ADC78H90CIMT

ADC78H90 8-Channel, 500 kSPS, 12-Bit A/D Converter March 2005 ADC78H90 8-Channel, 500 kSPS, 12-Bit A/D Converter General Description The ADC78H90 is a low-power, eight-channel CMOS 12-bit analog-to-digital converter with a conversion throughput of 500 kSPS. The converter is based on a successiveapproximation register architecture with an internal trackand-hold circuit. It can be configured to accept up to eight input signals at inputs AIN1 through AIN8. The output serial data is straight binary, and is compatible with several standards, such as SPI™, QSPI™, MICROWIRE™, and many common DSP serial interfaces. The ADC78H90 may be operated with independent analog and digital supplies. The analog supply (AVDD) can range from +2.7V to +5.25V, and the digital supply (DVDD) can range from +2.7V to AVDD. Normal power consumption using a +3V or +5V supply is 1.5 mW and 8.3 mW, respectively. The power-down feature reduces the power consumption to just 0.3 µW using a +3V supply, or 0.5 µW using a +5V supply. The ADC78H90 is packaged in a 16-lead TSSOP package. Operation over the industrial temperature range of −40˚C to +85˚C is guaranteed. Features n n n n n Eight input channels Variable power management Independent analog and digital supplies SPI™/QSPI™/MICROWIRE™/DSP compatible Packaged in 16-lead TSSOP Key Specifications n n n n Conversion Rate DNL INL Power Consumption — 3V Supply — 5V Supply 500 kSPS ± 1 LSB (max) ± 1 LSB (max) 1.5 mW (typ) 8.3 mW (typ) Applications n n n n n Automotive Navigation Portable Systems Medical Instruments Mobile Communications Instrumentation and Control Systems Connection Diagram 20079305 Ordering Information Order Code ADC78H90CIMT ADC78H90CIMTX ADC78H90EVAL Temperature Range −40˚C to +85˚C −40˚C to +85˚C Description 16-Lead TSSOP Package 16-Lead TSSOP Package, Tape & Reel Evaluation Board TRI-STATE ® is a trademark of National Semiconductor Corporation. MICROWIRE™ is a trademark of National Semiconductor Corporation. QSPI™ and SPI™ are trademarks of Motorola, Inc. © 2005 National Semiconductor Corporation DS200793 www.national.com ADC78H90 Block Diagram 20079307 Pin Descriptions and Equivalent Circuits Pin No. ANALOG I/O 4 - 11 DIGITAL I/O Digital clock input. The range of frequencies for this input is 50 kHz to 8 MHz, with guaranteed performance at 8 MHz. This clock directly controls the conversion and readout processes. Digital data output. The output samples are clocked out of this pin on falling edges of the SCLK pin. Digital data input. The ADC78H90’s Control Register is loaded through this pin on rising edges of the SCLK pin. Chip select. On the falling edge of CS, a conversion process begins. Conversions continue as long as CS is held low. Positive analog supply pin. This pin should be connected to a quiet +2.7V to +5.25V source and bypassed to GND with a 1 µF tantalum capacitor and a 0.1 µF ceramic monolithic capacitor located within 1 cm of the power pin. Positive digital supply pin. This pin should be connected to a +2.7V to AVDD supply, and bypassed to GND with a 0.1 µF ceramic monolithic capacitor located within 1 cm of the power pin. The ground return for the analog supply and signals. The ground return for the digital supply and signals. AIN1 to AIN8 Analog inputs. These signals can range from 0V to AVDD. Symbol Equivalent Circuit Description 16 SCLK 15 14 1 POWER SUPPLY DOUT DIN CS 2 AVDD 13 DVDD 3 12 AGND DGND www.national.com 2 ADC78H90 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Analog Supply Voltage AVDD Digital Supply Voltage DVDD Voltage on Any Pin to GND Input Current at Any Pin (Note 3) Package Input Current(Note 3) Power Dissipation at TA = 25˚C ESD Susceptibility (Note 5) Human Body Model Machine Model Soldering Temperature, Infrared, 10 seconds (Note 