ADC121S021CIMF-TI

National Semiconductor is now part of Texas Instruments. Search http://www.ti.com/ for the latest technical information and details on our current products and services. ADC121S021 Single Channel, 50 to 200 ksps, 12-Bit A/D Converter General Description Features The ADC121S021 is a low-power, single channel CMOS 12bit analog-to-digital converter with a high-speed serial interface. Unlike the conventional practice of specifying performance at a single sample rate only, the ADC121S021 is fully specified over a sample rate range of 50 ksps to 200 ksps. The converter is based upon a successive-approximation register architecture with an internal track-and-hold circuit. 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 ADC121S021 operates with a single supply that can range from +2.7V to +5.25V. Normal power consumption using a +3.6V or +5.25V supply is 1.5 mW and 7.9 mW, respectively. The power-down feature reduces the power consumption to as low as 2.6 µW using a +5.25V supply. The ADC121S021 is packaged in 6-lead LLP and SOT-23 packages. Operation over the industrial temperature range of −40°C to +85°C is guaranteed. ■ ■ ■ ■ ■ Specified over a range of sample rates. 6-lead LLP and SOT-23 packages Variable power management Single power supply with 2.7V - 5.25V range SPI™/QSPI™/MICROWIRE/DSP compatible Key Specifications ■ ■ ■ ■ ■ DNL INL SNR Power Consumption — 3.6V Supply — 5.25V Supply +0.45 / -0.25 LSB (typ) +0.45 / -0.4 LSB (typ) 72.3 dB (typ) 1.5 mW (typ) 7.9 mW (typ) Applications ■ Portable Systems ■ Remote Data Acquisition ■ Instrumentation and Control Systems Pin-Compatible Alternatives by Resolution and Speed All devices are fully pin and function compatible. Resolution Specified for Sample Rate Range of: 50 to 200 ksps 200 to 500 ksps 500 ksps to 1 Msps 12-bit ADC121S021 ADC121S051 ADC121S101 10-bit ADC101S021 ADC101S051 ADC101S101 8-bit ADC081S021 ADC081S051 ADC081S101 Connection Diagram 20145105 Ordering Information Temperature Range Description Top Mark ADC121S021CISD Order Code −40°C to +85°C 6-Lead LLP Package X7C ADC121S021CISDX −40°C to +85°C 6-Lead LLP Package, Tape and Reel X7C ADC121S021CIMF −40°C to +85°C 6-Lead SOT-23 Package X07C ADC121S021CIMFX −40°C to +85°C 6-Lead SOT-23 Package, Tape & Reel X07C ADC121S021EVAL SOT-23 Evaluation Board TRI-STATE® is a registered trademark of National Semiconductor Corporation. © 2010 National Semiconductor Corporation 201451 www.national.com ADC121S021 Single Channel, 50 to 200 ksps, 12-Bit A/D Converter January 13, 2010 ADC121S021 Block Diagram 20145107 Pin Descriptions and Equivalent Circuits Pin No. Symbol Description ANALOG I/O VIN 3 Analog input. This signal can range from 0V to VA. DIGITAL I/O 4 SCLK Digital clock input. This clock directly controls the conversion and readout processes. 5 SDATA 6 CS Chip select. On the falling edge of CS, a conversion process begins. 1 VA Positive supply pin. This pin should be connected to a quiet +2.7V to +5.25V source and bypassed to GND with a 1 µF capacitor and a 0.1 µF monolithic capacitor located within 1 cm of the power pin. 2 GND The ground return for the supply and signals. PAD GND For package suffix CISD(X) only, it is recommended that the center pad should be connected to ground. Digital data output. The output samples are clocked out of this pin on falling edges of the SCLK pin. POWER SUPPLY www.national.com 2 (Note 1, Note 2) Operating Temperature Range 2) VA Supply Voltage Digital Input Pins Voltage Range (regardless of supply voltage) Analog Input Pins Voltage Range Clock Frequency Sample Rate If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Analog Supply Voltage VA Voltage on Any Digital Pin to GND Voltage on Any Analog Pin to GND Input Current at Any Pin (Note 3) Package Input Current (Note 3) Power Consumption at TA = 25°C ESD Susceptibility (Note 5) Human Body Model Machine Model −0.3V to 6.5V −0.3V to 6.5V −0.3V to (VA +0.3V) ±10 mA ±20 mA See (Note 4) Junction Temperature Storage Temperature −40°C ≤ TA ≤ +85°C +2.7V to +5.25V −0.3V to +5.25V 0V to VA 25 kHz to 20 MHz up to 1Msps Package Thermal Resistance   3500V 300V +150°C −65°C to +150°C Package θJA 6-lead LLP 6-lead SOT-23 94°C / W 265°C / W Soldering process must comply with National Semiconductor's Reflow Temperature Profile specifications. Refer to www.national.com/packaging. (Note 6) ADC121S021 Converter Electrical Characteristics (Note 7, Note 