ADC78H89EVAL

ADC78H89 7-Channel, 500 KSPS, 12-Bit A/D Converter March 2005 ADC78H89 7-Channel, 500 KSPS, 12-Bit A/D Converter General Description The ADC78H89 is a low-power, seven-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 seven input signals on pins AIN1 through AIN7. 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 ADC78H89 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 ADC78H89 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 Seven 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 20061605 Ordering Information Order Code ADC78H89CIMT ADC78H89CIMTX ADC78H89EVAL 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 DS200616 www.national.com ADC78H89 Block Diagram 20061607 Pin Descriptions and Equivalent Circuits Pin No. ANALOG I/O 5 - 11 2 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 ADC78H89’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 0.1 µF ceramic monolithic and 1 µF tantalum capacitors 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 both analog and digital supplies. These pins are tied directly together internally, so must be connected to the same potential. If any potential exists across these pins, large currents will flow through the device. AIN1 to AIN7 NC Analog inputs. These signals can range from 0V to AVDD. This pin is not connected internally, and can be left floating, or tied to ground. Symbol Equivalent Circuit Description 16 SCLK 15 14 1 POWER SUPPLY DOUT DIN CS 3 AVDD 13 DVDD 4, 12 GND www.national.com 2 ADC78H89 Absolute Maximum Ratings (Notes 1, 2) 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, 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 Units (Note 7) Bits LSB (max) LSB (max) LSB (max) LSB (max) LSB (max) LSB (max) ADC78H89 Converter Electrical Characteristics STATIC CONVERTER CHARACTERISTICS Resolution with No Missing Codes INL DNL OE OEM GE GEM Integral Non-Linearity Differential Non-Linearity Offset Error Offset Error Match Gain Error Gain Error Match AVDD = +5.0V, DVDD = +3.3V AVDD = +5.0V, DVDD = +3.3V AVDD = +5.0V, DVDD = +3.3V AVDD = +5.0V, DVDD = +3.3V AVDD = +5.0V, DVDD = +3.3V AVDD = +5.0V, DVDD = +3.3V AVDD = +5.0V, DVDD = +3.3V 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, fIN = 40.2 kHz, −0.02 dBFS 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 12 ±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 72.6 72.8 -86 88 11.8 -82 -93 -90 11 8 dB dB dB dB bits dB dB dB MHz MHz IMD FPBW 3 www.national.com ADC78H89 ADC78H89 Converter Electrical Characteristics (Note 8) (Continued) The following specifications apply for AVDD = DVDD = +2.7V to 5.25V, 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 Units (Note 7) V Symbol ANALOG INPUT CHARACTERISTICS VIN IDCL CINA Input Range DC Leakage Current Input Capacitance In Track Mode In 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 ±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 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 0.1 0.1 8.3 1.5 0.5 0.3 8 50 500 13 50 Full-Scale Step Input Conversion Time + Acquisition Time 4 13 40 60 3 16 500 12 8.3 2.7 5.25 2.3 2.3 V (min) V (max) mA (max) mA (max) µA µA mW (max) mW (max) µW µW MHz (max) 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) IDD Total Supply Current, Shutdown (CS high) Power Consumption, Normal Mode (Operational, CS low) Power Consumption, Shutdown (CS high) Maximum Clock Frequency Minimum Clock Frequency fS tCONV DC tACQ fRATE tAD Maximum Sample Rate Conversion Time Duty Cycle Track/Hold Acquisition Time Throughput Time Throughput Rate Aperture Delay PD AC ELECTRICAL CHARACTERISTICS fSCLK www.national.com 4 ADC78H89 ADC78H89 Timing Specifications The following specifications apply for AVDD = DVDD = +2.7V to 5.25V, fSCLK = 8 MHz, 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 SCLK High to CS Fall Setup Time SCLK Low to CS Fall Hold Time Delay from CS Until DOUT TRI-STATE ® Disabled 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 10) (Note 10) Conditions Typical Limits 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 (max) ns (max) ns (min) ns (min) ns (max) Note 1: Absolute maximum ratings are limiting values which 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 50 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 five. 