ADC08161 500 ns A/D Converter with S/H Function and 2.5V Bandgap Reference General Description Key Specifications Using a patented multi-step A/D conversion technique, the 8-bit ADC08161 CMOS A/D converter offers 500 ns conversion time, internal sample-and-hold (S/H), a 2.5V bandgap reference, and dissipates only 100 mW of power. The ADC08161 performs an 8-bit conversion with a 2-bit voltage estimator that generates the 2 MSBs and two low-resolution (3-bit) flashes that generate the 6 LBSs. Input signals are tracked and held by the input sampling circuitry, eliminating the need for an external sample-and-hold. The ADC08161 can perform accurate conversions of full-scale input signals at frequencies from DC to typically more than 300 kHz (full power bandwidth) without the need of an external sample-and-hold (S/H). For ease of interface to microprocessors, this part has been designed to appear as a memory location or I/O port without the need for external interfacing logic. n n n n n n Resolution Conversion time (tCONV) Full power bandwidth Throughput rate Power dissipation Total unadjusted error 8 Bits 560 ns max (WR-RD Mode) 300 kHz (typ) 1.5 MHz min 100 mW max ± 1⁄2 LSB and ± 1 LSB max Features n No external clock required n Analog input voltage range from GND to V+ n 2.5V bandgap reference Applications n n n n Mobile telecommunications Hard-disk drives Instrumentation High-speed data acquisition systems Block Diagram DS011149-1 TRI-STATE ® is a registered trademark of National Semiconductor Corporation. © 1999 National Semiconductor Corporation DS011149 www.national.com ADC08161 500 ns A/D Converter with S/H Function and 2.5V Bandgap Reference June 1999 Connection Diagram RD Mode (logic low on the MODE pin) Wide-Body Small-Outline Package INT GND VREF−, VREF+ These are the reference voltage inputs. They may be placed at any voltage between GND − 50 mV and V+ + 50 mV, but VREF+ must be greater than VREF−. Ideally, an input voltage equal to VREF− produces an output code of 0, and an input voltage greater than VREF+ − 1.5 LSB produces an output code of 255. For the ADC08161 an input voltage that exceeds V+ by more than 100 mV or is below GND by more than 100 mV will create conversion errors. CS This is the active low Chip Select input. A logic low signal applied to this input pin enables the RD and WR inputs. Internally, the CS signal is ORed with RD and WR signals. Overflow Output. If the analog input is higher than VREF+, OFL will be low at the end of conversion. It can be used when cascading two ADC08161s to achieve higher resolution (9 bits). This output is always active and does not go into TRI-STATE as DB0–DB7 do. When OFL is set, all data outputs remain high when the ADC08061’s output data is read. DS011149-14 See NS Package Number M20B Ordering Information Industrial (−40˚C ≤ TA ≤ 85˚C) ADC08161CIWM Package M20B Pin Description VIN This is the analog input. The input range is GND–50 mV ≤ VINPUT ≤ V+ + 50 mV. DB0–DB7 TRI-STATE data outputs — bit 0 (LSB) through bit 7 (MSB). WR /RDY WR-RD Mode (Logic high applied to MODE pin) WR: With CS low, the conversion is started on the rising edge of WR. The digital result will be strobed into the output latch at the end of conversion (Figures 2, 3, 4). RD Mode (Logic low applied to MODE pin) RDY: This is an open drain output (no internal pull-up device). RDY will go low after the falling edge of CS and returns high at the end of conversion. Mode: Mode (RD or WR-RD ) selection input– This pin is pulled to a logic low through an internal 50 µA current sink when left unconnected. RD Mode is selected if the MODE pin is left unconnected or externally forced low. A complete conversion is accomplished by pulling RD low until output data appears. MODE RD www.national.com OFL WR-RD Mode is selected when a high is applied to the MODE pin. A conversion starts with the WR signal’s rising edge and then using RD to access the data. WR-RD Mode (logic high on the MODE pin) This is the active low Read input. With a logic low applied to the CS pin, the TRI-STATE data outputs (DB0–DB7) will be activated when RD goes low (Figures 2, 3, 4). 