ADC08832IN

ADC08831/ADC08832 8-Bit Serial I/O CMOS A/D Converters with Multiplexer and Sample/Hold Function September 1999 ADC08831/ADC08832 8-Bit Serial I/O CMOS A/D Converters with Multiplexer and Sample/Hold Function General Description The ADC08831/ADC08832 are 8-bit successive approximation Analog to Digital converters with 3-wire serial interfaces and a configurable input multiplexer for 2 channels. The serial I/O will interface to COPS™family of micro-controllers, PLD’s, microprocessors, DSP’s, or shift registers. The serial I/O is configured to comply with the NSC MICROWIRE™ serial data exchange standard. To minimize total power consumption, the ADC08831/ADC08832 automatically go into low power mode whenever they are not performing conversions. A track/hold function allows the analog voltage at the positive input to vary during the actual A/D conversion. The analog inputs can be configured to operate in various combinations of single-ended, differential, or pseudo-differential modes. The voltage reference input can be adjusted to allow encoding of small analog voltage spans to the full 8-bits of resolution. n Remote sensing in noisy environments n Instrumentation n Embedded Systems Features n n n n n n n 3-wire serial digital data link requires few I/O pins Analog input track/hold function 2-channel input multiplexer option with address logic Analog input voltage range from GND to VCC No zero or full scale adjustment required TTL/CMOS input/output compatible Superior pin compatible replacement for ADC0831/2 Key Specifications n n n n n n n Resolution: 8 bits Conversion time (fC = 2 MHz): 4µs (max) Power dissipation: 8.5mW (typ) Low power mode: 3.0mW (typ) Single supply: 5VDC Total unadjusted error: ± 1LSB No missing codes over temperature Applications n Digitizing sensors and waveforms n Process control monitoring Typical Application DS100108-44 DS100108-43 COPS™ is a trademark of National Semiconductor Corporation. MICROWIRE™ is a trademark of National Semiconductor Corporation. TRI-STATE™ © 1999 National Semiconductor Corporation DS100108 www.national.com Connection Diagrams ADC08831 Wide Body SO Packages ADC08832 Wide Body SO Packages DS100108-4 DS100108-3 ADC08831 N,M,MM Packages ADC08832 N,M,MM Packages DS100108-2 DS100108-1 Ordering Information Temperature Range Industrial (−40˚C ≤ TJ ≤ +85˚C) ADC08831IN ADC08832IN ADC08831IWM, ADC08832IWM, ADC08831IM, ADC08832IM, ADC08831IMM, ADC08832IMM, N08E M14B M08A MUA08A Package www.national.com 2 Absolute Maximum Ratings (Notes 1, 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (VCC) Voltage at Inputs and Outputs Input Current at Any Pin (Note 4) Package Input Current (Note 4) ESD Susceptibility (Note 6) Human Body Model Machine Model Junction Temperature (Note 5) Storage Temperature Range 6.5V −0.3V to VCC + 0.3V ± 5 mA ± 20 mA 2000V 200V 150˚C −65˚ C to 150˚C Mounting Temperature Lead Temp. (soldering, 10 sec) Infrared (10 sec) 260˚C 215˚C Operating Ratings(Notes 2, 3) Temperature Range Supply Voltage Thermal Resistance (θjA) SO Package, 8-pin Surface Mount MSOP, 8-pin Surface Mount SO Package, 14-pin Surface Mount N Package, 8-pin Clock Frequency −40˚C ≤ TJ ≤ +85˚C 4.5 V to 6.0 V 190˚C/W 235˚C/W 145˚C/W 122˚C/W 10kHz≤fCLK≤2MHz Electrical Characteristics The following specifications apply for VCC = VREF = +5VDC, and fCLK = 2 MHz unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol Parameter Conditions Typical (Note 8) Limits (Note 9) Units (Limits) LSB (max) LSB LSB LSB LSB 2.8 5.9 (VCC + 0.05) (GND − 0.05) kΩ (min) kΩ (max) V (max) V (min) LSB (max) LSB (max) LSB (max) µA (max) µA (min) µA (min) µA (max) CONVERTER AND MULTIPLEXER CHARACTERISTICS TUE Total Unadjusted Error Offset Error DNL INL FS RREF VIN Differential NonLinearity Integral NonLinearity Full Scale Error Reference Input Resistance Analog Input Voltage DC Common-Mode Error Power Supply Sensitivity On Channel Leakage Current (Note 13) VCC = 5V ± 10%, VCC = 5V ± 5% On Channel = 5V, Off Channel = 0V On Channel = 0V Off Channel = 5V On Channel = 5V, Off Channel = 0V On Channel = 0V, Off Channel = 5V (Note 11) (Note 12) (Note 10) ± 0.3 ± 0.2 ± 0.2 ± 0.2 ± 0.3 3.5 ±1 ± 1⁄4 ± 1⁄4 ± 1⁄4 0.2 1 −0.2 −1 −0.2 −1 0.2 