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. The values for maximum power dissipation listed above will be reached only when the ADC78H89 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: Except power supply pins. Note 10: 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 ADC78H89 Timing Diagrams 20061651 FIGURE 1. ADC78H89 Operational Timing Diagram 20061608 Timing Test Circuit 20061606 ADC78H89 Serial Timing Diagram www.national.com 6 ADC78H89 Timing Diagrams (Continued) 20061650 SCLK and CS Timing Parameters 7 www.national.com ADC78H89 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 both second order (or all four third order) intermodulation products to the sum of the power in both of the original frequencies. IMD is usually expressed in dBFS. MISSING CODES are those output codes that will never appear at the ADC outputs. The ADC78H89 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. TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB, 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 ADC78H89, this is 16 SCLK periods. www.national.com 8 ADC78H89 Typical Performance Characteristics kHz unless otherwise stated. DNL TA = +25˚C, fSAMPLE = 500 KSPS, fSCLK = 8 MHz, fIN = 40.2 DNL 20061640 20061641 INL INL 20061642 20061643 DNL vs. Supply INL vs. Supply 20061621 20061620 9 www.national.com ADC78H89 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 20061622 20061632 ENOB vs. Supply SNR vs. Input Frequency 20061633 20061623 THD vs. Input Frequency ENOB vs. Input Frequency 20061624 20061625 www.national.com 10 ADC78H89 Typical Performance Characteristics TA = +25˚C, fSAMPLE = 500 KSPS, fSCLK = 8 MHz, fIN = 40.2 kHz unless otherwise stated. (Continued) Spectral Response Spectral Response 20061630 20061631 Power Consumption vs. Throughput 20061644 11 www.national.com ADC78H89 Applications Information 1.0 USING THE ADC78H89 An operational timing diagram and a serial interface timing diagram for the ADC78H89 are shown in the Timing Diagrams section. CS is chip select, which 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 ADC78H89’s Control Register is placed on DIN, the serial data in pin. The conversion process and serial data timing are controlled by the SCLK. Each conversion requires 16 SCLK cycles to complete. Conversions are begun by bringing CS low. Several conversions can be executed sequentially in a single serial frame, which is defined as the time between falling and rising edges of CS. If CS is held low continuously, the ADC78H89 will perform conversions continuously. Each time CS goes low, a conversion process is initiated simultaneously with a load of the Control Register. The new contents of the Control Register will affect the next conversion. There is thus a one sample delay between selecting a new input channel and observing the corresponding output. Basic operation of the ADC78H89 begins with CS going low and initiating a conversion process and data transfer. At this time the DOUT pin comes out of the high impedance state. The converter enters track mode at the first falling edge of SCLK after CS is brought low, and begins to acquire the input signal. Acquisition of the input signal continues during the first three SCLK cycles after the falling edge of CS. This acquisition time is denoted by tACQ. The converter goes from track to hold mode on the fourth falling edge of SCLK, and the analog input signal is sampled at this time (see Figure 1). The ADC78H89 supports idling SCLK either high or low between conversions, when CS is high. The SCLK may also run continuously while CS is high. Regardless of whether the clock is idled, SCLK is internally gated off when CS is brought high. If SCLK is in the 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 as the first falling edge of SCLK. If SCLK is in the 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 (see Figure 1). In both cases, a total of sixteen falling edges are required to complete the acquisition and conversion process. Sixteen SCLK cycles are required to read a complete sample from the ADC78H89. Each bit of the sample (including leading zeros) is valid on subsequent rising edges of SCLK. The ADC78H89 will produce four leading zeros on DOUT, followed by twelve data bits, most significant first. The final data bit, DB0, will be clocked out on the 16th SCLK falling edge, and will be valid on the following rising edge. Depending upon the application, the first edge on SCLK after CS goes low may be either a falling edge or a rising edge. If the first SCLK edge after CS goes low is a falling edge, all four leading zeros will be valid on the first four rising edges of SCLK. If the first SCLK edge after CS goes low is a rising edge, the first leading zero may not be set up in time for a microprocessor or DSP to read it correctly. The remaining data bits are still clocked out on the falling edges of SCLK, so that they are valid on the rising edges of SCLK. Control information must be written to the Control Register whenever a conversion is performed. Information is written to the Control Register on the first eight rising edges of SCLK of each conversion. It is important that the DIN line is set up with the correct information when reading data from the ADC78H89. The input channel to be sampled in the next conversion process is determined by writing information to the Control Register in the current conversion. On the rising edges of SCLK after CS is brought low, data is loaded through the DIN pin to the Control Register, MSB first. Since the data on the DIN pin is transferred while the conversion data is being read, 16 serial clocks are required for each data transfer. The