2 With CS low, a conversion starts on the falling edge of RD. Output data appears on DB0–DB7 at the end of conversion (Figures 1, 5). This is an active low output that indicates that a conversion is complete and the data is in the output latch. INT is reset by the rising edge of RD. This is the power supply ground pin. The ground pin should be connected to a “clean” ground reference point. V+ Positive power supply voltage input. Nominal operating supply voltage is +5V. The supply pin should be bypassed with a 10 µF bead tantalum in parallel with a 0.1 ceramic capacitor. Lead length should be as short as possible. VREFOUT The internal bandgap reference’s 2.5V output is available on this pin. Use a 220 µF bypass capacitor between this pin and analog ground. Absolute Maximum Ratings (Notes 1, 2) Power Dissipation (Note 4) Lead Temperature (Note 5) (Vapor Phase, 60 sec.) (Infrared, 15 sec.) Storage Temperature ESD Susceptibility (Note 6) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (V+) Logic Control Inputs Voltage at Other Inputs and Outputs Input Current at Any Pin (Note 3) Package Input Current (Note 3) 6V −0.3V to V+ + 0.3V −0.3V to V+ + 0.3V 5 mA 20 mA 875 mW +215˚C +220˚C −65˚C to +150˚C 750V Operating Ratings(Notes 1, 2) TMIN ≤ TA ≤ TMAX −40˚C ≤ TA ≤ 85˚C 4.5V to 5.5V Temperature Range ADC08161CIWM Supply Voltage, (V+) Converter Characteristics The following specifications apply for RD Mode, V+ = 5V, VREF+ = 5V, and VREF− = GND unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol Parameter INL Integral Non Linearity TUE Total Unadjusted Error (Note 9) INL Integral Non Linearity TUE Total Unadjusted Error Missing Codes Conditions Typical Limits Units (Note 7) (Note 8) (Limit) VREF = 5V VREF = 5V VREF = 2.5V VREF = 2.5V ±1 ±1 ±1 ±1 LSB (max) VREF = 5V VREF = 2.5V 0 Bits (max) 0 Bits (max) Reference Input Resistance VREF+ VREF− VIN LSB (max) LSB (max) LSB (max) 700 500 Ω (min) 700 1250 Ω (max) Positive Reference Input Voltage VREF− V (min) V+ V (max) Negative Reference GND V (min) Input Voltage VREF+ V (max) Analog (Note 10) GND − 0.1 V (min) V+ + 0.1 V (max) −0.4 −20 µA (max) −0.4 −20 µA (max) ± 1/16 ± 1⁄2 LSB (max) Input Voltage On-Channel Input Current On Channel Input = 5V, Off Channel Input = 0V (Note 11) On Channel Input = 0V, Off Channel Input = 5V PSS Power Supply Sensitivity (Note 11) V+ = 5V ± 5%, Effective Bits All Codes Tested VIN = 4.85 Vp-p 7.8 Bits Full-Power Bandwidth fIN = 20 Hz to 20 kHz VIN = 4.85 Vp-p 300 kHz 0.5 % 50 dB 50 dB 25 pF VREF = 4.75V THD Total Harmonic Distortion S/N Signal-to-Noise Ratio VIN = 4.85 Vp-p fIN = 20 Hz to 20 kHz VIN = 4.85 Vp-p Intermodulation Distortion fIN = 20 Hz to 20 kHz VIN = 4.85 Vp-p IMD fIN = 20 Hz to 20 kHz CVIN Analog Input Capacitance 3 www.national.com AC Electrical Characteristics The following specifications apply for V+ = 5V, tr = tf = 10 ns, VREF+ = 5V, VREF− = 0V unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol tWR Typical (Note 7) Limit (Note 8) Units (Limit) 100 100 ns (min) Mode Pin to V+, (Figure 2) 350 350 ns (min) Mode Pin to GND (Figure 5 ) 200 250 ns (min) 400 400 ns (max) Mode Pin to V+, (Figure 2 ) 500 560 ns (max) Parameter Conditions Mode Pin to V+ Write Time (Figures 2, 3, 4) tRD Read Time (Time from Rising Edge of WR to Falling Edge of RD ) tRDW tCONV RD Width WR -RD Mode Conversion Time (tWR + tRD + tACC1) tCRD RD Mode Conversion Time Mode Pin to GND, (Figure 1 ) 655 900 ns (max) tACCO Access Time (Delay from Falling CL ≤ 100 pF, Mode Pin to GND 640 900 ns (max) Edge of RD to Output Valid) (Figure 1 ) Access Time (Delay from Falling Edge of RD CL ≤ 10 pF CL = 100 pF 110 ns (max) to Output Valid) Mode Pin to V+, tRD ≤ tINTL 30 55 ns (max) tACC1 45 50 ns (Figure 2 ) tACC2 t1H, t0H Falling Edge of RD CL ≤ 10 pF CL = 100 pF to Output Valid) tRD > tINTL, TRI-STATE ® Control (Figures 3, 5) RL = 3 kΩ, CL = 10 pF (Delay from Rising Edge (Figures 1, 2, 3, 4, 5) 30 60 ns (max) Delay from Rising Edge of Mode Pin = V+, CL = 50 pF 520 690 ns (max) WR to Falling Edge of INT (Figures 3, 4) CL = 50 pF, 50 95 ns (max) (Figures 1, 2, 3, 5) CL = 50 pF, (Figure 4) 45 95 ns (max) 25 45 ns (max) 0 15 ns (max) 60 115 ns (max) (Figures 1, 2, 3, 4, 5) 50 50 ns (min) Access Time (Delay from 25 ns of RD to HI-Z State) tINTL tINTH Delay from Rising Edge of RD to Rising Edge of INT tINTH Delay from Rising Edge of WR to Rising Edge of INT tRDY tID tRI Delay from INT Mode Pin = 0V, CL = 50 pF, RL = 3 kΩ, (Figure 1) RL = 3 kΩ, CL = 100 pF to Output Valid (Figure 4) Delay from CS