AD775

a FEATURES CMOS 8-Bit 20 MSPS Sampling A/D Converter Low Power Dissipation: 60 mW +5 V Single Supply Operation Differential Nonlinearity: 0.3 LSB Differential Gain: 1% Differential Phase: 0.5 Degrees Three-State Outputs On-Chip Reference Bias Resistors Adjustable Reference Input Video Industry Standard Pinout Small Packages: 24-Pin 300 Mil SOIC Surface Mount 24-Pin 400 Mil Plastic DIP 18 AV DD VRTS 16 V RT 17 15 8-Bit 20 MSPS, 60 mW Sampling A/D Converter AD775 FUNCTIONAL BLOCK DIAGRAM AV DD 14 15 V IN 19 DV DD 13 11 10 D7 (MSB) 3-STATE OUTPUT LATCHES AD775 CORRECTION LOGIC COARSE COMPARATORS 4 9 8 7 6 5 4 3 D0 (LSB) 8 RREF FINE COMPARATORS BANK A LSB MULTIPLEXOR SWITCH MATRIX 5 255 V RB 23 FINE COMPARATORS BANK B V RBS 22 CLOCK LOGIC 20 21 12 CLK 1 AV SS OE 2 24 AV SS DV SS PRODUCT DESCRIPTION PRODUCT HIGHLIGHTS The AD775 is a CMOS, low power, 8-bit, 20 MSPS sampling analog-to-digital converter (ADC). The AD775 features a builtin sampling function and on-chip reference bias resistors to provide a complete 8-bit ADC solution. The AD775 utilizes a pipelined/ping pong two-step flash architecture to provide high sampling rates (up to 35 MHz) while maintaining very low power consumption (60 mW). Its combination of excellent DNL, fast sampling rate, low differential gain and phase errors, extremely low power dissipation, and single +5 V supply operation make it ideally suited for a variety of video and image acquisition applications, including portable equipment. The AD775’s reference ladder may be connected in a variety of configurations to accommodate different input ranges. The low input capacitance (11 pF typical) provides an easy-to-drive input load compared to conventional flash converters. The AD775 is offered in both 300 mil SOIC and 400 mil DIP plastic packages, and is designed to operate over an extended commercial temperature range (–20°C to +75°C). Low Power: The AD775 has a typical supply current of 12 mA, for a power consumption of 60 mW. Reference ladder current is also low: 6.6 mA typical, minimizing the reference power consumption. Complete Solution: The AD775’s switched capacitor design features an inherent sample/hold function: no external SHA is required. On-chip reference bias resistors are included to allow a supply-based reference to be generated without any external resistors. Excellent Differential Nonlinearity: The AD775 features a typical DNL of 0.3 LSBs, with a maximum limit of 0.5 LSBs. No missing codes is guaranteed. Single +5 V Supply Operation: The AD775 is designed to operate on a single +5 V supply, and the reference ladder may be configured to accommodate analog inputs inclusive of ground. Low Input Capacitance: The 11 pF input capacitance of the AD775 can significantly decrease the cost and complexity of input driving circuitry, compared with conventional 8-bit flash ADCs. 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 Fax: 617/326-8703 (T = 25 with AV , = +5 AV AD775–SPECIFICATIONS CLOCK+= 20C MHz unlessDVotherwiseV,noted), DV A DD DD SS SS = 0 V, VRT = 2.6 V, VRB = +0.6 V, Parameter RESOLUTION DC ACCURACY Integral Nonlinearity (INL) Differential Nonlinearity (DNL) No Missing Codes Offset To Top of Ladder VRT To Bottom of Ladder VRB VIDEO ACCURACY1 Differential Gain Error Differential Phase Error ANALOG INPUT Input Range (VRT–VRB) Input Capacitance AC SPECIFICATIONS2 Signal-to-Noise and Distortion (S/(N + D)) fIN = 1 MHz fIN = 5 MHz Total Harmonic Distortion (THD) fIN = 1 MHz fIN = 5 MHz REFERENCE INPUT Reference Input Resistance (RREF) Case 1: VRT = VRTS, VRB = VRBS Reference Bottom Voltage (VRB) Reference Span (VRT–VRB) Reference Ladder Current (IREF) Case 2: VRT = VRTS, VRB = AVSS Reference Span (VRT–VRB) Reference Ladder Current (IREF) POWER SUPPLIES Operating