control register only loads the information on the first 8 rising SCLK edges; DIN is ignored for the last 8 rising edges. Table 1 describes the bit functions, where MSB indicates the first bit of information in the loaded data. At power-up, the control register defaults to all zeros in the bit locations. www.national.com 12 ADC78H89 Applications Information Bit 7 (MSB) DONTC Bit 6 DONTC (Continued) TABLE 1. Control Register Bits Bit 5 ADD2 Bit 4 ADD1 Bit 3 ADD0 Bit 2 DONTC Bit 1 DONTC Bit 0 DONTC 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 this bit does 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 2. ADC78H89 is in track mode: switch SW1 connects the sampling capacitor to one of seven analog input channels through the multiplexer, and SW2 balances the comparator inputs. The ADC78H89 is in this state for the first three SCLK cycles after CS is brought low. The user does not need to worry about any kind of power-up delays or dummy conversions with the ADC78H89. The part is able to acquire input to full resolution in the first conversion immediately following power-up. The first conversion after power up will be that of the first channel. Figure 3 shows the ADC78H89 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 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 ADC78H89 is in this state for the last thirteen SCLK cycles after CS is brought low. TABLE 2. Input Channel Selection 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 Input Channel AIN1 (Default) AIN2 AIN3 AIN4 AIN5 AIN6 AIN7 GND 2.0 ADC78H89 OPERATION The ADC78H89 is a successive-approximation analog-todigital converter designed around a charge-redistribution digital-to-analog converter. Simplified schematics of the ADC78H89 in both track and hold modes are shown in Figure 2 and Figure 3, respectively. In Figure 2, the 20061609 FIGURE 2. ADC78H89 in Track Mode 20061610 FIGURE 3. ADC78H89 in Hold Mode 13 www.national.com ADC78H89 Applications Information 3.0 ADC78H89 TRANSFER FUNCTION (Continued) The output format of the ADC89H89 is straight binary. Code transitions occur midway between successive integer LSB values. The LSB width for the ADC78H89 is AVDD / 4096. The ideal transfer characteristic is shown in Figure 4. 4.0 TYPICAL APPLICATION CIRCUIT A typical application of the ADC78H89 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 ADC78H89. The digital supply is separated from the analog supply by an isolation resistor and conditioned with additional bypass capacitors. The ADC78H89 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 ADC78H89’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. 20061611 FIGURE 4. Ideal Transfer Characteristic 20061613 FIGURE 5. Typical Application Circuit 5.0 ANALOG INPUTS An equivalent circuit for one of the ADC78H89’s input channels is shown in Figure 6. At the start of each conversion, one of the ADC78H89’s seven channels are selected. Diodes D1 and D2 provide ESD protection for the analog inputs. At no time should an analog input be beyond (AVDD + 300 mV) or (GND - 300 mV), as these ESD diodes will begin conducting, which could cause erratic operation. The capacitor C1 in Figure 6 typically has a 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 ADC78H89 sampling capacitor, and is typically 30 pF. The ADC78H89 will deliver best performance when driven by a lowimpedance source to eliminate distortion caused by the charging of the sampling capacitor. 20061614 FIGURE 6. Equivalent Input Circuit In applications where dynamic performance is critical, the ADC78H89 might need to be driven with a low outputimpedance amplifier. In addition, when using the ADC78H89 www.national.com 14 ADC78H89 Applications Information (Continued) to sample AC signals, a band-pass or low-pass filter will reduce harmonics and noise, improving dynamic performance. 6.0 DIGITAL INPUTS AND OUTPUTS The ADC78H89’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 ADC78H89 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 ADC78H89 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 supply (DVDD) cannot exceed the analog supply (AVDD) by more than 300 mV. The ADC78H89’s analog power supply must, therefore, be applied before (or concurrently with) the digital power supply. The ADC78H89 is fully powered-up whenever CS is low, and fully powered-down whenever CS is high, with one exception: the ADC78H89 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 ADC78H89 can perform multiple conversions back to back; each conversion requires 16 SCLK cycles. The ADC78H89 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 ADC78H89 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. 15 www.national.com ADC78H89 7-Channel, 500 KSPS, 12-Bit A/D Converter Physical Dimensions inches (millimeters) unless otherwise noted 16-Lead TSSOP Order Number ADC78H89CIMT, ADC78H89CIMTX 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. 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