TSM1285DB

TSM1285 A 300ksps, Single-supply, Low-Power 12-Bit Serial-output ADC FEATURES DESCRIPTION  The TSM1285 – a single-supply, single-channel, 12bit analog-to-digital converter (ADC) - is an alternate source for the MAX1285 and a higher-speed upgrade to the MAX1240 and MAX1240 ADCs. The TSM1285 combines a high-bandwidth track-and-hold (T/H), a high-speed serial digital interface, an internal +2.5V reference, and low conversion-mode power consumption. The TSM1285 operates from a single +2.7V to+3.6V supply and draws less than 2.5mA at 300ksps.          Alternate Source for MAX1285 and higher-speed upgrade to MAX1240 and MAX1241 Single-Supply Operation: +2.7V to +3.6V DNL & INL: ±1LSB (max) 300ksps Sampling Rate Low Conversion-Mode Supply Current: 2.5mA @ 300ksps Low Supply Current in Shutdown: 2µA Internal Track-and-Hold Internal +2.5V Reference SPI®/QSPI™/MICROWIRE™ 3-Wire SerialInterface1 8-Pin SOIC Package APPLICATIONS Process Control and Factory Automation Data and Low-frequency Signal Acquisition Portable Data Logging Pen Digitizers & Tablet Computers Medical Instrumentation Battery-powered Instruments 1 SPI and QSPI are trademarks of Motorola, Inc. MICROWIRE is a trademark of National Semiconductor Corporation. Connecting directly to any SPI, QSPI, MICROWIRE™ microcontrollers and other interface-compatible computing devices, the TSM1285’s 3-wire serial interface is easy to use and doesn’t require separate, external logic. An external serial-interface clock controls the TSM1285’s conversion process and its output shift register operation. In PCB-space-conscious, low-power remote-sensor and data-acquisition applications, the TSM1285 is an excellent choice for its low-power, ease-of-use, and small-package-footprint attributes. The TSM1285BC is fully specified over the 0°C to +70°C temperature range. TSM1285BE is fully specified over the -40°C to +85°C temperature range. Both products are available in a 8-pin SOIC package. FUNCTIONAL BLOCK DIAGRAM Page 1 © 2014 Silicon Laboratories, Inc. All rights reserved. TSM1285 ABSOLUTE MAXIMUM RATINGS VDD to GND.................................................................... -0.3V to +6V AIN to GND ...................................................... -0.3V to (VDD + 0.3V) REF to GND ..................................................... -0.3V to (VDD + 0.3V) Digital Inputs to GND ....................................... -0.3V to (VDD + 0.3V) DOUT to GND .................................................. -0.3V to (VDD + 0.3V) DOUT Current ........................................................................ ±25mA Continuous Power Dissipation (TA = +70°C): 8-Pin SOIC (Derate 5.88mW/°C above +70°C) .............. 471mW Operating Temperature Ranges: TSM1285BC ...........................................................0°C to +70°C TSM1285BE ....................................................... -40°C to +85°C Storage Temperature Range .................................. -60°C to +150°C Lead Temperature (Soldering, 10s)....................................... +300°C Soldering Temperature (Reflow) ........................................... +260°C Electrical and thermal stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only and functional operation of the device at these or any other condition beyond those indicated in the operational sections of the specifications is not implied. Exposure to any absolute maximum rating conditions for extended periods may affect device reliability and lifetime. PACKAGE/ORDERING INFORMATION ORDER NUMBER PART MARKING TEMPERATURE CARRIER QUANTITY RANGE TSM1285BCSA+ Tube 97 