1 2.0 0.8 Off Channel Leakage Current (Note 13) DC CHARACTERISTICS VIN(1) VIN(0) IIN(1) IIN(0) VOUT(1) Logical “1” Input Voltage Logical “0” Input Voltage Logical “1” Input Current Logical “0” Input Current Logical “1” Output Voltage VIN = 5.0V VIN = 0V VCC = 4.75V: IOUT = −360 µA IOUT = −10 µA VCC = 4.75V IOUT = 1.6 mA VOUT = 0V VOUT = 5V VOUT = 0V VOUT = VCC 3 V (min) V (max) µA (max) µA (max) V (min) V (min) V (max) µA (max) µA (max) mA (max) mA (min) www.national.com 0.05 0.05 +1 −1 2.4 4.5 0.4 −3.0 3.0 −6.5 8.0 VOUT(0) IOUT ISOURCE ISINK Logical “0” Output Voltage TRI-STATE Output Current Output Source Current Output Sink Current Electrical Characteristics Symbol Parameter (Continued) The following specifications apply for VCC = VREF = +5VDC, and fCLK = 2 MHz unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Conditions Typical (Note 8) 0.6 1.7 1.3 2.4 Limits (Note 9) 1.0 2.4 1.8 3.5 Units (Limits) mA (max) mA (max) mA (max) mA (max) DC CHARACTERISTICS ICC ICC Supply Current CLK = VCC ADC08831 CS = VCC CS = LOW CS = VCC CS = LOW Supply Current ADC08832 CLK = VCC (Note 16) Electrical Characteristics The following specifications apply for VCC = VREF = +5 VDC, and tr = tf = 20 ns unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol fCLK Parameter Clock Frequency Clock Duty Cycle (Note 14) TC tCA tSET-UP tHOLD tpd1, tpd0 Conversion Time (Not Including MUX Addressing Time) Acquisition Time CS Falling Edge or Data Input Valid to CLK Rising Edge Data Input Valid after CLK Rising Edge CLK Falling Edge to Output Data Valid (Note 15) TRI-STATE Delay from Rising Edge of CS to Data Output and SARS Hi-Z Capacitance of Analog Input (Note 17) Capacitance of Logic Inputs Capacitance of Logic Outputs CL = 100 pF: Data MSB First Data LSB First CL = 10 pF, RL = 10 kΩ (see TRI-STATE Test Circuits) CL = 100 pF, RL = 2 kΩ fCLK = 2MHz Conditions Typical (Note 8) Limits (Note 9) 2 40 60 8 4 12 Units (Limits) MHz (max) % (min) % (max) 1/fCLK (max) µs (max) 1/fCLK (max) ns (min) ns (min) ⁄ 25 20 250 200 50 180 13 5 5 ns (max) ns (max) ns ns (max) pF pF pF t1H, t0H CIN CIN COUT www.national.com 4 Dynamic Characteristics The following specifications apply for VCC = 5V, fCLK = 2MHz, TA = 25˚C, RSOURCE = 50Ω, fIN = 45kHz, VIN = 5VP, VREF = 5V, non-coherent 2048 samples with windowing. Symbol fS SNR THD SINAD ENOB SFDR Sampling Rate Parameter ADC08831 ADC08832 Conditions fCLK/11 fCLK/13 (Note 21) 48.5 −59.5 48.0 7.7 62.5 Typical (Note 8) Limits (Note 9) 181 153 Units (Limits) ksps ksps dB dB dB Bits dB Signal-to -Noise Ratio (Note 19) Total Harmonic Distortion (Note 20) Signal-to -Noise and Distortion Effective Number Of Bits (Note 18) Spurious Free Dynamic Range Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Note 2: Operating Ratings indicate conditions for which the device is functional. These ratings 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 3: All voltages are measured with respect to GND = 0 VDC, unless otherwise specified. Note 4: When the input voltage VIN at any pin exceeds the power supplies (VIN < (GND) or VIN > VCC,) the current at that pin should be limited to 5 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 5 mA to four pins. Note 5: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA and the ambient temperature, TA. The maximum allowable power dissipation at any temperature is PD = (TJMAX − TA)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. Note 6: Human body model, 100 pF capacitor discharged through a 1.5 kΩ resistor. The machine mode is a 200pF capacitor discharged directly into each pin. Note 7: See AN450 “Surface Mounting Methods and Their Effect on Product Reliability” or Linear Data Book section “Surface Mount” for other methods of soldering surface mount devices. Note 8: Typicals are at TJ = 25˚C and represent the most likely parametric norm. Note 9: Guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 10: Total Unadjusted Error (TUE) includes offset, full-scale, linearity, multiplexer errors. Note 11: It is not tested for the ADC08832. Note 12: For VIN(−) ≥ VIN(+) the digital code will be 0000 0000. Two on-chip diodes are tied to each analog input (see Functional Block Diagram) which will