changes are effected immediately on writing a “1” to location CR8: • Any conversion in progress is terminated. • If a conversion is in progress, the leading edge of WR immediately drives the BUSY/INT output high. • All the linear circuitry is turned off. • The REFOUT output stops being driven and is weakly (5 kΩ) pulled to analog ground. REV. 0 Control inputs CS, WR and RD retain their purpose while the AD7776/ AD7777/AD7778 is in powerdown. If no conversions are in progress when the AD7776/AD7777/AD7778 is placed into the powerdown modes, the contents of the ADC registers, ADCREG1 and ADCREG2, are retained during powerdown and can be read as normal. On returning to normal operating mode a new conversion (or conversions, dependent on CR6) is automatically started. On completion, the invalid conversion results are loaded into the ADC registers losing the previous valid results. • Because each analog input channel sees a resistive divider to AGND, the input resistance of which does not change between normal and powerdown modes, driving the analog input signals to 0 V or as close as possible to 0 V will minimize the power dissipated in the input signal conditioning circuitry. • Similarly, the REFIN input sees a resistive divider to AGND, the input resistance of which does not change between normal and powerdown modes. If an external reference is being used, then driving this reference input to 0 V or as close as possible to 0 V will minimize the power dissipated in the input signal conditioning circuitry. • Since the REFOUT pin is pulled to AGND via, typically, a 5 kΩ resistor, any voltage above 0 V that this output may be pulled to by external circuitry will dissipate unnecessary power. • Digital inputs CS, WR & RD should all be held at VCC or as close as possible. CLKIN should be held as close as possible to either 0 V or VCC. • Since the BUSY/INT output is actively driven to a logic high, any loading on this pin to 0 V will dissipate power. The AD7776/AD7777/AD7778 comes out of the powerdown mode when a Logic “0” is written to location CR8 of the control register. Note that the contents of the other locations in the control register are retained when the device is placed in powerdown and are valid when power is restored. However, coming out of powerdown provides an opportunity to reload the complete contents of the control register without any extra instructions. –7– AD7776/AD7777/AD7778 Figure 10 shows the interface with the 80C196KB @ 12 MHz and the 80C196KC @ 16 MHz. One wait state is required with the 16 MHz machine. The 80C196 is configured to operate with a 16-bit multiplexed address/data bus. Microprocessor Interfacing Circuits The AD7776/AD7777/AD7778 family of ADCs is intended to interface to DSP machines such as the ADSP-2101, ADSP-2105, the TMS320 family and microcontrollers such as the 80C196 family. Table I gives a truth table for the AD7776/AD7777/AD7778 and summarizes their microprocessor interfacing features. Note that a read instruction to any of the devices while a conversation is in progress will immediately stop that conversion and return unreliable data over the data bus. Figure 7 shows the AD7776/AD7777/AD7778 interfaced to the TMS320C10 @ 20.5 MHz and the TMS320C14 @ 25 MHz. Figure 8 shows the interface with the TMS320C25 @ 40 MHz. Note that one wait state is required with this interface. The ADSP-2101-50 and the ADSP-2105-40 interface is shown in Figure 9. One wait state is required with either of these machines. A11–A0 TMS320C10-20.5 TMS320C14-25 A13–A0 ADDRESS BUS ADDR DECODE ADDR DECODE CS WR WR RD RD RD D23–D6 DATA BUS Figure 9. AD7776/AD7777/AD7778 to ADSP-2101 and ADSP-2105 Interface Figure 7. AD7776/AD7777/AD7778 to TMS320C10 and TMS320C14 Interface AD15–AD6 (PORT 4) ADDRESS BUS IS READY ADDR DECODE MSC ALE 80C196KB-12 80C196KC-16 WR RD TMS320C25-40 '373 LATCH AD7776/7/8* ADDR DECODER CS WR WR RD RD DB9–DB0 AD7–AD0 (PORT 3) D15–D0 ADDRESS BUS CS AD7776/7/8* STRB R/W DATA BUS * ADDITIONAL PINS OMITTED FOR CLARITY * ADDITIONAL PINS OMITTED FOR CLARITY A15–A0 DB9–DB0 ADSP-2101-50 ADSP-2105-40 DB9–DB0 D15–D0 CS AD7776/7/8* WR (C10) DEN (C14) REN EN DMS AD7776/7/8* WE ADDRESS BUS DATA BUS DATA BUS (10) DB9–DB0 * ADDITIONAL PINS OMITTED FOR CLARITY * ADDITIONAL PINS OMITTED FOR CLARITY Figure 10. AD7776/AD7777/AD7778 to 80C196 Interface Figure 8. AD7776/AD7777/AD7778 to TMS320C25 Interface –8– REV. 0 AD7776/AD7777/AD7778 Table I. AD7776/AD7777/AD7778 Truth Table for Microprocessor Interfacing CS RD WR 1 X* X* High Z Data Port High Impedance 0 1 j CR Data Load control register (CR) data to control register and start a conversion. 0 k 1 ADC Data ADC data placed on data bus. Depending upon location CR6 of the control register, one or two Read instructions will be required. DB0–DB9 Function/Comments If CR6 is low, i.e., single channel conversion selected, a read instruction returns the contents of ADCREG1. Succeeding read instructions continue to return the contents of ADCREG1. If CR6 is high, i.e., simultaneous sampling (double conversion) selected, the first read instruction returns the contents of ADCREG1 while the second read instruction returns the contents of ADCREG2. A third read instruction returns ADCREG1 again, the fourth ADCREG2, etc. *X = Don’t Care DESIGN INFORMATION Layout Hints TERMINOLOGY Relative Accuracy Ensure that the layout for the printed circuit board has the digital and analog grounds separated as much as possible. Take care not to run any digital track alongside an analog signal track. Guard (screen) the analog input(s) with RTN. For the AD7776, AD7777 and AD7778, relative accuracy or endpoint nonlinearity is the maximum deviation, in LSBs, of the ADC’s actual code transition points from a straight line drawn between the endpoints of the ADC transfer function. Establish a single point analog ground separate from the logic system ground and as close as possible to the AD7776/AD7777/ AD7778. Both the RTN and AGND pins on the AD7776/ AD7777/AD7778 and all other signal grounds should be connected to this single point analog ground. In turn, this star ground should be connected to the digital ground at one point only—preferably at the low impedance power supply itself. Differential Nonlinearity Low impedance analog and digital power supply common returns are important for correct operation of the devices, so make the foil width for these tracks as wide as possible. In order to ensure a low impedance +5 V power supply at the actual VCC pin, it will be necessary to employ bypass capacitors from the pin itself to DGND. A 4.7 µF tantalum capacitor in parallel with a 0.1 µF ceramic capacitor is sufficient. ADC Corruption Executing a read instruction to the AD7776/AD7777/AD7778 while a conversion is in progress will immediately halt the conversion and return invalid data over the data bus. The BUSY/ INT output pin should be monitored closely and all read instructions to the AD7776/AD7777/AD7778 prevented while this output shows that a conversion is in progress. Executing a write instruction to the AD7776/AD7777/AD7778 while a conversion is in progress immediately halts the conversion, the falling edge of WR driving the BUSY/INT output high. The analog input(s) is sampled as normal and a new conversion sequence (dependent upon CR6) is started. ADC Conversion Time Although each conversion takes only 14 CLKIN cycles, it can take between 4.5 to 5.5 CLKIN cycles to acquire the analog input(s) after the WR input goes high and before any conversions start. REV. 0 Differential nonlinearity is the difference between the measured change and the ideal 1 LSB change between any two adjacent codes. A specified maximum differential nonlinearity of ± 1 LSB ensures no missed codes. Bias Offset Error For an ideal 10-bit ADC, the output code for an input voltage equal to