TSM1285BCSA+T Tape & Reel 2500 TSM1285BESA+ Tube 97 Tape & Reel 2500 T1285B T1285BE 0ºC to 70ºC -40ºC to +85ºC TSM1285BESA+T Lead-free Program: Silicon Labs supplies only lead-free packaging. Consult Silicon Labs for products specified with wider operating temperature ranges. Page 2 TSM1285 Rev. 1.0 TSM1285 ELECTRICAL SPECIFICATIONS VDD = +2.7V to +3.6V; fSCLK = 4.8MHz, 50% duty cycle, 16 clocks/conversion cycle, 300ksps; 4.7μF capacitor at REF; TA = -40ºC to +85ºC, unless otherwise noted. Typical values apply at TA = +25°C. PARAMETER DC ACCURACY (See Note 1) Resolution Relative Accuracy Differential Nonlinearity Offset Error Gain Error Gain-Error Temperature Coefficient SYMBOL CONDITIONS TYP MAX UNITS ±1.0 ±1.0 ±6.0 ±6.0 Bits LSB LSB LSB LSB 12 INL DNL ZE GE See Note 2 No missing codes over temperature See Note 3 TCGE DYNAMIC SPECIFICATIONS (fIN = 75kHz sine wave, 2.5VPP, fSAMPLE = 300ksps, fSCLK = 4.8MHz) Signal-to-Noise SINAD Plus Distortion Ratio Total Harmonic Distortion THD Including the 5th harmonic Spurious-Free Dynamic Range SFDR Intermodulation Distortion IMD fA = 73kHz, fB = 77kHz Full-Power Bandwidth FPBW -3dB point Full-Linear Bandwidth FLBW SINAD > 68dB CONVERSION RATE Conversion Time tCONV See Note 4 Track/Hold Acquisition Time tACQ Aperture Delay tAD Aperture Jitter tAJ Serial Clock Frequency tSCLK Duty Cycle ANALOG INPUT (AIN) Input Voltage Range VIN Input Capacitance CINA INTERNAL REFERENCE REF Output Voltage VREF REF Short-Circuit Current TA = +25°C REF Output Tempco TCVREF Load Regulation See Note 5; 0 to 0.75mA output load Capacitive Bypass at REF DIGITAL INPUTS (SCLK, CS, SHDN) Input High Voltage VINH Input Low Voltage VINL Input Hysteresis VHYST Input Leakage IIN VINL = 0V or VINH = VDD Input Capacitance CIND DIGITAL OUTPUT (DOUT) Output Voltage Low VOL ISINK = 5mA Output Voltage High VOH ISOURCE = 0.5mA Three-State Leakage Current IL VCS = +3V Three-State Output Capacitance COUT VCS = +3V POWER SUPPLY Positive Supply Voltage VDD See Note 6 Positive Supply Current IDD See Note 7; VDD = +3.6V Shutdown Supply Current ISHDN SCLK = VDD, SHDN = GND Power-Supply Rejection PSR VDD = +2.7V to 3.6V, midscale input TSM1285 Rev. 1.0 MIN ±1.6 ppm/°C 70 dB -80 80 76 3 250 dB dB dB MHz kHz 3.3 625 10 < 50 0.5 40 4.8 60 0 2.5 V pF 2.52 V mA ppm/°C mV/mA μF 10 2.48 2.50 15 ±15 0.1 4.7 2.0 10 2.0 0.8 0.2 ±1 15 0.4 VDD - 0.5 ±10 15 2.7 2.5 2 ±0.5 μs ns ns ps MHz % 3.6 3.5 10 V V V μA pF V V μA pF V mA μA mV Page 3 TSM1285 TIMING SPECIFICATIONS VDD = +2.7V to +3.6V, TA = -40ºC to +85ºC, unless otherwise noted. PARAMETER SCLK Period SCLK Pulse-Width High SCLK Pulse-Width Low CS Fall to SCLK Rise Setup SCLK Rise to CS Rise Hold SCLK Rise to CS Fall Ignore CS Rise to SCLK Rise Ignore SCLK Rise to DOUT Hold SCLK Rise to DOUT Valid CS Rise to DOUT Disable CS Fall to DOUT Enable CS Pulse-Width High SYMBOL tCP tCH tCL tCSS tCSH tCSO tCS1 tDOH tDOV tDOD tDOE tCSW CONDITIONS CLOAD = 20pF CLOAD = 20pF CLOAD = 20pF; Refer to Figure 2 CLOAD = 20pF; Refer to Figure 1 MIN 208 83 83 45 0 45 45 13 13 TYP MAX 100 85 85 100 UNITS ns ns ns ns ns ns ns ns ns ns ns ns Note 1: Tested at VDD = VDD(MIN). Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has been calibrated. Note 3: Internal reference, offset, and reference errors nulled. Note 4: Conversion time is defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle. Note 5: External load should not change during conversion for specified accuracy. Guaranteed specification limit of 2mV/mA because of production test limitations. Note 6: Electrical characteristics are guaranteed from VDD(MIN) to VDD(MAX). For operations beyond this range, see Typical Operating Characteristics. Note 7: TSM1285 tested with 20pF on DOUT and fSCLK = 4.8MHz, 0 to 3V. DOUT = full scale. Page 4 TSM1285 Rev. 1.0 TSM1285 TYPICAL PERFORMANCE CHARACTERISTICS VDD = +3V; fSCLK = 4.8MHz; CLOAD = 20pF; 4.7μF capacitor at REF; TA = 25ºC, unless otherwise noted. Differential Nonlinearity Integral Nonlinearity 0.4 0.25 0.2 0.3 0.15 0.2 DNL - LSB INL - LSB 0.2 0.1 0 -0.1 0.05 0 -0.05 -0.1 -0.2 -0.15 -0.3 -0.2 -0.4 -0.25 0 1k 2k 3k 4k 5k 0 4k 3k DIGITAL OUTPUT CODE Offset Error vs Supply Voltage Offset Error vs Temperature 5k 1 -0.4 0.5 OFFSET ERROR - LSB OFFSET ERROR - LSB 2k DIGITAL OUTPUT CODE -0.2 -0.6 -0.8 -1 -1.2 -1.4 0 -0.5 -1 -1.5 -1.6 -1.8 -2 2.7 2.88 3.06 3.24 3.42 -40 3.6 -15 10 35 60 POWER SUPPLY VOLTAGE - Volt TEMPERATURE - ºC Gain Error vs Supply Voltage Gain Error vs Temperature 1.2 1.2 1 1 0.8 GAIN ERROR - LSB GAIN ERROR - LSB 1k 0.6 0.4 0.2 0 85 0.8 0.6 0.4 0.2 0 -0.2 -0.2 2.7 2.88 3.06 3.24 3.42 POWER SUPPLY VOLTAGE - Volt TSM1285 Rev. 1.0 3.6 -0.4 -40 -15 10 35 60 85 TEMPERATURE - ºC Page 5 TSM1285 TYPICAL PERFORMANCE CHARACTERISTICS VDD = +3V; fSCLK = 4.8MHz; CLOAD = 20pF; 4.7μF capacitor at REF; TA = 25ºC, unless otherwise noted. Internal Reference Output vs Temperature 2.506 2.510 2.504 2.508 REFERENCE OUTPUT - V REFERENCE OUTPUT - V Internal Reference Output vs Supply Voltage 2.502 2.5 2.498 2.496 2.494 2.7 2.88 3.06 3.24 3.42 2.504 2.502 2.5 2.498 -40 3.6 -15 10 35 60 85 POWER SUPPLY VOLTAGE - Volt TEMPERATURE - ºC Power Supply Current vs Power Supply Voltage Power Supply Current vs Temperature 1.4 1.2 1.2 CODE = 1111 1111 1111 RLOAD = ∞ CLOAD = 10pF 1.1 CONVERTING SCLK = 4.8MHz 1 0.9 0.8 0.7 STATIC 1 0.9 0.8 0.7 STATIC, VDD = 3V 0.6 0.6 0.5 0.5 2.7 2.88 3.06 3.24 3.42 POWER SUPPLY VOLTAGE - Volt Page 6 CONVERTING, VDD = 3V 1.1 SUPPLY CURENT - mA 1.3 SUPPLY CURENT - mA 2.506 3.6 -40 -15 10 35 60 85 TEMPERATURE - ºC TSM1285 Rev. 1.0 TSM1285 PIN FUNCTIONS PIN 1 2 NAME VDD AIN 3 SHDN 4 REF 5 GND 6 DOUT 7 CS 8 SCLK FUNCTION Power Supply Voltage, +2.7V to +3.6V. Analog Signal Input; Unipolar, 0 to VREF input range. Active-Low Shutdown Input. Toggling SHDN high-to-low powers down the TSM1285 and reduces the supply current to 2μA (typ). Reference Voltage for Analog-to-Digital Conversion – an internal 2.5V reference output. Bypass with a good-quality 4.7μF capacitor. Analog and Digital Ground. Connect the TSM1285’s GND pin at one and only one point to the system analog ground plane. Serial-Data Output. DOUT toggles state on SCLK’s rising edge and is high impedance when CS is logic high. Active-Low Chip Select. The CS signal initiates the conversion process on its falling edge. When the CS input is logic high, DOUT is high impedance. Serial-Clock Input. The SCLK signal controls the conversion process and transfers output data at rates up to 4.8MHz. Figure 1: Output Loading Circuits for DOUT Enable Time (tDOE). Figure 2: Output Loading Circuits for DOUT Disable Time (tDOD). TSM1285 Rev. 1.0 Page 7 TSM1285 DESCRIPTION OF OPERATION Converter Operation Analog Input The TSM1285 uses an input track-and-hold (T/H) and a successive-approximation register (SAR) circuitry to convert an analog input signal to a digital 12-bit output. No external-hold capacitor is needed for the track/hold circuit. Figure 3 illustrates the TSM1285 in its simplest configuration. The TSM1285 converts Figure 4 illustrates the sampling architecture of the Figure 4: TSM1285 Equivalent Input Circuit Details. Figure 3: TSM1285 Typical Application Circuit. analog-to-digital converter’s comparator. The fullscale input voltage is set by