forward-conduct for analog input voltages one diode drop below ground or one diode drop greater than VCC supply. During testing at low VCC levels (e.g., 4.5V), high level analog inputs (e.g., 5V) can cause an input diode to conduct, especially at elevated temperatures, which will cause errors for analog inputs near full-scale. The spec allows 50 mV forward bias of either diode; this means that as long as the analog VIN does not exceed the supply voltage by more than 50 mV, the output code will be correct. Exceeding this range on an unselected channel will corrupt the reading of a selected channel. Achievement of an absolute 0 VDC to 5 VDC input voltage range will therefore require a minimum supply voltage of 4.950 VDC over temperature variations, initial tolerance and loading. Note 13: Channel leakage current is measured after a single-ended channel is selected and the clock is turned off. For off channel leakage current the following two cases are considered: one, with the selected channel tied high (5 VDC) and the remaining off channel tied low (0 VDC), total current flow through the off channel is measured; two, with the selected channel tied low and the off channels tied high, total current flow through the off channel is again measured. The two cases considered for determining on channel leakage current are the same except total current flow through the selected channel is measured. Note 14: A 40% to 60% duty cycle range insures proper operation at all clock frequencies. In the case that an available clock has a duty cycle outside of these limits the minimum time the clock is high or low must be at least 250 ns. The maximum time the clock can be high or low is 60 µs. Note 15: Since data, MSB first, is the output of the comparator used in the successive approximation loop, an additional delay is built in to allow for comparator response time. Note 16: For the ADC08832 Vref is internally tied to VCC, therefore, for the ADC08832 reference current is included in the supply current. Note 17: Analog inputs are typically 300 ohms input resistance to a 13pF sample and hold capacitor. Note 18: Effective Number Of Bits (ENOB) is calculated from the measured signal-to-noise plus distortion ratio (SINAD) using the equation ENOB = (SINAD-1.76)/ 6.02. Note 19: The signal-to-noise ratio is the ratio of the signal amplitude to the background noise level. Harmonics of the input signal are not included in it’s calculation. Note 20: The contributions from the first 6 harmonics are used in the calculation of the THD. Note 21: The maximum sampling rate is slightly less than fCLK/11 if CS is reset in less than one clock period. 5 www.national.com Block Diagram DS100108-47 *For ADC08831 VREF pin is available, for ADC08832 DI pin is available, and VREF is tied to VCC Pin names in parentheses refer to ADC08832 www.national.com 6 Typical Performance Characteristics 5V, unless otherwise specified. Linearity Error (TUE) vs Reference Voltage The following specifications apply for TA = 25˚C, VCC = VREF = Linearity Error (TUE) vs Temperature Linearity Error (TUE) vs Clock Frequency DS100108-27 DS100108-15 DS100108-14 Power Supply Current vs Temperature (ADC08831) Power Supply Current vs Temperature (ADC08832) Power Supply Current vs Clock Frequency, CS = Low, ADC08831 DS100108-35 DS100108-36 DS100108-37 Output Current vs Temperature DS100108-33 7 www.national.com Typical Performance Characteristics 5V, unless otherwise specified. (Continued) Spectral Response with 10KHz Sine Wave Input The following specifications apply for TA = 25˚C, VCC = VREF = Spectral Response with 55 KHz Sine Wave Input Spectral Response with 90 KHz Sine Wave Input DS100108-13 DS100108-34 DS100108-16 Total Unadjuster Error Plot DS100108-38 Leakage Current Test Circuit DS100108-5 www.national.com 8 TRI-STATE Test Circuits and Waveforms DS100108-20 DS100108-21 Timing Diagrams Data Input Timing DS100108-22 Data Output Timing DS100108-23 9 www.national.com Timing Diagrams (Continued) ADC08831 Start Conversion Timing DS100108-24 ADC08831 Timing DS100108-25 *LSB first output not available on ADC08831. ADC08832 Timing DS100108-26 www.national.com 10 ADC08832 Functional Block Diagram 11 DS100108-12 