to RDY Mode Pin = V+, tRD ≤ tINTL Delay from RD to INT (Figure 2) tN Time between End of RD and Start of New Conversion tCSS CS Setup Time (Figures 1, 2, 3, 4, 5) 0 0 ns (max) tCSH CS Hold Time (Figures 1, 2, 3, 4, 5) 0 0 ns (max) DC Electrical Characteristics The following specifications apply for V+ = 5V unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol VIH Limit (Note 8) Units (Limit) CS, WR, RD, A0, A1, A2 Pins 2.0 V (min) Mode Pin 3.5 Parameter Logic “1” Input Voltage www.national.com Typical (Note 7) Conditions V+ = 5.5 V 4 DC Electrical Characteristics (Continued) The following specifications apply for V+ = 5V unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol VIL IIH Limit (Note 8) Units (Limit) CS, WR, RD, A0, A1, A2 Pins 0.8 V (max) Mode Pin VH = 5V 1.5 Parameter Logic “0” Input Voltage Logic “1” Input Current Typical (Note 7) Conditions V+ = 4.5V CS, RD, A0, A, A2 Pins IIL 0.005 1 WR Pin 0.1 3 50 200 Logic “0” Input Current Mode Pin VL = 0V −0.005 −2 µA (max) Logic “1” Output Voltage Mode Pins V+ = 4.75V IOUT = −360 µA 2.4 V (min) DB0–DB7, OFL, INT IOUT = −10 µA 4.5 V (min) 0.4 V (max) µA (max) CS, RD, WR, A0, A1, A2 VOH Logic “0” Output Voltage DB0–DB7, OFL, INT V+ = 4.75V TRI-STATE Output Current DB0–DB7, OFL, INT, RDY VOUT = 5.0V 0.1 3 µA (max) DB0–DB7, RDY VOUT = 0V −0.1 −3 µA (max) ISOURCE Output Source Current DB0–DB7, RDY VOUT = 0V −26 −6 mA (min) ISINK Output Sink Current DB0–DB7, OFL, INT VOUT = 5V 24 7 mA (min) DB0–DB7, OFL, INT, RDY CS = WR = RD = 0 11.5 20 mA (max) VOL IOUT = 1.6 mA IO IC Supply Current COUT Logic Output Capacitance 5 pF CIN Logic Input Capacitance 5 pF Bandgap Reference Electrical Characteristics The following specifications apply for V+ = 5V unless otherwise specified. Boldface limits apply for TMIN to TMAX; all other limits TA = TJ = 25˚C. Symbol Parameter VREFOUT Internal Reference Output Voltage ∆VREF/∆T Internal Reference Temperature Conditions Typical Limits Units (Note 7) (Note 8) (Limit) 2.5 ± 2.0% V (max) 40 ppm/˚C Coefficient ∆VREF/∆IL Internal Reference Load Sourcing (0 ≤ IL ≤ +10 mA) 0.01 0.1 %/mA (max) 4.75V ≤ V+ ≤ 5.25V VREV = 0V 0.5 6.0 mV (max) Regulation Line Regulation ISC Short Circuit Current ∆VREF/∆t Long Term Stability Start-Up Time V+: 0V→5V, CL = 220 µF 35 mA (max) 200 ppm/kHr 40 ms Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating the device beyond its specified operating ratings. Operating Ratings indicate conditions for which the device is functional, but do not guarantee 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. 5 www.national.com Bandgap Reference Electrical Characteristics (Continued) Note 2: All voltages are measured with respect to the GND pin, unless otherwise specified. Note 3: When the input voltage (VIN) at any pin exceeds the power supply voltage (VIN < GND or VIN > V+), the absolute value of the current at that pin should be limited to 5 mA or less. The 20 mA package input current specification limits the number of pins that can exceed the power supply boundaries with a 5 mA current limit to four. Note 4: The power dissipation of this device under normal operation should never exceed 875 mW (Quiescent Power Dissipation + TTL Loads on the digital outputs). Caution should be taken not to exceed absolute maximum power rating when the device is operating in a severe fault condition (e.g., when any input or output exceeds the power supply). The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX (maximum junction temperature), θJA (package junction to ambient thermal resistance), and TA (ambient temperature). The maximum allowable power dissipation at any temperature is PDmax = (TJMAX − TA)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For this device, TJMAX = 105˚C and θJA = 85˚C/W. Note 5: See AN-450 “Surface Mounting Methods and Their Effect on Product Reliability” for other methods of soldering surface mount devices. Note 6: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Note 7: Typicals are at 25˚C and represent most likely parametric norm. Note 8: Limits are guaranteed to National’s AOQL (Average Output Quality Level). Note 9: Total unadjusted error includes offset, full-scale, and linearity errors. Note 10: Two on-chip diodes are tied to each analog input and are reversed biased during normal operation. One is connected to V+ and