6) Junction Temperature Storage Temperature −0.3V to 6.5V −0.3V to AVDD + 0.3V, max 6.5V −0.3V to AVDD +0.3V Operating Ratings (Notes 1, 2) Operating Temperature Range AVDD Supply Voltage DVDD Supply Voltage Digital Input Pins Voltage Range Clock Frequency Analog Input Voltage −40˚C ≤ TA ≤ +85˚C +2.7V to +5.25V +2.7V to AVDD -0.3V to AVDD 50 kHz to 8 MHz 0V to AVDD ± 10 mA ± 20 mA See (Note 4) 2500V 250V 260˚C +150˚C −65˚C to +150˚C Package Thermal Resistance Package 16-lead TSSOP on 4-layer, 2 oz. PCB θJA 96˚C / W (Note 8) The following specifications apply for AVDD = DVDD = +2.7V to 5.25V, AGND = DGND = 0V, fSCLK = 8 MHz, fSAMPLE = 500 KSPS, unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C. Symbol Parameter Conditions Typical Limits (Note 7) 12 AVDD = +5.0V, DVDD = +3.0V AVDD = +5.0V, DVDD = +3.0V AVDD = +5.0V, DVDD = +3.0V AVDD = +5.0V, DVDD = +3.0V AVDD = +5.0V, DVDD = +3.0V AVDD = +5.0V, DVDD = +3.0V AVDD = +5.0V, DVDD = +3.0V, fIN = 40.2 kHz, −0.02 dBFS AVDD = +5.0V, DVDD = +3.0V, fIN = 40.2 kHz, −0.02 dBFS AVDD = +5.0V, DVDD = +3.0V, fIN = 40.2 kHz, −0.02 dBFS AVDD = +5.0V, DVDD = +3.0V, fIN = 40.2 kHz, −0.02 dBFS AVDD = +5.0V, DVDD = +3.0V, AVDD = +5.0V, DVDD = +3.0V, fIN = 40.2 kHz AVDD = +5.0V, DVDD = +3.0V, fa = 40.161 kHz, fb = 41.015 kHz AVDD = +5.0V, DVDD = +3.0V, fa = 40.161 kHz, fb = 41.015 kHz AVDD = +5V AVDD = +3V Units ADC78H90 Converter Electrical Characteristics STATIC CONVERTER CHARACTERISTICS Resolution with No Missing Codes INL DNL VOFF OEM GE GEM Integral Non-Linearity Differential Non-Linearity Offset Error Offset Error Match Gain Error Gain Error Match Bits LSB (max) LSB (max) LSB (max) LSB (max) LSB (max) LSB (max) ±1 ±1 ±2 ±2 ±3 ±3 DYNAMIC CONVERTER CHARACTERISTICS SINAD SNR THD SFDR ENOB Signal-to-Noise Plus Distortion Ratio Signal-to-Noise Ratio Total Harmonic Distortion Spurious-Free Dynamic Range Effective Number of Bits Channel-to-Channel Crosstalk Intermodulation Distortion, Second Order Terms Intermodulation Distortion, Third Order Terms -3 dB Full Power Bandwidth 73 73 −86 88 11.8 -82 -93 -90 11 8 70 70.8 −74 75.6 11.3 dB (min) dB (min) dB (max) dB (min) Bits (min) dB dB dB MHz MHz IMD FPBW 3 www.national.com ADC78H90 ADC78H90 Converter Electrical Characteristics (Note 8) (Continued) The following specifications apply for AVDD = DVDD = +2.7V to 5.25V, AGND = DGND = 0V, fSCLK = 8 MHz, fSAMPLE = 500 KSPS, unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C. Parameter Conditions Typical Limits (Note 7) Units Symbol ANALOG INPUT CHARACTERISTICS VIN IDCL CINA Input Range DC Leakage Current Input Capacitance Track Mode Hold Mode DVDD = +4.75Vto +5.25V DVDD = +2.7V to +3.6V DVDD = +2.7V to +5.25V VIN = 0V or DVDD 33 3 2.4 2.1 0.8 0 to AVDD V ±1 µA (max) pF pF V (min) V (min) V (max) µA (max) pF (max) DIGITAL INPUT CHARACTERISTICS VIH VIL IIN CIND Input High Voltage Input Low Voltage Input Current Digital Input Capacitance ISOURCE = 200 µA, DVDD = +2.7V to +5.25V ISINK = 200 µA ± 0.01 2 ±1 4 DIGITAL OUTPUT CHARACTERISTICS VOH VOL IOZH, IOZL COUT Output High Voltage Output Low Voltage TRI-STATE ® Leakage Current TRI-STATE ® Output Capacitance Output Coding POWER SUPPLY CHARACTERISTICS (CL = 10 pF) AVDD, DVDD Analog and Digital Supply Voltages AVDD ≥ DVDD AVDD = DVDD = +4.75V to +5.25V, fSAMPLE = 500 kSPS, fIN = 40 kHz AVDD = DVDD = +2.7V to +3.6V, fSAMPLE = 500 kSPS, fIN = 40 kHz AVDD = DVDD = +4.75V to +5.25V, fSAMPLE = 0 kSPS AVDD = DVDD = +2.7V to +3.6V, fSAMPLE = 0 kSPS \AVDD = DVDD = +4.75V to +5.25V AVDD = DVDD = +2.7V to +3.6V AVDD = DVDD = +4.75V to +5.25V AVDD = DVDD = +2.7V to +3.6V 1.65 0.5 200 200 8.3 1.5 0.5 0.3 8 50 500 13 50 Full-Scale Step Input Acquisition Time + Conversion Time 4 40 60 3 16 500 12 8.3 2.7 5.25 2.3 2.3 V (min) V (max) mA (max) mA (max) nA nA mW (max) mW (max) µW µW MHz (min) kHz KSPS (min) SCLK cycles % (min) % (max) SCLK cycles SCLK cycles kSPS (min) ns 2 DVDD −0.5 0.4 V (min) V (max) µA (max) pF (max) ±1 4 Straight (Natural) Binary Total Supply Current, Normal Mode (Operational, CS low) I A + ID Total Supply Current, Shutdown (CS high) Power Consumption, Normal Mode (Operational, CS low) Power Consumption, Shutdown (CS high) Maximum Clock Frequency Minimum Clock Frequency Maximum Sample Rate Conversion Time SCLK Duty Cycle Track/Hold Acquisition Time Throughput Time fRATE tAD Throughput Rate Aperture Delay PD AC ELECTRICAL CHARACTERISTICS fSCLK fSMIN fS tCONV DC tACQ www.national.com 4 ADC78H90 ADC78H90 Timing Specifications The following specifications apply for AVDD = DVDD = +2.7V to 5.25V, AGND = DGND = 0V, fSCLK = 8 MHz, fSAMPLE = 500 KSPS, CL = 50 pF, Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25˚C. Symbol t1a t1b t2 t3 t4 t5 t6 t7 t8 Parameter Setup Time SCLK High to CS Falling Edge Hold time SCLK Low to CS Falling Edge Delay from CS Until DOUT active Data Access Time after SCLK Falling Edge Data Setup Time Prior to SCLK Rising Edge Data Valid SCLK Hold Time SCLK High Pulse Width SCLK Low Pulse Width CS Rising Edge to DOUT High-Impedance (Note 9) (Note 9) Conditions Typical Limits (Note 7) 10 10 30 30 10 10 0.4 x tSCLK 0.4 x tSCLK 20 Units ns (min) ns (min) ns (max) ns (max) ns (min) ns (min) ns (min) ns (min) ns (max) Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions. Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified. Note 3: When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND or VIN > VA or VD), the current at that pin should be limited to 10 mA. The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 10 mA to two. Note 4: The absolute maximum junction temperature (TJmax) for this device is 150˚C. The maximum allowable power dissipation is dictated by TJmax, the junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax − TA)/θJA. In the 16-pin TSSOP, θJA is 96˚C/W, so PDMAX = 1,200 mW at 25˚C and 625 mW at the maximum operating ambient temperature of 85˚C. Note that the power consumption of this device under normal operation is a maximum of 12 mW. The values for maximum power dissipation listed above will be reached only when the ADC78H90 is operated in a severe fault condition (e.g. when input or output pins are driven beyond the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided. Note 5: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through ZERO ohms Note 6: See AN450, “Surface Mounting Methods and Their Effect on Product Reliability”, or the section entitled “Surface Mount” found in any post 1986 National Semiconductor Linear Data Book, for other methods of soldering surface mount devices. Note 7: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 8: Data sheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 9: Clock may be in any state (high or low) when CS is asserted, with the restrictions on setup and hold time given by t1a and t1b. 5 www.national.com ADC78H90 Timing Diagrams 20079308 Timing Test Circuit 20079351 FIGURE 1. ADC78H90 Operational Timing Diagram 20079306 ADC78H90 Serial Timing Diagram www.national.com 6 ADC78H90 Timing Diagrams (Continued) 20079350 SCLK and CS Timing Parameters 7 www.national.com ADC78H90 Specification Definitions ACQUISITION TIME is the time required to acquire the input voltage. That is, it is time required for the hold capacitor to charge up to the input voltage. APERTURE DELAY is the time between the fourth falling SCLK edge of a conversion and the time when the input signal is acquired or held for conversion. CONVERSION TIME is the time required, after the input voltage is acquired, for the ADC to convert the input voltage to a digital word. CROSSTALK is the coupling