9) The following specifications apply for VA = +2.7V to 5.25V, fSCLK = 1 MHz to 4 MHz, fSAMPLE = 50 ksps to 200 ksps, CL = 15 pF, unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25°C. Symbol Parameter Conditions Typical Limits (Note 9) Units STATIC CONVERTER CHARACTERISTICS Resolution with No Missing Codes 12 VA = +2.7V to +3.6V INL Integral Non-Linearity VA = +4.75v to +5.25V VA = +2.7V to +3.6V DNL Differential Non-Linearity VA = +4.75v to +5.25V VOFF Offset Error GE Gain Error +0.45 −0.40 ±1.0 +0.55 Bits LSB (max) LSB (min) LSB (max) −0.40 LSB (min) +0.45 +1.0 −0.25 −0.8 LSB (max) LSB (min) +0.60 LSB (max) −0.30 LSB (min) VA = +2.7v to +3.6V −0.18 VA = +4.75v to +5.25V −0.26 VA = +2.7 to +3.6V −0.75 VA = +4.75v to +5.25V −1.6 ±1.2 ±1.5 LSB (max) LSB (max) DYNAMIC CONVERTER CHARACTERISTICS Signal-to-Noise Plus Distortion Ratio VA = +2.7 to 5.25V fIN = 100 kHz, −0.02 dBFS 72 70 dBFS (min) SNR Signal-to-Noise Ratio VA = +2.7 to 5.25V fIN = 100 kHz, −0.02 dBFS 72.3 70.8 dBFS (min) THD Total Harmonic Distortion VA = +2.7 to 5.25V fIN = 100 kHz, −0.02 dBFS −83 dBFS SFDR Spurious-Free Dynamic Range VA = +2.7 to 5.25V fIN = 100 kHz, −0.02 dBFS 85 dB ENOB Effective Number of Bits VA = +2.7 to 5.25V fIN = 100 kHz, −0.02 dBFS 11.7 Intermodulation Distortion, Second Order Terms VA = +5.25V fa = 103.5 kHz, fb = 113.5 kHz −83 dBFS Intermodulation Distortion, Third Order VA = +5.25V Terms fa = 103.5 kHz, fb = 113.5 kHz −82 dBFS SINAD IMD 3 11.3 Bits (min) www.national.com ADC121S021 Operating Ratings Absolute Maximum Ratings (Note 1, Note ADC121S021 Symbol FPBW Parameter -3 dB Full Power Bandwidth Conditions Typical Limits (Note 9) Units VA = +5V 11 MHz VA = +3V 8 MHz ANALOG INPUT CHARACTERISTICS VIN Input Range IDCL DC Leakage Current CINA Input Capacitance 0 to VA V ±1 µA (max) Track Mode 30 pF Hold Mode 4 pF DIGITAL INPUT CHARACTERISTICS VIH Input High Voltage VIL Input Low Voltage IIN Input Current CIND VA = +5.25V 2.4 V (min) VA = +3.6V 2.1 V (min) VA = +5V 0.8 V (max) VA = +3V 0.4 V (max) ±0.1 ±1 µA (max) 2 4 pF (max) ISOURCE = 200 µA VA − 0.07 VA − 0.2 V (min) ISOURCE = 1 mA VA − 0.1 ISINK = 200 µA 0.03 0.4 V (max) ISINK = 1 mA 0.1 VIN = 0V or VA Digital Input Capacitance DIGITAL OUTPUT CHARACTERISTICS VOH VOL Output High Voltage Output Low Voltage IOZH, IOZL TRI-STATE® Leakage Current COUT TRI-STATE® Output Capacitance V V ±0.1 ±10 µA (max) 2 4 pF (max) Output Coding Straight (Natural) Binary POWER SUPPLY CHARACTERISTICS VA Supply Voltage Supply Current, Normal Mode (Operational, CS low) IA Supply Current, Shutdown (CS high) Power Consumption, Normal Mode (Operational, CS low) PD Power Consumption, Shutdown (CS high) 2.7 V (min) 5.25 V (max) VA = +5.25V, fSAMPLE = 200 ksps 1.5 2.8 mA (max) VA = +3.6V, fSAMPLE = 200 ksps 0.40 1.2 mA (max) fSCLK = 0 MHz, VA = +5.25V fSAMPLE = 0 ksps 500 nA VA = +5.25V, fSCLK = 4 MHz, fSAMPLE = 0 ksps 60 µA VA = +5.25V 7.9 14.7 mW (max) VA = +3.6V 1.5 4.3 mW (max) fSCLK = 0 MHz, VA = +5.25V fSAMPLE = 0 ksps 2.6 µW VA = +5.25V, fSCLK = 4 MHz, fSAMPLE = 0 ksps 315 µW AC ELECTRICAL CHARACTERISTICS fSCLK Clock Frequency (Note 8) Sample Rate (Note 8) DC SCLK Duty Cycle fSCLK = 4 MHz tACQ fS tQUIET 1.0 MHz (min) 4.0 MHz (max) 50 ksps (min) 200 ksps (max) 40 % (min) 60 % (max) Minimum Time Required for Acquisition 350 ns (max) (Note 10) 50 ns (min) www.national.com 50 4 Parameter Conditions Typical Limits (Note 9) Units tAD Aperture Delay 3 ns tAJ Aperture Jitter 30 ps ADC121S021 Timing Specifications The following specifications apply for VA = +2.7V to 5.25V, GND = 0V, fSCLK = 1.0 MHz to 4.0 MHz, CL = 25 pF, fSAMPLE = 50 ksps to 200 ksps, Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25°C. Symbol Parameter Conditions Typical Limits Units tCS Minimum CS Pulse Width 10 ns (min) tSU CS to SCLK Setup Time 10 ns (min) 20 ns (max) 40 ns (max) tEN Delay from CS Until SDATA (Note 11) tACC Data Access Time after SCLK Falling Edge (Note 12) tCL SCLK Low Pulse Width tCH SCLK High Pulse Width tH tDIS TRI-STATE® Disabled SCLK to Data Valid Hold Time SCLK Falling Edge to SDATA High Impedance (Note 13) VA = +2.7V to +3.6V VA = +4.75V to +5.25V VA = +2.7V to +3.6V VA = +4.75V to +5.25V VA = +2.7V to +3.6V VA = +4.75V to +5.25V tPOWER-UP Power-Up Time from Full Power-Down 20 ns (max) 0.4 x tSCLK ns (min) 0.4 x tSCLK ns (min) 7 ns (min) 5 ns (min) 25 ns (max) 6 ns (min) 25 ns (max) 5 ns (min) 1 µs Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions. Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified. Note 3: When the input voltage at any pin exceeds the power supply (that is, VIN < GND or VIN > VA), the current at that pin should be limited to 10 mA. The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 10 mA to two. The Absolute Maximum Rating specification does not apply to the VA pin. The current into the VA pin is limited by the Analog Supply Voltage specification. 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 device 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: Reflow temperature profiles are different for lead-free and non-lead-free packages. Note 7: Tested limits are guaranteed to National's AOQL (Average Outgoing Quality Level). Note 8: This is the frequency range over which the electrical performance is guaranteed. The device is functional over a wider range which is specified under Operating Ratings. Note 9: Data sheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 10: Minimum Quiet Time required by bus relinquish and the start of the next conversion. Note 11: Measured with the timing test circuit shown in Figure 1 and defined as the time taken by the output signal to cross 1.0V. Note 12: Measured with the timing test circuit shown in Figure 1 and defined as the time taken by the output signal to cross 1.0V or 2.0V. Note 13: tDIS is derived from the time taken by the outputs to change by 0.5V with the timing test circuit shown in Figure 1. The measured number is then adjusted to remove the effects of charging or discharging the output capacitance. This means that tDIS is the true bus relinquish time, independent of the bus loading. 5 www.national.com ADC121S021 Symbol ADC121S021 Timing Diagrams 20145108 FIGURE 1. Timing Test Circuit 20145106 FIGURE 2. ADC121S021 Serial Timing Diagram www.national.com 6 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. Acquisition time is measured backwards from the falling edge of CS when the signal is sampled and the part moves from track to hold. The start of the time interval that contains TACQ is the 13th rising edge of SCLK of the previous conversion when the part moves from hold to track. The user must ensure that the time between the 13th rising edge of SCLK and the falling edge of the next CS is not less than TACQ to meet performance specifications. APERTURE DELAY is the time after the falling edge of CS to when the input signal is acquired or held for conversion. APERTURE JITTER (APERTURE UNCERTAINTY) is the variation in aperture delay from sample to sample. Aperture jitter manifests itself as noise in the output. CONVERSION TIME is the time required, after the input voltage is acquired, for the ADC to convert the input voltage to a digital word. This is from the falling edge of CS when the input signal is sampled to the 16th falling edge of SCLK when the SDATA output goes into TRI-STATE. 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 (½ LSB below the first code transition) through positive full scale (½ 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. 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. 7 www.national.com ADC121S021 INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two sinusoidal frequencies being applied to the ADC input at the same time. It is defined as the ratio of the power in the second and third order intermodulation products to the sum of the power in both of the original frequencies. IMD is usually expressed in dB. MISSING CODES are those output codes that will never appear at the ADC outputs. The ADC121S021 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 desired signal amplitude to the amplitude of the peak spurious spectral component, where a spurious spectral component is any signal present in the output spectrum that is not present at the input and may or may not be a harmonic. TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB or dBc, of the rms total of the first five harmonic components at the output to the rms level of the input signal frequency as seen at the output. THD is calculated as Specification Definitions ADC121S021 Typical Performance Characteristics TA = +25°C, fSAMPLE = 50 ksps to 200 ksps, fSCLK = 1 MHz to 4 MHz, fIN = 100 kHz unless otherwise stated. DNL fSCLK = 1 MHz INL fSCLK = 1 MHz 20145120 20145121 DNL fSCLK = 4 MHz INL fSCLK = 4 MHz 20145160 20145161 DNL vs. Clock Frequency INL vs. Clock