Voltages AVDD DVDD Operating Current IAVDD IDVDD IAVDD + IDVDD POWER CONSUMPTION TEMPERATURE RANGE Operating NOTES 1 NSTC 40 IRE modulation ramp, CLOCK = 14.3 MSPS. 2 fIN amplitude = 0.3 dB full scale. Min 8 AD775J Typ Max Units Bits +0.5 1.3 ± 0.3 ± 0.5 GUARANTEED –10 0 –35 +15 1.0 0.5 2.0 11 –60 +45 LSB LSB mV mV % Degrees V p-p pF 47 41 –51 –42 230 0.60 1.96 4.4 2.25 5 300 0.64 2.09 7.0 2.39 8 450 0.68 2.21 9.6 2.53 11 dB dB dB dB Ω V V mA V mA +4.75 +4.75 9.5 2.5 12 60 –20 +5.25 +5.25 Volts Volts mA mA mA mW °C 17 85 +75 Specifications subject to change without notice. See Definition of Specifications for additional information. –2– REV. 0 DIGITAL SPECIFICATIONS CLOCK = 20 MHz unless otherwise noted) Parameter LOGIC INPUT High Level Input Voltage Low Level Input Voltage High Level Input Current (VIH = DVDD) Low Level Input Current (VIL = 0 V) Logic Input Capacitance LOGIC OUTPUTS High Level Output Current OE = DVSS, VOH = DVDD–0.5 V OE = DVDD, VOH = DVDD Low Level Output Current OE = DVSS, VOL = 0.4 V OE = DVDD, VOL = 0 V Symbol VIH VIL IIH IIL CIN DVDD 5.0 5.0 5.25 5.25 (TA = +25 C with AVDD, DVDD = +5 V, AVSS, DVSS = 0 V, VRT = 2.6 V, VRB = +0.6 V, AD775J Typ AD775 Units V V µA µA pF Min 4.0 Max 1.0 5 –5 5 IOH IOZ IOL IOZ 4.75 5.25 4.75 5.25 3.7 –1.1 16 mA µA mA µA 16 TIMING SPECIFICATIONS Symbol Maximum Conversion Rate Clock Period Clock High Clock Low Output Delay Pipeline Delay (Latency) Sampling Delay Aperture Jitter Specifications subject to change without notice. SAMPLE N VIN SAMPLE N+1 SAMPLE N+2 Min 20 50 25 25 Typ 35 Max Units MHz ns ns ns ns Clock Cycles ns ps tC tCH tCL tOD tDS 18 4 30 30 2.5 tDS tCH CLK tCL tC tOD OUT DATA N-3 DATA N-2 DATA N-1 DATA N Figure 1. AD775 Timing Diagram REV. 0 – 3– AD775 PIN DESCRIPTION Pin No. 1 2, 24 3 4–9 10 11, 13 12 16 17 23 22 14, 15, 18 19 20, 21 Symbol OE DVSS D0 (LSB) D1–D6 D7 (MSB) DVDD CLK VRTS VRT VRB VRBS AVDD VIN AVSS Type Name and Function DI P DO DO DO P DI AI AI AI AI P AI P OE = Low OE = High Normal Operating Mode. High Impedance Outputs. Digital Ground. Note: DVSS and AVSS pins should share a common ground plane on the circuit board. Least Significant Bit, Data Bit 0. Data Bits 1 Through 6. Most Significant Bit, Data Bit 7. +5 V Digital Supply. Note: DVDD and AVDD pins should share a common supply on the circuit board. Clock Input. Reference Top Bias. Short to VRT for Self-Bias. Reference Ladder Top. Reference Ladder Bottom. Reference Bottom Bias. Short to VRB for Self-Bias. +5 V Analog Supply. Note: DVDD and AVDD pins should share a common supply within 0.5 inches of the AD775. Analog Input. Input Span = VRT–VRB. Analog Ground. Note: DVSS and AVSS pins should share a common ground within 0.5 inches of the AD775. NOTE Type: AI = Analog Input; DI = Digital Input; DO = Digital Output; P = Power. PIN CONFIGURATION (DIP and SOIC) MAXIMUM RATINGS* Supply Voltage (AVDD, DVDD) . . . . . . . . . . . . . . . . . . . . 7 V Supply Difference (AVDD–DVDD) . . . . . . . . . . . . . . . . . . 0 V Ground Difference (AVSS–DVSS) . . . . . . . . . . . . . . . . . . . 0 V Reference Voltage (VRT, VRB) . . . . . . . . . . . . . . . . VDD to VSS Analog Input Voltage (VIN) . . . . . . . . . . . . . . . . . . VDD to VSS Digital Input Voltage (CLK) . . . . . . . . . . . . . . . . . VDD to VSS Digital Output Voltage (VOH, VOL) . . . . . . . . . . . . VDD to VSS Storage Temperature . . . . . . . . . . . . . . . . . . –55°C to +150°C *Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ORDERING GUIDE Temperature Range Package Description Package Option Model AD775JN –20°C to +75°C 24-Pin 400 Mil Plastic DIP N-24B AD775JR –20°C to +75°C 24-Pin 300 Mil SOIC R-24A 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 AD775 features 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 –4– REV. 0 AD775 54 48 42 36 –36 –30 S/(N + D) – dB 30 24 18 12 6 0 0.1 THD – dB –42 –48 1 fIN – MHz 10 –54 0.1 1 fIN – MHz 10 Figure 2. S/(N + D) vs. Input Frequency at 20 MSPS Clock Rate (VIN = –0.3 dB) Figure 5. THD vs. Input Frequency at 20 MSPS Clock Rate (VIN = –0.3 dB) 0 –10 –20 –30 –40 dB 0 –10 –20 –30 –40 dB 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 –50 –60 –70 –80 –90 –100 FREQUENCY – MHz –50 –60 –70 –80 –90 –100 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 FREQUENCY – MHz Figure 3. Typical FFT at 1 MHz Input, 20 MSPS Clock Rate (VIN = –0.5 dB) Figure 6. Typical FFT at 5 MHz Input, 20 MSPS Clock Rate (VIN = –0.5 dB) +0.4 +0.3 +0.2 +0.1 0 –0.1 –0.2 –0.3 –0.4 –FULLSCALE INL – LSB +1 DNL – LSB 0 +FULLSCALE –1 –FULLSCALE +FULLSCALE Figure 4. Typical Differential Nonlinearity (DNL) Figure 7. Typical Integral Nonlinearity (INL) REV. 0 –5– AD775 DEFINITIONS OF SPECIFICATIONS Integral Nonlinearity (INL) Differential Phase Integral nonlinearity refers to the deviation of each individual code from a line drawn from “zero” through “full scale.” The point used as “zero” occurs 1/2 LSB before the first code transition. “Full scale” is defined as a level 1 1/2 LSB beyond the last code transition. The deviation is measured from the center of each particular code to the true straight line. Differential Nonlinearity (DNL, No Missing Codes) The difference in the output phase of a small high frequency sine wave at two stated levels of a low frequency signal on which it is superimposed. Pipeline Delay (Latency) The number of clock cycles between conversion initiation and the associated output data being made available. New output data is provided every clock cycle. Signal-to-Noise Plus Distortion Ratio (S/N+D) An ideal ADC exhibits code transitions that are exactly 1 LSB apart. DNL is the deviation from this ideal value. It is often specified in terms of the resolution for which no missing codes (NMC) is guaranteed. Offset Error S/N+D is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components including harmonics but excluding dc. The value for S/N+D is expressed in decibels. Total Harmonic Distortion (THD) The first code transition should occur at a level 1/2 LSB above nominal negative full scale. Offset referred to the Bottom of Ladder VRB is defined as the deviation from this ideal. The last code transition should occur 1 1/2 LSB below the nominal positive full scale. Offset referred to the Top of Ladder VRT is defined as the deviation from this ideal. Differential Gain THD is the ratio of the rms sum of the first six harmonic components to the rms value of the measured input signal and is expressed as a percentage or in decibels. The percentage difference between the output amplitudes of a small high frequency sine wave at two stated levels of a low frequency signal on which it is superimposed. THEORY OF OPERATION APPLYING THE AD775 REFERENCE INPUT The AD775 uses a pipelined two-step (subranging) flash architecture to achieve significantly lower power and lower input capacitance than conventional full flash converters while still maintaining high throughput. The analog input is sampled by the switched capacitor comparators on the falling edge of the input clock: no external sample and hold is required. The coarse comparators determine the top four bits (MSBs), and select the appropriate reference ladder taps for the fine comparators. With the next falling edge of the clock, the fine comparators determine the bottom four bits (LSBs). Since the LSB comparators require a full clock cycle between their sampling instant and their decision, the converter alternates between two sets of fine comparators in a “ping-pong” fashion. This multiplexing allows a new input sample to be taken on every