the other is connected to GND. They will become forward biased and conduct when an analog input voltage is equal to or greater than one diode drop above V+ or below GND. Therefore, caution should be exercised when testing with V+ = 4.5V. Analog inputs with magnitudes equal to 5V can cause an input diode to conduct, especially at elevated temperatures. This can create conversion errors for analog signals near full-scale. The specification allows 50 mV forward bias on either diode; e.g., the output code will be correct as long as the analog input signal does not exceed the supply voltage by more than 50 mV. Exceeding this range on an unselected channel will corrupt the reading of a selected channel. An absolute analog input signal voltage range of 0V ≤ VIN ≤ 5V can be achieved by ensuring that the minimum supply voltage applied to V+ is 4.950V over temperature variations, initial tolerance, and loading. Note 11: Off-channel leakage current is measured on the on-channel selection. www.national.com 6 TRI-STATE Test Circuit and Waveforms t1H, CL = 10 pF t1H DS011149-2 DS011149-4 tr = 10 ns t0H, CL = 10 pF t0H DS011149-5 DS011149-3 tr = 10 ns DS011149-6 FIGURE 1. RD Mode (Mode Pin is Low) 7 www.national.com TRI-STATE Test Circuit and Waveforms (Continued) DS011149-7 FIGURE 2. WR-RD Mode with tRD ≤ tINTL (Mode Pin is High) DS011149-8 FIGURE 3. WR-RD Mode with tRD > tINTL (Mode Pin is High) www.national.com 8 TRI-STATE Test Circuit and Waveforms (Continued) DS011149-9 FIGURE 4. WR-RD Mode Reduced Interface System Connection with CS = RD = 0 (Mode Pin is High) DS011149-10 FIGURE 5. RD Mode (Pipeline Operation); tRDW must be between 200 ns and 400 ns. (Mode Pin is Low) Typical Performance Characteristics tCRD vs Temperature Linearity Error vs Reference Voltage Offset Error vs Reference Voltage DS011149-23 DS011149-24 9 DS011149-25 www.national.com Typical Performance Characteristics Supply Current vs Temperature (Continued) Reference Output Voltage vs Temperature Logic Threshold vs Temperature DS011149-26 DS011149-27 Output Current vs Temperature DS011149-29 www.national.com 10 DS011149-28 Application Information DS011149-17 FIGURE 6. Block Diagram of the ADC08161 Multi-Step Flash Architecture points can be connected, in groups of eight, to the eight comparators shown at the right of Figure 6. This function provides the necessary reference voltages to the comparators during each flash conversion. The six comparators, seven-resistor string (estimator DAC), and Estimator Decoder at the left of Figure 6 form the Voltage Estimator. The estimator DAC connected between VREF+ and VREF− generates the reference voltages for the six Voltage Estimator comparators. These comparators perform a very low resolution A/D conversion to obtain an “estimate” of the input voltage. This estimate is then used to control the Comparator Multiplexer, connecting the appropriate MSB Ladder section to the eight flash comparators. Only 14 comparators, six in the Voltage Estimator and eight in the flash converter, are needed to achieve the full eight-bit resolution, instead of 32 comparators that would be needed by traditional half-flash methods. A conversion begins with the Voltage Estimator comparing the analog input signal against the six tap voltages on the estimator DAC. The estimator decoder then selects one of the groups of tap points along the MSB Ladder. These eight tap points are then connected to the eight flash comparators. For example, if the analog input signal applied to VIN is be- 1.0 FUNCTIONAL DESCRIPTION The ADC08161 performs an 8-bit analog-to-digital conversion using a multi-step flash technique. The first flash generates the five most significant bits (MSBs) and the second flash generates the three least significant bits (LSBs). Figure 6 shows the major functional blocks of the ADC08161 multi-step flash converter. It consists of an over-encoded 21⁄2-bit Voltage Estimator, an internal DAC with two different voltage spans, a 3-bit half-flash converter and a comparator multiplexer. The resistor string near the center of the block diagram in Figure 6 forms the internal main DAC. Each of the eight resistors at the bottom of the string is equal to 1/256 of the total string resistance. These resistors form the LSB Ladder and have a voltage drop of 1/256 of the total reference