www.national.com *Some of these functions/pins are not available with other options. Functional Description 1.0 MULTIPLEXER ADDRESSING The design of these converters utilizes a comparator structure with built-in sample-and-hold which provides for a differential analog input to be converted by a successive approximation routine. The actual voltage converted is always the difference between an assigned “+” input terminal and a “−” input terminal. The polarity of each input terminal of the pair indicates which line the converter expects to be the most positive. If the assigned “+” input voltage is less than the “−” input voltage the converter responds with an all zeros output code. A unique input multiplexing scheme has been utilized to provide multiple analog channels with software-configurable single-ended, or differential operation. The analog signal conditioning required in transducer-based data acquisition systems is significantly simplified with this type of input flexibility. One converter package can now handle ground referenced inputs, differential inputs, as well as signals with some arbitrary reference voltage. A particular input configuration is assigned during the MUX addressing sequence, prior to the start of a conversion. The MUX address selects which of the analog inputs are to be enabled and whether this input is single-ended or differential. In addition to selecting differential mode the polarity may also be selected. Channel 0 may be selected as the positive input and channel 1 as the negative input or vice versa. This programmability is illustrated by the MUX addressing codes for the ADC08832. The MUX address is shifted into the converter via the DI line. Because the ADC08831 contains only one differential input channel with a fixed polarity assignment, it does not require addressing. TABLE 1. Multiplexer/Package Options Part Number ADC08831 ADC08832 Number of Analog Channels Single-Ended 1 2 Differential 1 1 Number of Package Pins 8 or 14 8 or 14 Since the input configuration is under software control, it can be modified as required before each conversion. A channel can be treated as a single-ended, ground referenced input for one conversion; then it can be reconfigured as part of a differential channel for another conversion. The analog input voltages for each channel can range from 50mV below ground to 50mV above VCC (typically 5V) without degrading conversion accuracy. 2.0 THE DIGITAL INTERFACE A most important characteristic of these converters is their serial data link with the controlling processor. Using a serial communication format offers two very significant system improvements. It allows many functions to be included in a small package and it can eliminate the transmission of low level analog signals by locating the converter right at the analog sensor; transmitting highly noise immune digital data back to the host processor. To understand the operation of these converters it is best to refer to the Timing Diagrams and Functional Block Diagram and to follow a complete conversion sequence. For clarity, a separate timing diagram is shown for each device. 1. A conversion is initiated by pulling the CS (chip select) line low. This line must be held low for the entire conversion. The converter is now waiting for a start bit and its MUX assignment word, if applicable. 2. On each rising edge of the clock the status of the data in (DI) line is clocked into the MUX address shift register. The start bit is the first logic “1” that appears on this line (all leading zeros are ignored). Following the start bit the converter expects the next 2 bits to be the MUX assignment word. 3. When the start bit has been shifted into the start location of the MUX register, and the input channel has been assigned, a conversion is about to begin. An interval of 1⁄2 clock period (where nothing happens) is automatically inserted to allow the selected MUX channel to settle to a final analog input value. The DI line is disabled at this time. It no longer accepts data. 4. The data out (DO) line now comes out of TRI-STATE and provides a leading zero for this one clock period of MUX settling time. 5. During the conversion the output of the SAR comparator indicates whether the analog input is greater than (high) or less than (low) a series of successive voltages generated internally from a ratioed capacitor array (first 5 bits) and a resistor ladder (last 3 bits). After each comparison the comparator’s output is shipped to the DO line on the falling edge of CLK. This data is the result of the conversion being shifted out (with the MSB first) and can be read by the processor immediately. 