the TSM1285’s internal 2.5-V reference. Track-and-Hold Operation input signals within the 0V to VREF range in 3.3μs including the track-and-hold’s acquisition time. The serial interface requires only three digital lines (SCLK, CS, and DOUT) and provides an easy interface to microprocessors (μPs) and microcontrollers (μCs). The TSM1285 has two operating modes: normal and shutdown. Toggling (or driving) the SHDN pin low shuts down the ADCs and reduces supply current below 1 μA when VDD ≤ 3.6V. Open-circuiting or toggling (or driving) the SHDN pin high or places the ADCs into operational mode. Toggling the CS pin to logic low initiates a conversion where the conversion result is available at DOUT in unipolar serial format. The serial data stream consists of three leading zeros followed by the data bits with the MSB first. All transitions on the DOUT pin occur within 20ns after the low-to-high transition of SCLK. Serial interface timing details of the TSM1285 are illustrated in Figures 8 and 9. Page 8 During track mode, the analog signal is acquired and stored on the internal hold capacitor. During hold mode, the track/hold switches SW1 and SW2 are opened thereby maintaining a constant input level to the converter’s SAR subcircuit. During the acquisition phase with SW1 and SW2 on TRACK, the input capacitor, CHOLD, is charged to the analog input (AIN). Toggling the CS pin low causes the acquisition process to stop. At this instant, track/hold switches SW1 and SW2 are moved to HOLD position and the input side of CHOLD is then switched to GND. Unbalancing Node ZERO at the comparator’s input, the retained charge on CHOLD represents a sample of the input signal applied to the converter. In hold mode and to restore Node ZERO to 0V within the limits of the converter’s 12- bit resolution, the output of the capacitive digital-to-analog converter (the CDAC) is adjusted during the remainder of the conversion cycle. In other words, the stored charge on CHOLD is transferred to the binary-weighted CDAC where it is converted into a digital representation of the analog input signal. At end of the conversion TSM1285 Rev. 1.0 TSM1285 process, the input side of CHOLD is switched back to AIN so as to be charged to the input signal again. An ADC’s acquisition time is function of how fast its input capacitance can be charged. If an input signal’s driving-point source impedance is high, the acquisition time is lengthened and more time must be allowed between conversions. The acquisition time (tACQ) is the maximum time the ADC requires to acquire the signal and is also the minimum time needed for the signal to be acquired. The TSM1285’s acquisition time is calculated from the following expression: tACQ = 9 x (RS + RIN) x 10pF where RIN = 100Ω (the TSM1285’s internal track/hold switch resistance), RS = the input signal’s source impedance, and tACQ is never less than 625ns. Because of the input structure of the TSM1285, sources with output impedances of 1kΩ or less do not affect significantly the AC performance of the TSM1285. The TSM1285 can still be used in applications where the source impedance is higher so long as a 0.01μF capacitor is connected between the analog input and GND. Limiting the ADC’s input signal bandwidth, the use of an external, input capacitor forms an RC filter with the input’s source impedance. Input Bandwidth Considerations Since the TSM1285’s input track-and-hold circuit exhibits a 10 MHz small-signal bandwidth, it is possible to measure periodic signals and to digitize high-speed transient events with signal bandwidths higher than the TSM1285’s sampling rate by