falling clock edge, thereby providing 20 MSPS operation. The data is accumulated in the correction logic and output through a three-state output latch on the rising edge of the clock. The latency between input sampling and the corresponding converted output is 2.5 clock cycles. All three comparator banks utilize the same resistive ladder for their reference input. The analog input range is determined by the voltages applied to the bottom and top of the ladder, and the AD775 can digitize inputs down to 0 V using a single supply. On-chip application resistors are provided to allow the ladder to be conveniently biased by the supply voltage. The AD775 uses switched capacitor autozeroing techniques to cancel the comparators’ offsets and achieve excellent differential nonlinearity performance: typically ± 0.3 LSB. The integral nonlinearity is determined by the linearity of the reference ladder and is typically +0.5 LSB. The AD775 features a resistive reference ladder similar to that found in most conventional flash converters. The analog input range of the converter falls between the top (VRT) and bottom (VRB) voltages of this ladder. The nominal resistance of the ladder is 300 ohms, though this may vary from 230 ohms to 450 ohms. The minimum recommended voltage for VRB is 0 V; the linearity performance of the converter may deteriorate for input spans (VRB–VRB) below 1.8 V. While 2.8 V is the recommended maximum ladder top voltage (VRT), the top of the ladder may be as high as the positive supply voltage (AVDD) with minimal linearity degradation. AVDD 325Ω 16 0.1µF 17 AD775 300Ω *VALUES FOR RESISTANCE ARE TYPICAL 90Ω AV SS 23 0.1µF 22 Figure 8. Reference Configuration: 0.64 V to 2.73 V To simplify biasing of the AD775, on-chip reference bias resistors are provided on Pins 16 and 22. The two recommended configurations for these resistors are shown in Figures 8 and 9. –6– REV. 0 AD775 In the topology shown in Figure 8, the top of the ladder (VRT) is shorted to the top bias resistor (VRTS) (Pin 17 shorted to Pin 16), while the bottom of the ladder (VRB) is shorted to the bottom bias resistor (VRBS) (Pin 23 shorted to Pin 22). This creates a resistive path (nominally 725 ohms) between AVDD and AVSS. For nominal supply voltages (5 V and 0 V respectively), this creates an input range of 0.64 V to 2.73 V. Both top and bottom of the reference ladder should be decoupled, preferably with a chip capacitor to ground to minimize reference noise. The topology shown in Figure 9 provides a ground-inclusive input range. The bottom of the ladder (VRB) is shorted to AVSS. (0 V), while the top of the ladder (VRT) is connected to the onboard bias resistor (VRTS). This provides a nominal input range of 0 V to +2.4 V for AVDD of 5 V. The VRBS pin may be left floating, or shorted to AVSS. AVDD 325Ω 16 0.1µF 17 10kΩ 10kΩ NC 16 VRTS 17 VRT +5V 0.1µF AD680 3 VIN 1 3 0.1µF VOUT GND 0.1µF 2 422Ω 2 500pF AD775 20Ω AD822 1 10kΩ 500pF NC 22 VRBS 23 VRB 0.1µF NC = NO CONNECT 422Ω 10kΩ 6 20Ω AD822 5 7 140Ω Figure 11. Reference Configuration: 0.7 V to 3.2 V ANALOG INPUT AD775 300Ω 23 AV SS 22 90Ω AVSS *VALUES FOR RESISTANCE ARE TYPICAL The impedance looking into the analog input is essentially capacitive, as shown in the equivalent circuit of Figure 12, typically totalling around 11 pF. A portion of this capacitance is parasitic; the remainder is part of the switched capacitor structure of the comparator arrays. The switches close on the rising edge of the clock, acquire the input voltage, and open on the clock’s falling edge (the sampling instant). The charge that must be moved onto the capacitors during acquisition will be a function of the converter’s previous two samples, but there should be no sample-to-sample crosstalk so long as ample driving