6. After 8 clock periods the conversion is completed. 7. The stored data in the successive approximation register is loaded into an internal shift register. The data, LSB first, is automatically shifted out the DO line after the MSB first data stream. The DO line then goes low and stays low until CS is returned high. The ADC08831 is an exception in that its data is only output in MSB first format. The DI and DO lines can be tied together and controlled through a bidirectional processor I/O bit with one wire. This is possible because the DI input is only “looked-at” during the MUX addressing interval while the DO line is still in a high impedance state. MUX Addressing: ADC08832 Single-Ended MUX Mode MUX Address Start Bit 1 1 SGL/ DIF 1 1 ODD/ SIGN 0 1 Channel # 0 + + 1 Differential MUX Mode MUX Address Start Bit 1 1 SGL/ DIF 0 0 ODD/ SIGN 0 1 Channel # 0 + − 1 − + 8. www.national.com 12 Functional Description 3.0 Reducing Power Consumption (Continued) digital logic enter static current modes. However power dissipation from the reference ladder occurs, regardless of the signal on CS 4.0 REFERENCE CONSIDERATIONS The voltage applied to the reference input on these converters, VREF, defines the voltage span of the analog input (the difference between VIN(MAX) and VIN(MIN) over which the 256 possible output codes apply. The devices can be used either in ratiometric applications or in systems requiring absolute accuracy. The reference pin must be connected to a voltage source capable of driving the reference input resistance which can be as low as 2.8kΩ. This pin is the top of a resistor divider string and capacitor array used for the successive approximation conversion. In a ratiometric system the analog input voltage is proportional to the voltage used for the A/D reference. This voltage is typically the system power supply, so the VREF pin can be tied to VCC (done internally on the ADC08832). This technique relaxes the stability requirements of the system reference as the analog input and A/D reference move together maintaining the same output code for a given input condition. For absolute accuracy, where the analog input varies between very specific voltage limits, the reference pin can be biased with a time and temperature stable voltage source. The LM385, LM336 and LM4040 reference diodes are good low current devices to use with these converters. The maximum value of the reference is limited to the VCC supply voltage. The minimum value, however, can be quite small (see Typical Performance Characteristics) to allow direct conversions of transducer outputs providing less than a 5V output span. Particular care must be taken with regard to noise pickup, circuit layout and system error voltage sources when operating with a reduced span due to the increased sensitivity of the converter (1 LSB equals VREF/256). The ADC08831 operate up to a 2MHz clock frequency, or about 181 ksps. At 5V supply, it consumes about 1.7 mA or 8.5 mW when CS is logic low. The ADC08831 has a low power mode to minimize total power consumption. When the chip select is asserted with a logic high, some analog circuitry and digital logic are pulled to a static, low power condition. Also, DOUT, the output driver is taken into TRI-STATE mode. To optimize static power consumption, special attention is needed to the digital input logic signals: CLK, CS, DI. Each digital input has a large CMOS buffer between VCC and GND. A traditional TTL level high (2.4V) will be sufficient for each input to read a logical “1”. However, there could be a large VIH to VCC voltage difference at each input. Such a voltage difference would cause static power dissipation, even when chip select pin is high and the part is in low power mode. Therefore, to