using undersampling techniques. To avoid the aliasing of high-frequency signals into the frequency band of interest, the use of external anti-alias filter circuits (discrete or integrated) is recommended. The time constant of the external anti-alias filter should be set so as not to interfere with the desired signal bandwidth. Analog Input Protection The TSM1285 incorporates internal protection diodes that clamp the analog input between VDD and GND. These internal protection diodes allow the AIN pin to swing from GND - 0.3V to VDD + 0.3V without causing TSM1285 Rev. 1.0 damage to the TSM1285. However, for accurate conversions near full scale, the input signal must not exceed VDD by more than 50mV or be lower than GND by 50mV. If the analog inputs can exceed 50mV beyond the supplies, then the current in the forward-biased protection diodes should be limited to less than 2mA since large fault currents can affect conversion results. Internal Reference Considerations The TSM1285 has an internal voltage reference that is factory-trimmed to 2.5V. The internal reference output is connected to the REF pin and is also connected to the ADC’s internal CDAC. The REF output can be used as a reference voltage source for other components external to the ADC and can source up to 750μA. To maintain conversion accuracy to within 1 LSB, a 4.7μF capacitor from the REF pin to GND is recommended. While largervalued capacitors can be used to further reduce reference wide-band noise, larger capacitor values can increase the TSM1285’s wake-up time when exiting from shutdown mode (see the “Using SHDN to Reduce Operating Supply Current” section for more information). When in shutdown (that is, when SHDN = 0), the TSM1285’s internal 2.5-V reference is disabled. Serial Digital Interface Initialization Conversion after Power-Up and Starting a If the SHDN pin is not driven low upon an initial, coldstart condition, it may take up to 2.5ms for a fullydischarged 4.7μF reference bypass capacitor to provide adequate charge for specified conversion accuracy. As a result, conversions should not be initiated during this reference capacitor charge-up delay. To initiate a conversion, the CS pin is toggled (or driven) low. At the CS’s falling edge, the TSM1285’s internal track-and-hold is placed in hold mode and a conversion is initiated. Data can then be transferred out of the ADC using an external serial clock. Page 9 TSM1285 Using the ADC’s SHDN to Reduce Operating Supply Current Power consumption can be reduced significantly by turning off the TSM1285 in between conversions. Figure 5: TSM1285 Supply Current vs Conversion Rate SUPPLY CURENT - mA 1k VDD = 3V DOUT = FS RL = ∞ CL = 10pF 100 10 1 0.1 0.1 1 10 100 1k CONVERSION RATE - ksps Figure 5 illustrates the TSM1285’s average supply current versus conversion rate. The wake-up delay time (tWAKE) is the time from when the SHDN pin is deasserted to the time when a conversion may be initiated (Refer to Figure 6). This delay time depends on how long the ADC was in shutdown (Refer to Figure 7) because the external 4.7μF reference bypass capacitor is discharged slowly when SHDN = 0. Timing and Control Details The CS and SCLK digital inputs control the TSM1285’s conversion-start and data-read operations. The ADC’s serial-interface operations are illustrated in Figures 8 and 9. A CS high-to-low transition initiates the conversion sequence - the input track-and-hold samples the input signal level, the ADC begins to convert, and the DOUT pin changes state from high impedance to logic low. The external SCLK signal is used to drive the conversion process and