impedance and acquisition time are provided. SWITCHES EACH CLOCK CYCLE Figure 9. Reference Configuration: 0 V to +2.4 V AVDD More elaborate topologies can be used for those wishing to provide an input span based on an external reference voltage. The circuit in Figure 10 uses the AD780 2.5 V reference to drive the top of the ladder (VRT), with the bottom (VRB) of the ladder grounded to provide an input span of 0 V to +2.5 V. This is modified in Figure 11 to shift the 2.5 V span up 700 mV. +5V NC 1 2 0.1µF NC 3 4 C2 VIN C1 SWITCHES ON ALTERNATE CLOCK CYCLES AVSS C3 C1 + C2 + C3 ≈ 11pF AD780 8 7 6 5 NC NC NC 16 17 NC 0.1µF AD775 Figure 12. Equivalent Analog Input Circuit (VIN) AD775 NC 22 23 NC = NO CONNECT Figure 10. Reference Configuration: 0 V to 2.5 V The AD775 can accommodate dynamic changes in the reference voltage for gain or offset adjustment. However, conversions that are in progress, including those in the converter pipeline, while the reference voltages are changing will be invalid. For example, to ensure accurate acquisition (to 1/4 bit accuracy) of a full-scale input step in less than 20 ns, a source impedance of less than 100 ohms is recommended. Figure 13 shows one option of input buffer circuitry using the AD817. The AD817 acts as both an inverting buffer and level shifting circuit. In order to level shift the ground-based input signal to the dc level required by the input of the AD775, the supply voltage is resistively divided to produce the appropriate voltage at the noninverting input of the AD817. For most applications, the AD817 provides a low cost, high performance level shifter. The AD811 is recommended for systems which require faster settling times. REV. 0 –7– AD775 1kΩ 100 1kΩ 0V DC 5.6kΩ +5V 10µF 1kΩ 1.5VDC AD775 90 POWER DISSIPATION – mW AD817 19 AIN 80 70 60 50 40 Figure 13. Level Shifting Input Buffer The analog input range is set by the voltage at the top and bottom of the reference ladder. In general, the larger the span (VRT–VRB), the better the differential nonlinearity (DNL) of the converter; a 1.8 V span is suggested as a minimum to realize good linearity performance. AS the input voltage exceeds 2.8 V (for AVDD = 4.75 V), the input circuitry may start to slightly degrade the acquisition performance. CLOCK INPUT 30 0 10 30 20 CLOCK FREQUENCY – MHz 40 Figure 15. Power Dissipation vs. Clock Frequency The AD775’s internal control circuitry makes use of both clock edges to generate on-chip timing signals. To ensure proper settling and linearity performance, both tCH and tCL times should be 25 ns or greater. For sampling frequencies at or near 20 MSPS, a 50% duty cycle clock is recommended. For slower sampling applications, the AD775 can accommodate a wider range of duty cycles, provided each clock phase is as least 25 ns. Under certain conditions, the AD775 can be operated at sampling rates above 20 MSPS. Figure 14 shows the signal-to-noise plus distortion (S/(N+D)) performance of a typical AD775 versus clock frequency. It is extremely important to note that the maximum clock rate will be a strong function of both temperature and supply voltage. In general, the part slows down with increasing temperature and decreasing supply voltage. 