Frequency 20145165 www.national.com 20145166 8 ADC121S021 SNR vs. Clock Frequency SINAD vs. Clock Frequency 20145163 20145164 SFDR vs. Clock Frequency THD vs. Clock Frequency 20145167 20145168 Spectral Response, VA = 5.25 V fSCLK = 4 MHz Power Consumption vs. Throughput, fSCLK = 4 MHz 20145155 20145170 9 www.national.com ADC121S021 Figure 4 shows the device 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 device moves from hold mode to track mode on the 13th rising edge of SCLK. Applications Information 1.0 ADC121S021 OPERATION The ADC121S021 is a successive-approximation analog-todigital converter designed around a charge-redistribution digital-to-analog converter core. Simplified schematics of the ADC121S021 in both track and hold modes are shown in Figure 3 and Figure 4, respectively. In Figure 3, the device is in track mode: switch SW1 connects the sampling capacitor to the input, and SW2 balances the comparator inputs. The device is in this state until CS is brought low, at which point the device moves to the hold mode. 20145109 FIGURE 3. ADC121S021 in Track Mode 20145110 FIGURE 4. ADC121S021 in Hold Mode edge and the next falling edge of SCLK. The SDATA pin will be placed back into TRI-STATE after the 16th falling edge of SCLK, or at the rising edge of CS, whichever occurs first. After a conversion is completed, the quiet time (tQUIET) must be satisfied before bringing CS low again to begin another conversion. Sixteen SCLK cycles are required to read a complete sample from the ADC. The sample bits (including leading zeroes) are clocked out on falling edges of SCLK, and are intended to be clocked in by a receiver on subsequent rising edges of SCLK. The ADC will produce three leading zero bits on SDATA, followed by twelve data bits, most significant first. If CS goes low before the rising edge of SCLK, an additional (fourth) zero bit may be captured by the next falling edge of SCLK. 2.0 USING THE ADC121S021 The serial interface timing diagram for the ADC is shown in Figure 2. CS is chip select, which initiates conversions on the ADC and frames the serial data transfers. SCLK (serial clock) controls both the conversion process and the timing of serial data. SDATA is the serial data out pin, where a conversion result is found as a serial data stream. Basic operation of the ADC begins with CS going low, which initiates a conversion process and data transfer. Subsequent rising and falling edges of SCLK will be labelled with reference to the falling edge of CS; for example, "the third falling edge of SCLK" shall refer to the third falling edge of SCLK after CS goes low. At the fall of CS, the SDATA pin comes out of TRI-STATE, and the converter moves from track mode to hold mode. The input signal is sampled and held for conversion on the falling edge of CS. The converter moves from hold mode to track mode on the 13th rising edge of SCLK (see Figure 2). It is at this point that the interval for the TACQ specification begins. In the worst case, 350ns must pass between the 13th rising www.national.com 2.1 Determining Throughput Throughput depends on the frequency of SCLK and how much time is allowed to elapse between the end of one conversion and the start of another. At the maximum specified 10 It is possible, however, to use fewer than 20 clock cycles provided the timing parameters are met. With a 1MHz SCLK, there are 2500ns in 2.5 SCLK cycles, which is greater than tACQ. After the last data bit has come out, the clock will need one full cycle to return to a falling edge. Thus the total time between falling edges of CS is 12.5*1μs +2.5*1μs +1*1μs=16μs which is a throughput of 62.5KSPS. 3.0 ADC121S021 TRANSFER FUNCTION The output format of the ADC is straight binary. Code transitions occur midway between successive integer LSB values. The LSB width for the ADC is VA/4096. The ideal transfer characteristic is shown in Figure 5. The transition from an output code of 0000 0000 0000 to a code of 0000 0000 0001 is at 1/2 LSB, or a voltage of VA/8192. Other code transitions occur at steps of one LSB. 12.5*50ns + 350ns + 0.5*50ns = 1000ns (12.5 SCLKs + tACQ + 1/2 SCLK) which corresponds to a maximum throughput of 1MSPS. At the slowest rate for this family, SCLK is 1MHz. Using a 20 cycle conversion frame as shown in Figure 2 yields a 20μs time between CS falling edges for a throughput of 50KSPS. 20145111 FIGURE 5. Ideal Transfer Characteristic