of energy from one channel into the other channel, or the amount of signal energy from one analog input that appears at the measured analog input. DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1 LSB. DUTY CYCLE is the ratio of the time that a repetitive digital waveform is high to the total time of one period. The specification here refers to the SCLK. EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE BITS) is another method of specifying Signal-to-Noise and Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02 and says that the converter is equivalent to a perfect ADC of this (ENOB) number of bits. FULL POWER BANDWIDTH is a measure of the frequency at which the reconstructed output fundamental drops 3 dB below its low frequency value for a full scale input. GAIN ERROR is the deviation of the last code transition (111...110) to (111...111) from the ideal (VREF - 1.5 LSB), after adjusting for offset error. INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from negative full scale (1⁄2 LSB below the first code transition) through positive full scale (1⁄2 LSB above the last code transition). The deviation of any given code from this straight line is measured from the center of that code value. INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two sinusoidal frequencies being applied to the ADC input at the same time. It is defined as the ratio of the power in the second and third order intermodulation products to the sum of the power in both of the original frequencies. IMD is usually expressed in dB. MISSING CODES are those output codes that will never appear at the ADC outputs. The ADC78H90 is guaranteed not to have any missing codes. OFFSET ERROR is the deviation of the first code transition (000...000) to (000...001) from the ideal (i.e. GND + 0.5 LSB). SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in dB, of the rms value of the input signal to the rms value of the sum of all other spectral components below one-half the sampling frequency, not including harmonics or d.c. SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD) Is the ratio, expressed in dB, of the rms value of the input signal to the rms value of all of the other spectral components below half the clock frequency, including harmonics but excluding d.c. SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the rms values of the input signal and the peak spurious signal where a spurious signal is any signal present in the output spectrum that is not present at the input, excluding d.c. TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB or dBc, of the rms total of the first five harmonic components at the output to the rms level of the input signal frequency as seen at the output. THD is calculated as where Af1 is the RMS power of the input frequency at the output and Af2 through Af6 are the RMS power in the first 5 harmonic frequencies. THROUGHPUT TIME is the minimum time required between the start of two successive conversion. It is the acquisition time plus the conversion time. In the case of the ADC78H90, this is 16 SCLK periods. www.national.com 8 ADC78H90 Typical Performance Characteristics kHz unless otherwise stated. DNL TA = +25˚C, fSAMPLE = 500 kSPS, fSCLK = 8 MHz, fIN = 40.2 DNL 20079340 20079341 INL INL 20079342 20079343 DNL vs. Supply INL vs. Supply 20079321 20079320 9 www.national.com ADC78H90 Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS, fSCLK = 8 MHz, fIN = 40.2 kHz unless otherwise stated. (Continued) SNR vs. Supply THD vs. Supply 20079322 20079332 ENOB vs. Supply SNR vs. Input Frequency 20079333 20079323 THD vs. Input Frequency ENOB vs. Input Frequency 20079324 20079325 www.national.com 10 ADC78H90 Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 kSPS, fSCLK = 8 MHz, fIN = 40.2 kHz unless