VBIAS should be midscale. The bias offset error is the difference between the actual midpoint voltage for midscale code and VBIAS, expressed in LSBs. Bias Offset Error Match This is a measure of how closely the bias offset errors of all channels track each other. The bias offset error match of any channel must be no further away than 10 LSBs from the bias offset error of any other channel, regardless of whether the channels are independently sampled or simultaneously sampled. Plus and Minus Full-Scale Error The input channels of the ADC can be considered to have bipolar (positive and negative) input ranges, but which are referred to VBIAS (or REFIN) instead of AGND. Positive full-scale error for the ADC is the difference between the actual input voltage required to produce the plus full-scale code transition and the ideal input voltage (VBIAS + VSWING –1.5 LSB), expressed in LSBs. Minus full-scale error is similarly specified for the minus full-scale code transition, relative to the ideal input voltage for this transition (VBIAS – VSWING + 0.5 LSB). Note that the full-scale errors for the ADC input channels are measured after their respective bias offset errors have been adjusted out. Plus and Minus Full-Scale Error Match This is a measure of how closely the full-scale errors of all channels track each other. The full-scale error match of any channel must be no further away than 10 LSBs from the respective fullscale error of any other channel, regardless of whether the channels are independently sampled or simultaneously sampled. –9– AD7776/AD7777/AD7778 Short Circuit Current Figure 11 shows a 2048 point FFT plot for a single channel of the AD7778 with an input signal of 99.88 kHz. The SNR is 58.71 dB. It can be seen that most of the harmonics are buried in the noise floor. It should be noted that the harmonics are taken into account when calculating the S/(N+D). This is defined as the maximum current which will flow either into or out of the REFOUT pin if this pin is shorted to any potential between 0 V and VCC. This condition can be allowed for up to 10 seconds provided that the power dissipation of the package is not exceeded. 0 Signal-to-Noise and Distortion Ratio, S/(N+D) SIGNAL AMPLITUDE – dB Signal-to-noise and distortion ratio, S/(N+D), is the ratio of the rms value of the measured 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 given in decibels. Total Harmonic Distortion, THD Total harmonic distortion 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. For the AD7776/AD7777/ AD7778, total harmonic distortion (THD) is defined as: 20 log = INPUT FREQUENCY = 99.88 kHz SAMPLE FREQUENCY = 380.95 kHz SNR = 58.7 dB TA = +25 °C –20 –40 –60 –80 –90 (V22 + V32 + V42 + V52 + V62)1/2 0 99.88 FREQUENCY – kHz V1 Figure 11. ADC FFT Plot where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the individual harmonics. The relationship between S/(N+D) and resolution (n) is expressed by the following equation: Intermodulation Distortion, IMD With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products, of order (m + n), at sum and difference frequencies of mfa + nfb, where m, n = 0, 1, 2, 3. Intermodulation terms are those for which m or n is not equal to zero. For example, the second order terms include (fa + fb) and (fa – fb) and the third order terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). S/(N+D) = (6.02n + 1.76) dB This is for an ideal part with no differential or integral linearity errors. These errors will cause a degradation in S/(N+D). By working backwards from the above equation, it is possible to get a measure of ADC performance expressed in effective number of bits (n). Channel-to-Channel Isolation Channel-to-channel isolation is a measure of the level of crosstalk between channels. It is measured by applying a full-scale 100 kHz sine wave signal to any one of the input channels and monitoring the remaining channels. The figure given is the worst case across all channels. S/(N+D) (dB) – 1.76 n(effective) = 6.02 The effective number of bits plotted vs. frequency for a single channel of the AD7778 is shown in Figure 12. The effective number of bits is typically 9.5. 