formance. To keep noise off the supply, use a dedicated linear regulator for this device, or provide sufficient decoupling from other circuitry to keep noise off the ADC supply pin. Because of the ADC's low power requirements, it is also possible to use a precision reference as a power supply to maximize performance. The three-wire interface is shown connected to a microprocessor or DSP. 4.0 TYPICAL APPLICATION CIRCUIT A typical application of the ADC is shown in Figure 6. Power is provided in this example by the National Semiconductor LP2950 low-dropout voltage regulator, available in a variety of fixed and adjustable output voltages. The power supply pin is bypassed with a capacitor network located close to the ADC. Because the reference for the ADC is the supply voltage, any noise on the supply will degrade device noise per- 11 www.national.com ADC121S021 SCLK frequency, the maximum guaranteed throughput is obtained by using a 20 SCLK frame. As shown in Figure 2, the minimum allowed time between CS falling edges is determined by 1) 12.5 SCLKs for Hold mode, 2) the larger of two quantities: either the minimum required time for Track mode (tACQ) or 2.5 SCLKs to finish reading the result and 3) 0, 1/2 or 1 SCLK padding to ensure an even number of SCLK cycles so there is a falling SCLK edge when CS next falls. For example, at the fastest rate for this family of parts, SCLK is 20MHz and 2.5 SCLKs are 125ns, so the minimum time between CS falling edges is calculated by ADC121S021 20145113 FIGURE 6. Typical Application Circuit conversion process is begun) when CS is pulled low. The device will enter shutdown mode if CS is pulled high before the tenth falling edge of SCLK after CS is pulled low, or will stay in normal mode if CS remains low. Once in shutdown mode, the device will stay there until CS is brought low again. By varying the ratio of time spent in the normal and shutdown modes, a system may trade-off throughput for power consumption, with a sample rate as low as zero. 5.0 ANALOG INPUTS An equivalent circuit for the ADC's input is shown in Figure 7. Diodes D1 and D2 provide ESD protection for the analog inputs. At no time should the analog input go beyond (VA + 300 mV) or (GND − 300 mV), as these ESD diodes will begin to conduct, which could result in erratic operation. For this reason, the ESD diodes should not be used to clamp the input signal. The capacitor C1 in Figure 7 has a typical value of 4 pF, and is mainly the package pin capacitance. Resistor R1 is the on resistance of the track / hold switch, and is typically 500Ω. Capacitor C2 is the ADC sampling capacitor and is typically 26 pF. The ADC will deliver best performance when driven by a low-impedance source to eliminate distortion caused by the charging of the sampling capacitance. This is especially important when using the ADC to sample AC signals. Also important when sampling dynamic signals is an anti-aliasing filter. 7.1 Normal Mode The fastest possible throughput is obtained by leaving the ADC in normal mode at all times, so there are no power-up delays. To keep the device in normal mode continuously, CS must be kept low until after the 10th falling edge of SCLK after the start of a conversion (remember that a conversion is initiated by bringing CS low). If CS is brought high after the 10th falling edge, but before the 16th falling edge, the device will remain in normal mode, but the current conversion will be aborted, and SDATA will return to TRI-STATE (truncating the output word). Sixteen SCLK cycles are required to read all of a conversion word from the device. After sixteen SCLK cycles have elapsed, CS may be idled either high or low until the next conversion. If CS is idled low, it must be brought high again before the start of the next conversion, which begins when CS is again brought low. After sixteen SCLK cycles, SDATA returns to TRI-STATE. Another conversion may be started, after tQUIET has elapsed, by bringing CS low again. 