50 In applications sensitive to aperture jitter, the clock signal should have a fall time of less than 3 ns. High speed CMOS logic families (HC/HCT) are recommended for their symmetrical swing and fast rise/fall times. Care should be taken to minimize the fanout and capacitive loading of the clock input line. DIGITAL INPUTS AND OUTPUTS The AD775’s digital interface uses standard CMOS, with logic thresholds roughly midway between the supplies (DVSS, DVDD). The digital output is presented in straight binary format, with full scale (1111 1111) corresponding to VIN = VRT, and zero (0000 0000) corresponding to VIN = VRB. Excessive capacitive loading of the digital output lines will increase the dynamic power dissipation as well as the on-chip digital noise. Logic fanout and parasitic capacitance on these lines should be minimized for optimum noise performance. The data output lines may be placed in a high output impedance state by bringing OE (Pin 1) to a logic high. Figure 16 indicates typical timing for access and float delay times (tHL and tDD respectively). Note that even when the outputs are in a high impedance state, activity on the digital bus can couple back to the sensitive analog portions of the AD775 and corrupt conversions in progress. OE 40 S(N + D) – dB 30 20 tDD DATA OUTPUT tHL 10 DATA ACTIVE THREE-STATE (HIGH IMPEDANCE) 0 0.1 tDD = 18ns TYPICAL 1 10 100 CLOCK FREQUENCY – MHz tHL = 12ns TYPICAL Figure 16. High Impedance Output Timing Figure 14. S(N + D) vs. Clock Frequency (Temperature = +25°C) A significant portion of the AD775’s power dissipation is proportional to the clock frequency: Figure 15 illustrates this tradeoff for a typical part. –8– REV. 0 AD775 POWER SUPPLY CONNECTIONS AND DECOUPLING APPLICATIONS AD775 EVALUATION BOARD The analog and digital supplies of the AD775 have been separate to prevent the typically large transients associated with the on-chip digital circuitry from coupling into the analog supplies (AVDD, AVSS). However, in order to avoid possible latch-up conditions, AVDD and DVDD must share a common supply external to the part, preferably a common source somewhere on the PC board. Each supply should be decoupled by a 0.1 µF capacitor located as close to the device pin as possible. Surface-mount capacitors, by virtue of their low parasitic inductance, are preferable to through-hole types. A larger capacitor (10 µF electrolytic) should be located somewhere on the board to help decouple large, low frequency supply noise. For specific layout information, refer to the AD775 Evaluation Board section of the data sheet. +5VA R15 499 R14 500 A VCC Q1 2N3904 TP5 VIN +5VA CR1 1N4148 TP9 R16 49.9 D Figures 17 through 22 show the schematic and printed circuit board (PCB) layout for the AD775 evaluation board. Referring to Figure 17, the input signal is buffered by U3, an AD817 op amp configured as a unity-gain follower. The signal is then accoupled and dc-biased by adjusting potentiometer R14. Video and imaging applications would typically use a dc-restoration circuit instead of the manual potentiometer adjustment. Q1, an emitter-follower, buffers the input signal and provides ample current to drive a simple low-pass filter. The filtering is included to limit wideband noise and highlight the fact that the AD775 can be driven from a nonzero source impedance. The reference circuit is similar to the one shown in Figure 11 with the exception that R1 and R2 allow precise adjustment of J8 CLOCK ( ) TP10 5 6 9 8 1/6 U7 1/6 U7 U3 J1 2 ANALOG TP2 INPUT 6 AD817 7 3 4 R4 TP1 49.9 C5 A VEE A VCC C6 R13 20 C8 22µF R12 4.99k R11 75 C7 10pF A +5VA C15 +5V C18 TP13 ENABLE P2-40 PIN IDC +5V 1 C22 20 19 18 17 16 15 14 13 12 11 D D D VEE D AD775 13 DVDD 14 AVDD CLK 12 DV DD 11 D7 10 D6 9 8 7 6 5 4 3 2 1 J9 D 74ALS541 J10 1 2 3 4 5 6 7 8 9 10 G1 A1 A2 A3 A4 A5 A6 A7 A8 GND VCC G2 Y1 TP4 VRT R8 10k R9 10k C4 390pF J3 C13 A 15 AVDD J6 16 VRTS 17 VRT 18 AVDD 19 VIN 20 AVSS 21 AVSS U5 D5 D4 D3 D2 D1 D0 U6 Y2 Y3 Y4 Y5 Y6 Y7 Y8 +5VA U1 A 3 AD680 VIN VOUT GND 1 2 2 3 1/2 U2 AD822 8 +5VA 1 C9 A R10 20 C14 C12 J5 C2 A A J4 22 VRBS 23 VRB A DV SS OE R3 499 C1 A J2 R6 10k C3 390pF TP3 VRB D 24 DVSS D R2 500 R7 10k 6 A 1/2 U2 R5 20 D AD822 5 4 A 7 R1 500 A TP12 J7 +5V NOTES 40 D VCC 1 2 TP7 VCC TP8 C19 C16 78M05 U4 VIN VOUT GND C21 D TP6 +5VA = 47µF ELECTROLYTIC CAPACITOR