voltage (VREF+ − VREF−) across them. The remaining resistors make up the MSB Ladder . They are made up of eight groups of four resistors connected in series. Each MSB Ladder section has 1⁄8 of the total reference voltage across it. Within a given MSB Ladder section, each of the MSB resistors has 8/256, or 1⁄32 of the total reference voltage across it. Tap points are found between all of the resistors in both the MSB and LSB Ladders. Through the Comparator Multiplexer these tap 11 www.national.com Application Information high at the end-of-conversion. It can be used to signal a processor that the converter is busy or serve as a system Transfer Acknowledge signal. (Continued) tween 0 and 3/16 of VREF (VREF = VREF+ − VREF−), the estimator decoder instructs the comparator multiplexer to select the eight tap points between 8/256 and 2/8 of VREF and connects them to the eight flash comparators. The first flash conversion is now performed, producing the five MSBs of data. The remaining three LSBs are generated next using the same eight comparators that were used for the first flash conversion. As determined by the results of the MSB flash, a voltage from the MSB Ladder equivalent to the magnitude of the five MSBs is subtracted from the analog input voltage as the upper switch is moved from position one to position two. The resulting remainder voltage is applied to the eight flash comparators and, with the lower switch in position two, compared with the eight tap points from the LSB Ladder. By using the same eight comparators for both flash conversions, the number of comparators needed by the multi-step converter is significantly reduced when compared to standard half-flash techniques. Voltage Estimator errors as large as 1/16 of VREF(16 LSBs) will be corrected since the flash comparators are connected to ladder voltages that extend beyond the range specified by the Voltage Estimator. For example, if 7/16 VREF < VIN < 9/16 VREF the Voltage Estimator’s comparators tied to the tap points below 9/16 VREF will output “1”s (000111). This is decoded by the estimator decoder to “10”. The eight flash comparators will be placed at the MSB Ladder tap points between 3⁄8 VREF and 5⁄8 VREF. The overlap of 1/16 VREF on each side of the Voltage Estimator’s span will automatically correct an error of up to 16 LSBs (16 LSBs = 312.5 mV for VREF = 5V). If the first flash conversion determines that the input voltage is between 3⁄8 VREF and 4/8 VREF − LSB/2, the Voltage Estimator’s output code will be corrected by subtracting “1”. This results in a corrected value of “01”. If the first flash conversion determines that the input voltage is between 8/16 VREF − LSB/2 and 5⁄8 VREF, the Voltage Estimator’s output code remains unchanged. After correction, the 2-bit data from both the Voltage Estimator and the first flash conversion are decoded to produce the five MSBs. Decoding is similar to that of a 5-bit flash converter since there are 32 tap points on the MSB Ladder. However, 31 comparators are not needed since the Voltage Estimator places the eight comparators along the MSB Ladder where reference tap voltages are present that fall above and below the magnitude of VIN. Comparators are not needed outside this selected range. If a comparator’s output is a “0”, all comparators above it will also have outputs of “0” and if a comparator’s output is a “1”, all comparators below it will also have outputs of “1”. 2.2 RD Mode Pipelined Operation Applications that require shorter RD pulse widths than those used in the Read mode as described above can be achieved by setting RD’s width between 200 ns–400 ns (Figure 5). RD pulse widths outside this range will create conversion linearity errors. These errors are caused by exercising internal interface logic circuitry using CS and/or RD during a conversion. When RD goes low, a conversion is initiated and the data from the previous conversion is available on the DB0–DB7 outputs. Reading DB0–DB7 for the first two times after power-up produces random data. The data will be valid during the third RD pulse that occurs after the first conversion. 