minimize static power dissipation, it is recommended that all digital input logic levels should equal the converter’s supply. Various CMOS logic is particularly well suited for this application. The reference pin on the ADC08831 is not affected by the power-down mode. To reduce static reference current during non-conversion time, there are a couple options. First, a low voltage external reference (ie, 2.5V could be used). A shunt reference, such as the LM385-2.5, could be powered by a logic gate that is the inverse of the signal on CS . When CS is high, the reference is off. As a second option, an external, low on-resistance switch could be used. The ADC08832 is similar to the ADC08831, except its reference is derived from VCC. The ADC08832 does enter a low-power mode when CS is logic high, as the analog and DS100108-28 DS100108-29 a) Ratiometric b) Absolute with a Reduced Span FIGURE 1. Reference Examples 13 www.national.com Functional Description 5.0 THE ANALOG INPUTS (Continued) 5.4 Board Layout Consideration, Grounding and Bypassing: The ADC08831/2 are easy to use with some board layout consideration. They should be used with an analog ground plane and single-point grounding techniques. The GND pin should be tied directly to the ground plane. The supply pin should be bypassed to the ground plane with a surface mount or ceramic capacitor with leads as short as possible. All analog inputs should be referenced directly to the single-point ground. Digital inputs and outputs should be shielded from and routed away from the reference and analog circuitry. 6.0 OPTIONAL ADJUSTMENTS 6.1 Zero Error The offset of the A/D does not require adjustment. If the minimum analog input voltage value, VIN(MIN), is not ground a zero offset can be done. The converter can be made to output 0000 0000 digital code for this minimum input voltage by biasing any VIN (−) input at this VIN(MIN) value. This utilizes the differential mode operation of the A/D. The zero error of the A/D converter relates to the location of the first riser of the transfer function and can be measured by grounding the VIN (−) input and applying a small magnitude positive voltage to the VIN (+) input. Zero error is the difference between the actual DC input voltage which is necessary to just cause an output digital code transition from 0000 0000 to 0000 0001 and the ideal 1⁄2 LSB value (1⁄2 LSB = 9.8mV for VREF = 5.000VDC). 6.2 Full Scale The full-scale adjustment can be made by applying a differential input voltage which is 11⁄2 LSB down from the desired analog full-scale voltage range and then adjusting the magnitude of the VREF input (or VCC for the ADC08832) for a digital output code which is just changing from 1111 1110 to 1111 1111. 6.3 Adjusting for an Arbitrary Analog Input Voltage Range If the analog zero voltage of the A/D is shifted away from ground (for example, to accommodate an analog input signal which does not go to ground), this new zero reference should be properly adjusted first. A VIN (+) voltage which equals this desired zero reference plus 1⁄2 LSB (where the LSB is calculated for the desired analog span, using 1 LSB = analog span/256) is applied to selected “+” input and the zero reference voltage at the corresponding “−” input should then be adjusted to just obtain the 00HEX to 01HEX code transition. The full-scale adjustment should be made [with the proper VIN (−) voltage applied] by forcing a voltage to the VIN (+) input which is given by: The most important feature of these converters is that they can be located right at the analog signal source and through just a few wires can communicate with a controlling processor with a highly noise immune serial bit stream. This in itself greatly minimizes circuitry to maintain analog signal accuracy which otherwise is most susceptible to noise pickup. However, a few words are in order with regard to the analog inputs should the input be noisy to begin with or possibly riding on a large common-mode voltage. The differential input of these converters actually reduces the effects of common-mode input noise, a signal common to both selected “+” and “−” inputs for a conversion (60 Hz is most typical). The time interval between