otherwise stated. (Continued) Spectral Response Spectral Response 20079330 20079331 Power Consumption vs. Throughput 20079344 11 www.national.com ADC78H90 Applications Information 1.0 ADC78H90 OPERATION The ADC78H90 is a successive-approximation analog-todigital converter designed around a charge-redistribution digital-to-analog converter. Simplified schematics of the ADC78H90 in both track and hold operation are shown in Figures 2, 3, respectively. In Figure 2, the ADC78H90 is in track mode: switch SW1 connects the sampling capacitor to one of eight analog input channels through the multiplexer, and SW2 balances the comparator inputs. The ADC78H90 is in this state for the first three SCLK cycles after CS is brought low. There is no power-up delay or dummy conversions with the ADC78H90. The ADC is able to sample and convert an input to full resolution in the first conversion immediately following power up. The first conversion result after power up will be that of the first channel. Figure 3 shows the ADC78H90 in hold mode: switch SW1 connects the sampling capacitor to ground, maintaining the sampled voltage, and switch SW2 unbalances the comparator. The control logic then instructs the charge-redistribution DAC to add or subtract fixed amounts of charge to or from the sampling capacitor until the comparator is balanced. When the comparator is balanced, the digital word supplied to the DAC is the digital representation of the analog input voltage. The ADC78H90 is in this state for the last thirteen SCLK cycles after CS is brought low. 20079309 FIGURE 2. ADC78H90 in Track Mode 20079310 FIGURE 3. ADC78H90 in Hold Mode The time when CS is low is considered a serial frame. Each of these frames should contain an integer multiple of 16 SCLK cycles, during which time a conversion is performed and clocked out at the DOUT pin and data is clocked into the DIN pin to indicate the multiplexer address for the next conversion. 2.0 USING THE ADC78H90 An operational timing diagram and a serial interface timing diagram for the ADC78H90 are shown in The Timing Diagrams section. CS, chip select, initiates conversions and frames the serial data transfers. SCLK (serial clock) controls both the conversion process and the timing of serial data. DOUT is the serial data output pin, where a conversion result is sent as a serial data stream, MSB first. Data to be written to the ADC78H90’s Control Register is placed on DIN, the serial data input pin. New data is written to DIN with each conversion. A serial frame is initiated on the falling edge of CS and ends on the rising edge of CS. Each frame must contain an integer multiple of 16 rising SCLK edges. The ADC output data (DOUT) is in a high impedance state when CS is high and is active when CS is low. Thus, CS acts as an output enable. Additionally, the device goes into a power down state when CS is high. During the first 3 cycles of SCLK, the ADC is in the track mode, acquiring the input voltage. For the next 13 SCLK cycles the conversion is accomplished and the data is clocked out, MSB first. If there is more than one conversion in a frame, the ADC will re-enter the track mode on the falling www.national.com 12 ADC78H90 Applications Information (Continued) edge of SCLK after the N*16th rising edge of SCLK, and re-enter the hold/convert mode on the N*16+4th falling edge of SCLK, where "N" must be an integer. When CS is brought high, SCLK is internally gated off. If SCLK is in a low state when CS goes high, the subsequent fall of CS will generate a falling edge of the internal version of SCLK, putting the ADC into the track mode. This is seen by the ADC as the first falling edge of SCLK. If SCLK is in a high state when CS goes high, the ADC enters the track mode on the first falling edge of SCLK after