2.3 WR-RD (WR then RD ) Mode The ADC08161 is in the WR-RD mode with the MODE pin tied high. A conversion starts on the rising edge of the WR signal. There are two options for reading the output data which relate to interface timing. If an interrupt-driven scheme is desired, the user can wait for the INT output to go low before reading the conversion result (Figure 3). Typically, INT will go low 690 ns, maximum, after WR’s rising edge. However, if a shorter conversion time is desired, the processor need not wait for INT and can exercise a read after only 350 ns (Figure 2). If RD is pulled low before INT goes low, INT will immediately go low and data will appear at the outputs. This is the fastest operating mode (tRD ≤ tINTL) with a conversion time, including data access time, of 560 ns. Allowing 100 ns for reading the conversion data and the delay between conversions gives a total throughput time of 660 ns (throughput rate of 1.5 MHz). 2.4 WR-RD Mode with Reduced Interface System Connection CS and RD can be tied low, using only WR to control the start of conversion for applications that require reduced digital interface while operating in the WR-RD mode (Figure 4). Data will be valid approximately 705 ns following WR’s rising edge. 3.0 REFERENCE INPUTS The ADC08161’s two VREF inputs are fully differential and define the zero to full-scale input range of the A to D converter. This allows the designer to vary the span of the analog input since this range will be equivalent to the voltage difference between VREF+and VREF−. Transducers that have outputs that minimum output voltages above GND can also be compensated by connecting VREF− to a voltage that is equal to this minimum voltage. By reducing VREF (VREF = VREF+–VREF−) to less than 5V, the sensitivity of the converter can be increased (i.e., if VREF = 2.5V, then 1 LSB = 9.8 mV). The reference arrangement also facilitates ratiometric operation and in may cases the power supply can be used for transducer power as well as the VREF source. Ratiometric operation is achieved by connecting VREF− to GND and connecting VREF+ and a transducer’s power supply input to V+. The ADC08161s accuracy degrades when VREF+–|VREF−| is less than 2.0V. The voltage at VREF− sets the input level that produces a digital output of all zeroes. Through VIN is not itself differen- 2.0 DIGITAL INTERFACE The ADC08161 has two basic interface modes which are selected by connecting the MODE pin to a logic high or low. 2.1 RD Mode With a logic low applied to the MODE pin, the converter is set to Read mode. In this configuration (Figure 1), a complete conversion is done by pulling RD low, and holding low, until the conversion is complete and output data appears. This typically takes 655 ns. The INT (interrupt) line goes low at the end of conversion. A typical delay of 50 ns is needed between the rising edge of CS (after the end of a conversion) and the start of the next conversion (by pulling RD low). The RDY output goes low after the falling edge of CS and goes www.national.com 12 Application Information ternal S/H. In a non-sampling converter, regardless of its speed, the input must remain stable to at least 1⁄2 LSB throughout the conversion process if full accuracy is to be maintained. Consequently, for many high speed signals, this signal must be externally sampled and held stationary during the conversion. The ADC08161 is suitable for DSP-based systems because of the direct control of the S/H through the WR signal. The WR input signal allows the A/D to be synchronized to a DSP system’s sampling rate or to other ADC08161s. The ADC08161 can perform accurate conversions of full-scale input signals at frequencies from DC to more than 300 kHz (full power bandwidth) without the need of an external sample-and-hold (S/H). (Continued) tial, the reference design affords nearly differential-input capability for some measurement applications. Figure 7 shows one possible differential configuration. It should be noted that, while the two VREF inputs are fully differential, the digital output will be zero for any analog input voltage if VREF− ≥ VREF+. 