sampling the “+” input and then the “−” input is 1⁄2 of a clock period. The change in the common-mode voltage during this short time interval can cause conversion errors. For a sinusoidal common-mode signal this error is: where fCM is the frequency of the common-mode signal, VPEAK is its peak voltage value and fCLK is the A/D clock frequency. For a 60Hz common-mode signal to generate a 1⁄4 LSB error ()5mV) with the converter running at 250kHz, its peak value would have to be 6.63V which would be larger than allowed as it exceeds the maximum analog input limits. Source resistance limitation is important with regard to the DC leakage currents of the input multiplexer. Bypass capacitors should not be used if the source resistance is greater than 1kΩ. The worst-case leakage current of ± 1µA over temperature will create a 1mV input error with a 1kΩ source resistance. An op amp RC active low pass filter can provide both impedance buffering and noise filtering should a high impedance signal source be required. 5.1 Sample and Hold The ADC08831/2 provide a built-in sample-and-hold to acquire the input signal. The sample and hold can sample input signals in either single-ended or pseudo differential mode. 5.2 Input Op Amps When driving the analog inputs with an op amp it is important that the op amp settle within the allowed time. To achieve the full sampling rate, the analog input should be driven with a low impedance source (100Ω) or a high-speed op amp such as the LM6142. Higher impedance sources or slower op amps can easily be accommodated by allowing more time for the analog input to settle. 5.3 Source Resistance The analog inputs of the ADC08831/2 look like a 13pF capacitor (CIN) in series with 300Ω resistor (Ron). CIN gets switched between the selected “+” and “−” inputs during each conversion cycle. Large external source resistors will slow the settling of the inputs. It is important that the overall RC time constants be short enough to allow the analog input to completely settle. where: VMAX = the high end of the analog input range and VMIN = the low end (the offset zero) of the analog range. (Both are ground referenced.) www.national.com 14 Functional Description (Continued) The VREFIN (or VCC) voltage is then adjusted to provide a code change from FEHEX to FFHEX. This completes the adjustment procedure. 7.0 DYNAMIC PERFORMANCE Dynamic performance specifications are often useful in applications requiring waveform sampling and digitization. Typically, a memory buffer is used to capture a stream of consecutive digital outputs for post processing. Capturing a number of samples that is a power of 2 (ie, 1024, 2048, 4096) allows the Fast Fourier Transform (FFT) to be used to digitally analyze the frequency components of the signal. Depending on the application, further digital filtering, windowing, or processing can be applied. 7.1 Sampling Rate The Sampling Rate, sometimes referred to as the Throughput Rate, is the time between repetitive samples by an Analog-to-Digital Converter. The sampling rate includes the conversion time, as well as other factors such a MUX setup time, acquisition time, and interfacing time delays. Typically, the sampling rate is specified in the number of samples taken per second, at the maximum Analog-to-Digital Converter clock frequency. Signals with frequencies exceeding the Nyquist frequency (1/2 the sampling rate), will be aliased into frequencies below the Nyquist frequency. To prevent signal degradation, sample at twice (or more) than the input signal and/or use of a low pass (anti-aliasing) filter on the front-end. Sampling at a much higher rate than the input signal will reduce the requirements of the anti-aliasing filter. Some applications require under-sampling the input signal. In this case, one expects the fundamental to be aliased into the frequency range below the Nyquist frequency. In order to be assured the frequency response accurately represents a harmonic of the fundamental, a band-pass filter should be used over the input range of interest. 7.2 Signal-to-Noise Ratio Signal-to-Noise Ratio (SNR) is the ratio of RMS magnitude of the fundamental to the RMS sum of all the non-fundamental signal, excluding the harmonics, up to 1/2 of the sampling frequency (Nyquist). 