the falling edge of CS. During each conversion, data is clocked into the DIN pin on the first 8 rising edges of SCLK after the fall of CS. For each conversion, it is necessary to clock in the data indicating the input that is selected for the conversion after the current one. See Tables 1, 2, 3 If CS and SCLK go low simultaneously, it is the following rising edge of SCLK that is considered the first rising edge for clocking data into DIN. TABLE 1. Control Register Bits Bit 7 (MSB) DONTC Bit 6 DONTC Bit 5 ADD2 Bit 4 ADD1 Bit 3 ADD0 Bit 2 DONTC Bit 1 DONTC Bit 0 DONTC TABLE 2. Control Register Bit Descriptions Bit #: 7, 6, 2, 1, 0 5 4 3 Symbol: DONTC ADD2 ADD1 ADD0 Description Don’t care. The value of these bit do not affect the device. These three bits determine which input channel will be sampled and converted on the next falling edge of CS. The mapping between codes and channels is shown in Table 3. 13 www.national.com ADC78H90 Applications Information ADD2 0 0 0 0 1 1 1 1 ADD1 0 0 1 1 0 0 1 1 ADD0 0 1 0 1 0 1 0 1 (Continued) 4.0 TYPICAL APPLICATION CIRCUIT A typical application of the ADC78H90 is shown in Figure 5. The split analog and digital supplies are both provided in this example by the National LP2950 low-dropout voltage regulator, available in a variety of fixed and adjustable output voltages. The analog supply is bypassed with a capacitor network located close to the ADC78H90. The digital supply is separated from the analog supply by an isolation resistor and conditioned with additional bypass capacitors. The ADC78H90 uses the analog supply (AVDD) as its reference voltage, so it is very important that AVDD be kept as clean as possible. Because of the ADC78H90’s low power requirements, it is also possible to use a precision reference as a power supply to maximize performance. The four-wire interface is also shown connected to a microprocessor or DSP. TABLE 3. Input Channel Selection Input Channel AIN1 (Default) AIN2 AIN3 AIN4 AIN5 AIN6 AIN7 AIN8 3.0 ADC78H90 TRANSFER FUNCTION The output format of the ADC89H90 is straight binary. Code transitions occur midway between successive integer LSB values. The LSB width for the ADC78H90 is AVDD / 4096. The ideal transfer characteristic is shown in Figure 4. The transition from an output code of 0000 0000 0000 to a code of 0000 0000 0001 is at 1/2 LSB, or a voltage of AVDD / 8192. Other code transitions occur at steps of one LSB. 20079311 FIGURE 4. Ideal Transfer Characteristic 20079313 FIGURE 5. Typical Application Circuit www.national.com 14 ADC78H90 Applications Information 5.0 ANALOG INPUTS (Continued) An equivalent circuit for one of the ADC78H90’s input channels is shown in Figure 6. Diodes D1 and D2 provide ESD protection for the analog inputs. At no time should an analog input go beyond (AVDD + 300 mV) or (GND - 300 mV), as these ESD diodes will begin conducting, which could result in erratic operation. The capacitor C1 in Figure 6 has a typical value of 3 pF, and is mainly the package pin capacitance. Resistor R1 is the on resistance of the multiplexer and track / hold switch, and is typically 500 ohms. Capacitor C2 is the ADC78H90 sampling capacitor, and is typically 30 pF. The ADC78H90 will deliver best performance when driven by a low-impedance source to eliminate distortion caused by the charging of the sampling capacitance. This is especially important when using the ADC78H90 to sample AC signals. Also important when sampling dynamic signals is a band-pass or low-pass filter to reduce harmonics and noise, improving dynamic performance. supply (DVDD) cannot exceed the analog supply (AVDD) by more than 300 mV. The ADC78H90’s analog power supply must, therefore, be applied before (or concurrently with) the digital power supply. The ADC78H90 is fully powered-up