10 DIGITAL SIGNAL PROCESSING APPLICATIONS EFFECTIVE NUMBER OF BITS In digital signal processing (DSP) application areas like voice recognition, echo cancellation and adaptive filtering, the dynamic characteristics S/(N+D), THD & IMD of the ADC are critical. The AD7776/AD7777/AD7778 are specified dynamically as well as with standard dc specifications. Because the track/hold amplifier has a wide bandwidth, an antialiasing filter should be placed on the analog inputs to avoid aliasing of high frequency noise back into the bands of interest. The dynamic performance of the ADC is evaluated by applying a sine wave signal of very low distortion to a single analog input which is sampled at a 380.95 kHz sampling rate. A fast Fourier transform (FFT) plot or histogram plot is then generated from which the signal to noise and distortion, harmonic distortion and dynamic differential nonlinearity data can be obtained. Similarly, for intermodulation distortion, an input signal consisting of two pure sine waves at different frequencies is applied to the AD7776/AD7777/AD7778. 9.5 9 SAMPLE FREQUENCY = 378.4 kHz TA = +24 °C 8.5 8 7.5 0 189.2 INPUT FREQUENCY – kHz Figure 12. Effective Number of Bits vs. Frequency –10– REV. 0 AD7776/AD7777/AD7778 Changing the Analog Input Voltage Range RTN is tied to REFOUT then the analog input range becomes 0 V to 2 V. The fixed 2 V analog input voltage span of the ADC can range from 1 V to 3 V (RTN = 0 V) to 0 V to 2 V (RTN = 2 V), i.e., with proper biasing, an input signal range from 0.3 V to 2.3 V can be covered. Both the relative accuracy and differential nonlinearity performance remains essentially unchanged in this mode while the SNR and THD performance are typically 2 dB to 3 dB worse than standard. By biasing the RTN pin above AGND it is possible to change the analog input voltage range from its VBIAS ± VSWING format to a more traditional 0 V to VREF range. The new input range can be described as VOFFSET to (VOFFSET + REFIN) where 0 V ≤ VOFFSET ≤ 1 V. To produce this range the RTN pin must be biased to (REFIN – 2 VOFFSET). For instance if OUTLINE DIMENSIONS Dimensions shown in inches and (mm). R-24 24-Lead Wide-Body SOIC 24 13 0.299 (7.6) 0.291 (7.4) 0.419 (10.65) 0.394 (10.00) PIN 1 1 12 0.614 (15.6) 0.598 (15.2) 0.104 (2.65) 0.093 (2.35) 0.012 (0.3) 0.004 (0.1) 0.050 (1.27) BSC 0.013 (0.32) 0.009 (0.23) 0.019 (0.49) 0.014 (0.35) 0.005 (1.27) 0.015 (0.40) R-28 28-Lead Wide-Body SOIC 28 15 0.299 (7.60) 0.291 (7.39) PIN 1 0.414 (10.52) 0.398 (10.10) 1 14 0.03 (0.76) 0.02 (0.51) 0.708 (18.02) 0.696 (17.67) 0.096 (2.44) 0.089 (2.26) 0.01 (0.254) 0.006 (0.15) 0.050 (1.27) BSC 0.013 (0.32) 0.009 (0.23) 0.019 (0.49) 0.014 (0.35) 1. LEAD NO. 1 IDENTIFIED BY A DOT. 2. SOIC LEADS WILL BE EITHER TIN PLATED OF SOLDER DIPPED IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS. REV. 0 –11– 0.042 (1.067) 0.018 (0.457) AD7776/AD7777/AD7778 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). N-28 28-Lead Plastic DIP 28 C1762–24–1/93 15 0.550 (13.97) 0.530 (13.462) PIN 1 14 1 0.606 (15.39) 0.594 (15.09) 1.450 (36.83) 1.440 (36.576) 0.200 (5.080) MAX 0.160 (4.06) 0.140 (3.56) 0.175 (4.45) 0.120 (3.05) 0.105 (2.67) 0.095 (2.41) 0° 0.065 (1.65) 0.045 (1.14) SEATING PLANE S-44 44-Pin PQFP 0.547 ± 0.01 SQ (13.9 ± 0.25) 0.096 (2.45) MAX 0.031 ± 0.006 (0.8 ± 0.15) 0.394 ± 0.004 SQ (10 ± 0.1) 4°± 4° 23 33 34 22 0.394 ± 0.004 (10 ± 0.1) TOP VIEW PIN 1 44 12 1 0.036 ± 0.004 (0.92 ± 0.1) 0.079 + 0.004/–0.002 (2 + 0.1/–0.05) 11 0.036 ± 0.004 (0.92 ± 0.1) 0.014 ± 0.002 (0.35 ± 0.05) 0.031 ± 0.002 (0.8 ± 0.05) PRINTED IN U.S.A. 0.020 (0.508) 0.015 (0.381) 0.012 (0.305) 0.008 (0.203) 15 ° –12– REV. 0