7.3 Total Harmonic Distortion Total Harmonic distortion is the ratio of the RMS sum of the amplitude of the harmonics to the fundamental input frequency. THD = 20 log [(V22 + V32+ V42+ V52+ V62) 1/2/V1] Where V1 is the RMS amplitude of the fundamental and V2,V3, V4, V5, V6 are the RMS amplitudes of the individual harmonics. In theory, all harmonics are included in THD calculations, but in practice only about the first 6 make significant contributions and require measurement. For under-sampling applications, the input signal should be band pass filtered (BPF) to prevent out of band signals, or their harmonics, to appear in the spectral response. The DC Linearity transfer function of an Analog-to-Digital Converter tends to influence the dominant harmonics. A parabolic Linearity curve would tend to create 2nd (and even) order harmonics, while an S-curve would tend to create 3rd (or odd) order harmonics. The magnitude of an DC linearity error correlates to the magnitude of the harmonics. 15 7.4 Signal-to-Noise and Distortion Signal-to-Noise And Distortion ratio (SINAD) is the ratio of RMS magnitude of the fundamental to the RMS sum of all the non-fundamental signals, including the noise and harmonics, up to 1/2 of the sampling frequency (Nyquist), excluding DC. SINAD is also dependent on the number of quantization levels in the A/D Converter used in the waveform sampling process. The more quantization levels, the smaller the quantization noise and theoretical noise performance. The theoretical SINAD for a N-Bit Analog-to-Digital Converter is given by: SINAD = (6.02 N + 1.76) dB Thus, for an 8-bit converter, the ideal SINAD = 49.92 dB 7.5 Effective Number of Bits Effective Number Of Bits (ENOB) is another specification to quantify dynamic performance. The equation for ENOB is given by: ENOB = [(SINAD - 1.76)] / 6.02] The Effective Number Of Bits portrays the cumulative effect of several errors, including quantization, non-linearities, noise, and distortion. 7.6 Spurious Free Dynamic Range Spurious Free Dynamic Range (SFDR) is the ratio of the signal amplitude to the amplitude of the highest harmonic or spurious noise component. If the amplitude is at full scale, the specification is simply the reciprocal of the peak harmonic or spurious noise. www.national.com Applications Low-Cost Remote Temperature Sensor DS100108-6 Operating with Ratiometric Transducers DS100108-7 *VIN(−) = 0.15 VCC 15% of VCC ≤ VXDR ≤ 85% of VCC www.national.com 16 Applications (Continued) Span Adjust; 0V ≤ VIN ≤ 3V DS100108-8 Zero-Shift and Span Adjust: 2V≤VIN ≤ 5V 17 www.national.com Applications (Continued) Protecting the Input DS100108-9 Diodes are 1N914 Digital Load Cell DS100108-10 • • • • Uses one more wire than load cell itself Two mini-DIPs could be mounted inside load cell for digital output transducer Electronic offset and gain trims relax mechanical specs for gauge factor and offset Low level cell output is converted immediately for high noise immunity www.national.com 18 Applications (Continued) 4 mA-20 mA Current Loop Converter DS100108-11 • • All power supplied by loop 1500V isolation at output Isolated Data Converter DS100108-40 19 www.national.com Applications (Continued) A “Stand-Alone” Hook-Up for ADC08832 Evaluation DS100108-39 www.national.com 20 Physical Dimensions inches (millimeters) unless otherwise noted Order Number ADC08831IWM, ADC08832IWM, NS Package Number M14B Order Number ADC08831IM or ADC08832IM NS Package Number M08A 21 www.national.com Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Order Number ADC08831IN, ADC08832IN NS Package Number N08E www.national.com 22 ADC08831/ADC08832 8-Bit Serial I/O CMOS A/D Converters with Multiplexer and Sample/Hold Function Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Order Number ADC08831IMM or ADC08832IMM NS Package Number MUA08A 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. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com www.national.com National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: sea.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 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.