whenever CS is low, and fully powered-down whenever CS is high, with one exception: the ADC78H90 automatically enters power-down mode between the 16th falling edge of a conversion and the 1st falling edge of the subsequent conversion (see Figure 1). The ADC78H90 can perform multiple conversions back to back; each conversion requires 16 SCLK cycles. The ADC78H90 will perform conversions continuously as long as CS is held low. The user may trade off throughput for power consumption by simply performing fewer conversions per unit time. The Power Consumption vs. Sample Rate curve in the Typical Performance Curves section shows the typical power consumption of the ADC78H90 versus throughput. To calculate the power consumption, simply multiply the fraction of time spent in the normal mode by the normal mode power consumption (8.3 mW with AVDD = DVDD = +3.6V, for example), and add the fraction of time spent in shutdown mode multiplied by the shutdown mode power dissipation (0.3 mW with AVDD = DVDD = +3.6V). 7.2 Power Supply Noise Considerations The charging of any output load capacitance requires current from the digital supply, DVDD. The current pulses required from the supply to charge the output capacitance will cause voltage variations on the digital supply. If these variations are large enough, they could cause degrade SNR and SINAD performance of the ADC. Furthermore, if the analog and digital supplies are tied directly together, the noise on the digital supply will be coupled directly into the analog supply, causing greater performance degradation than noise on the digital supply. Furthermore, discharging the output capacitance when the digital output goes from a logic high to a logic low will dump current into the die substrate, which is resistive. Load discharge currents will cause "ground bounce" noise in the substrate that will degrade noise performance if that current is large enough. The larger is the output capacitance, the more current flows through the die substrate and the greater is the noise coupled into the analog channel, degrading noise performance. The first solution is to decouple the analog and digital supplies from each other, or use separate supplies for them, to keep digital noise out of the analog supply. To keep noise out of the digital supply, keep the output load capacitance as small as practical. If the load capacitance is greater than 25 pF, use a 100 Ω series resistor at the ADC output, located as close to the ADC output pin as practical. This will limit the charge and discharge current of the output capacitance and improve noise performance. 20079314 FIGURE 6. Equivalent Input Circuit 6.0 DIGITAL INPUTS AND OUTPUTS The ADC78H90’s digital inputs (SCLK, CS, and DIN) are limited by and cannot exceed the analog supply voltage AVDD. The digital input pins are not prone to latch-up; SCLK, CS, and DIN may be asserted before DVDD without any risk. 7.0 POWER SUPPLY CONSIDERATIONS The ADC78H90 has two supplies, although they could both have the same potential. There are two major power supply concerns with this product. They are relative power supply levels, including power on sequencing, and the effect of digital supply noise on the analog supply. 7.1 Power Management The ADC78H90 is a dual-supply device. These two supplies share ESD resources, and thus care must be exercised to ensure that the power supplies are applied in the correct sequence. To avoid turning on the ESD diodes, the digital 15 www.national.com ADC78H90 8-Channel, 500 kSPS, 12-Bit A/D Converter Physical Dimensions inches (millimeters) unless otherwise noted 16-Lead TSSOP Order Number ADC78H90CIMT, ADC78H90CIMTX NS Package Number MTC16 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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