4.0 ANALOG INPUT AND SOURCE IMPEDANCE The ADC08161’s analog input circuitry includes an analog switch with an “on” resistance of 70Ω and a 1.4 pF capacitor (Figure 7). The switch is closed during the A/D’s input signal acquisition time (while WR is low when using the WR-RD Mode). A small transient current flows into the input pin each time the switch closes. A transient voltage, whose magnitude can increase as the source impedance increases, may be present at the input. So long as the source impedance is less than 500Ω, the input voltage transient will not cause errors and need not be filtered. Large source impedances can slow the charging of the sampling capacitors and degrade conversion accuracy. Therefore, only signal sources with output impedances less than 500Ω should be used if rated accuracy is to be achieved at the minimum sample time (100 ns maximum). A signal source with a high output impedance should have its output buffered with an operational amplifier. Any ringing or voltage shifts at the op amp’s output during the sampling period can result in conversion errors. Some suggested input configurations using the internal 2.5V reference, an external reference, and adjusting the input span are shown in Figure 8. Correct conversion results will be obtained for input voltages greater than GND − 100 mV and less than V+ + 100 mV. Do not allow the signal source to drive the analog input pin more than 300 mV higher than V+, or more than 300 mV lower than GND. The current flowing through any analog input pin should be limited to 5 mA or less to avoid permanent damage to the IC if an analog input pin is forced beyond these voltages. The sum of all the overdrive currents into all pins must be less than 20 mA. Some sort of protection scheme should be used when the input signal is expected to extend more than 300 mV beyond the power supply limits. A simple protection network using resistors and diodes is shown in Figure 9. 6.0 INTERNAL BANDGAP REFERENCE The ADC08161 has an internal bandgap 2.5V reference that can be used as the VREF+ input. A parallel combination of a 0.1 µF ceramic capacitor and a 220 µF tantalum capacitor should be used to bypass the VREFOUT pin. This reduces possible noise pickup that could cause conversion errors. 7.0 LAYOUT, GROUNDS, AND BYPASSING In order to ensure fast, accurate conversions from the ADC08161, it is necessary to use appropriate circuit board layout techniques. Ideally, the analog-to-digital converter’s ground reference should be low impedance and free of noise from other parts of the system. Digital circuits can produce a great deal of noise on their ground returns and, therefore, should have their own separate ground lines. Best performance is obtained using separate ground planes should be provided for the digital and analog parts of the system. The analog inputs should be isolated from noisy signal traces to avoid having spurious signals couple to the input. Any external component (e.g., an input filter capacitor) connected across the inputs should be returned to a very clean ground point. Incorrectly grounding the ADC08161 may result in reduced conversion accuracy. The V+ supply pin, VREF+, and VREF− (if not grounded) should be bypassed with a parallel combination of a 0.1 µF ceramic capacitor and a 10 µF tantalum capacitor placed as close as possible to the pins using short circuit board traces. See Figures 8, 9. 5.0 INHERENT SAMPLE-AND-HOLD An important benefit of the ADC08161’s input architecture is the inherent sample-and-hold (S/H) and its ability to measure relatively high speed signals without the help of an ex- DS011149-18 FIGURE 7. ADC08161 Equivalent Input Circuit Model 13 www.national.com Application Information Internal Reference 2.5V Full-Scale (Standard Application) (Continued) Power Supply as Reference Input Not Referred to GND DS011149-20 DS011149-21 DS011149-19 *Signal source driving VIN(−) must be capable of sinking 5 mA. Note: Bypass capacitors consist of a 0.1 µF ceramic in parallel with a 10 µF bead tantalum, unless otherwise specified. FIGURE 8. Analog Input Options DS011149-22 FIGURE 9. Typical Connection. Note the multiple bypass capacitors on the reference and power supply pins. VREF− should be bypassed to analog ground using multiple capacitors if it is not grounded (See Section 7.0 “LAYOUT, GROUNDS, and BYPASSING”). VIN1 is shown with an optional input protection network. www.national.com 14 inches (millimeters) unless otherwise noted Wide-Body Small-Outline Order Number ADC08161CIWM NS Package Number M20B 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. 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ADC08161 500 ns A/D Converter with S/H Function and 2.5V Bandgap Reference Physical Dimensions