20145114 7.2 Shutdown Mode Shutdown mode is appropriate for applications that either do not sample continuously, or it is acceptable to trade throughput for power consumption. When the ADC is in shutdown mode, all of the analog circuitry is turned off. To enter shutdown mode, a conversion must be interrupted by bringing CS high anytime between the second and tenth falling edges of SCLK, as shown in Figure 8. Once CS has been brought high in this manner, the device will enter shutdown mode; the current conversion will be aborted and SDATA will enter TRI-STATE. If CS is brought high before the second falling edge of SCLK, the device will not change mode; this is to avoid accidentally changing mode as a result of noise on the CS line. FIGURE 7. Equivalent Input Circuit 6.0 DIGITAL INPUTS AND OUTPUTS The ADC digital inputs (SCLK and CS) are not limited by the same maximum ratings as the analog inputs. The digital input pins are instead limited to +5.25V with respect to GND, regardless of VA, the supply voltage. This allows the ADC to be interfaced with a wide range of logic levels, independent of the supply voltage. 7.0 MODES OF OPERATION The ADC has two possible modes of operation: normal mode, and shutdown mode. The ADC enters normal mode (and a www.national.com 12 ADC121S021 20145116 FIGURE 8. Entering Shutdown Mode 20145117 FIGURE 9. Entering Normal Mode To exit shutdown mode, bring CS back low. Upon bringing CS low, the ADC will begin powering up (power-up time is specified in the Timing Specifications table). This power-up delay results in the first conversion result being unusable. The second conversion performed after power-up, however, is valid, as shown in Figure 9. If CS is brought back high before the 10th falling edge of SCLK, the device will return to shutdown mode. This is done to avoid accidentally entering normal mode as a result of noise on the CS line. To exit shutdown mode and remain in normal mode, CS must be kept low until after the 10th falling edge of SCLK. The ADC will be fully powered-up after 16 SCLK cycles. CS line after the 10th and before the 15th fall of SCLK of each conversion. A plot of typical power consumption versus throughput is shown in the Typical Performance Curves section. To calculate the power consumption for a given throughput, multiply the fraction of time spent in the normal mode by the normal mode power consumption and add the fraction of time spent in shutdown mode multiplied by the shutdown mode power consumption. Note that the curve of power consumption vs. throughput is essentially linear. This is because the power consumption in the shutdown mode is so small that it can be ignored for all practical purposes. 9.0 POWER SUPPLY NOISE CONSIDERATIONS The charging of any output load capacitance requires current from the power supply, VA. The current pulses required from the supply to charge the output capacitance will cause voltage variations on the supply. If these variations are large enough, they could degrade SNR and SINAD performance of the ADC. 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 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. To keep noise out of the power supply, keep the output load capacitance as small as practical. It is good practice to 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. 8.0 POWER MANAGEMENT The ADC takes time to power-up, either after first applying VA, or after returning to normal mode from shutdown mode. This corresponds to one "dummy" conversion for any SCLK frequency within the specifications in this document. After this first dummy conversion, the ADC will perform conversions properly. Note that the tQUIET time must still be included between the first dummy conversion and the second valid conversion. When the VA supply is first applied, the ADC may power up in either of the two modes: normal or shutdown. As such, one dummy conversion should be performed after start-up, as described in the previous paragraph. The part may then be placed into either normal mode or the shutdown mode, as described in Sections 7.1 and 7.2. When the ADC is operated continuously in normal mode, the maximum guaranteed throughput is FSCLK/20 at the maximum specified FSCLK. Throughput may be traded for power consumption by running fSCLK at its maximum specified rate and performing fewer conversions per unit time, raising the ADC 13 www.national.com ADC121S021 Physical Dimensions inches (millimeters) unless otherwise noted 6-Lead LLP Order Number ADC121S021CISD or ADC121S021CISDX NS Package Number SDB06A 6-Lead SOT-23 Order Number ADC121S021CIMF, ADC121S021CIMFX NS Package Number MF06A www.national.com 14 ADC121S021 Notes 15 www.national.com ADC121S021 Single Channel, 50 to 200 ksps, 12-Bit A/D Converter Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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