UNLESS OTHERWISE NOTED = 0.1µF CERAMIC CAPACITOR UNLESS OTHERWISE NOTED C11 3 4 5 6 TP11 VEE C20 A VEE Figure 17. AD775 Evaluation Board Schematic REV. 0 –9– AD775 VRT and VRB. Note that the VRT and VRB traces (see Figures 19 and 20) are run in parallel and in the same proximity. Any noise coupling is likely to be common mode to both signals and would result in an offset error but not a gain error. The entire reference circuit is powered by a single +5 V supply. The minimum voltage for VRB is determined by the impedance of the AD822 output stage and the amount of current flowing through the internal resistor ladder of the AD775. The sampling clock is buffered by U7, a 74HC04 inverter. It is recommended that the output loading of the inverter is minimized in order to maintain fast transition times on the clock. An additional inverter is used to provide a buffered clock signal whose rising edges indicate that data is valid. A 74ALS541 buffers the eight digital data outputs of the AD775 to improve the load driving capability. The multilayer PCB board layout shows some of the important design guidelines recommended for the AD775. The most important aspect is the power and ground distribution. While the AD775 has separate analog and digital power and ground pins, the AD775 should be treated as an entirely analog component. The ground plane is joined close to the ADC in order to maintain a low potential difference across the analog and digital ground pins. Because the power and grounds are derived from a common point, a slit in the ground plane is used to minimize any interaction between the analog and digital return currents. The power for the AD775, AVDD and DVDD, are derived from the same supply. Separate traces are run to AVDD and DVDD and joined together at the source. While not used on the evaluation board, a ferrite bead or inductor can effectively isolate noise generated by digital circuitry such as the output buffers. In cases where only a single supply is available, the inductor should not be placed between AVDD and DVDD. Instead, both supplies of the AD775 should be connected together and isolated from entirely digital components. Table I. Components List Reference Designator R1, R2, R14 R3, R15 R4, R13, R16 R5, R10 R6–R9 R11 R12 CR1 C1, C2, C5, C6, C9, C12–C15, C18 C20, C22, C23 C3, C4 C7 C8 C11, C16, C19, C21 Q1 U1 U2 U3 U4 U5 U6 U7 J1, J8 Description Potentiometer Resistor, 1%, 499 Ω Resistor, 1%, 49.9 Ω Resistor, 1%, 20 Ω Resistor, 1%, 10 kΩ Resistor, 1%, 75 Ω Resistor, 1%, 4.99 kΩ Diode, 1N4148 Ceramic Cap, Z5U, 0.1 µF Capacitor, Mica, 390 pF Capacitor, Mica, 10 pF Capacitor, Tantalum, 22 µF, 16 V Capacitor, Alum. Electrolytic, 47 µF, 16 V Transistor, 2N3904 AD680JT AD822AN AD817AN 78M05 AD775 74ALS541N 74HC04N BNC Jack Quantity 3 2 3 2 4 1 1 1 13 2 1 1 4 1 1 1 1 1 1 1 1 2 –10– REV. 0 AD775 Figure 18. Silkscreen Layer (Not to Scale) Figure 20. Solder Side PCB Layout (Not to Scale) Figure 19. Component Side PCB Layout (Not to Scale) Figure 21. Ground Plane PCB Layout (Not to Scale) REV. 0 –11– AD775 Figure 22. Power Plane PCB Layout (Not to Scale) OUTLINE DIMENSIONS Dimensions shown in inches and (mm). Plastic DIP (N-24B) 24 PIN 1 1 1.205 (30.60) 1.185 (30.10) 0.200 (5.05) 0.125 (3.18) 0.118 (3.00) MIN 0.024 (0.60) 0.016 (0.40) 0.100 (2.54) BSC 0.053 (1.35) 0.041 (1.05) SEATING PLANE 12 13 0.346 (8.80) 0.330 (8.40) PIN 1 1 24 SOIC (R-24A) 13 0.221 (5.6) 0.205 (5.2) 12 0.327 (8.3) 0.295 (7.5) 0.089 (2.25) 0.067 (1.70) 0.272 (6.9) 0.020 (0.50) MIN 0.400 (10.16) 0.195 (4.95) 0.125 (3.18) 0.606 (15.4) 0.586 (14.9) 15° 0° 0.014 (0.35) 0.008 (0.20) 0.012 (0.12) 0.002 (0.05) 0.050 (1.27) BSC 0.022 (0.55) 0.014 (0.35) 0.012 (0.30) 0.006 (0.15) 0.028 (0.7) 0.012 (0.3) –12– REV. 0 PRINTED IN U.S.A. C1830–18–8/93