is also used to transfer the converted data out of the ADC as each bit of conversion is determined. The SCLK signal transfers data after a low-to-high transition of the third (3rd) SCLK pulse. After each subsequent SCLK rising edge, transitions on the DOUT pin occur in 20ns. The third rising clock edge produces the MSB of the conversion at DOUT, followed by the remaining bits. Since there are twelve data bits and three leading zeros, at least fifteen rising clock edges are needed to transfer the entire data stream. Extra SCLK pulses occurring after the conversion result has been completely transferred out and, before to a new, low-to-high transition on CS, produce a string trailing zeros at DOUT. In addition, the extra SCLK pulses have no effect on converter operation. Minimum conversion cycle time can be accomplished by: (a) toggling the CS pin high after reading the conversion result’s LSB; and (b), after the specified minimum time defined by tCS has elapsed, toggling the CS pin low again to initiate the next conversion. Output Data Coding and Transfer Function Conversion results at the TSM1285’s DOUT pin are straight binary data. Figure 10 illustrates the nominal transfer function where code transitions occur halfway between successive integer LSB values. If VREF = +2.500V, then 1 LSB = 610μV or 2.500V/4096. Figure 6: TSM1285 Shutdown Operation. Page 10 TSM1285 Rev. 1.0 TSM1285 REFERENCE POWER-UP DELAY TIME - ms Figure 7: TSM1285 Reference Power-Up Delay vs Duration in Shutdown Mode APPLICATIONS INFORMATION Connection to Industry-Standard Serial Interfaces 2.5 The TSM1285’s serial interface is fully compatible with SPI/QSPI and MICROWIRE standard serial interfaces (Refer to Figure 11). For serial interface operation with these standards, the CPU’s serial interface should be set to master mode so the CPU then generates the serial clock. Second, the CPU’s serial clock should be configured to operate up to 4.8MHz. The process to configure the serial clock and data transfer operation is as follows: CREF = 4.7µF 2 1.5 1 0.5 0 0.1m 1m 10m 100m 1 10 TIME IN SHUTDOWN MODE - sec 1) Using a general-purpose I/O line from the CPU, the CS pin is driven low to start a conversion. DOUT transitions from high impedance to logic low. The SCLK polarity should be low to start the conversion process correctly. Figure 8: TSM1285 Serial Interface Timing Sequence Figure 9: TSM1285 Serial Interface Timing Specifications in Detail. 2) Next, SCLK is activated for a minimum of 15 SCLK cycles where the first two SCLKs produce zeros at the DOUT pin. Data at DOUT is formatted MSB first TSM1285 Rev. 1.0 and DOUT transitions occur 20ns after the third (3rd) SCLK low-to-high transition. Once the low-to-high SCLK transition has occurred, data is valid at DOUT Page 11 TSM1285 4) Once the CS pin is held at logic high for at least tCS, a new conversion cycle is started when the CS pin is toggled low. If a conversion is aborted by toggling the CS pin high before the current conversion has completed, a new conversion cycle can only be started after a the ADC has acquired the signal (tACQ). The CS pin must be held low and SCLK active until all data bits are transferred out of the ADC. As shown in Figure 8, data can be transferred in two 8-bit bytes or continuously. The bytes contain the result of the conversion padded with three leading 0s in the first 8bit byte and 1 trailing 0 in the second 8-bit byte. Figure 10: ADC Unipolar Transfer Function for Straight Binary Digital Data. SPI and MICROWIRE Interface Details When using an SPI or MICROWIRE interface, setting [CPOL:CPHA] = [0:0] configures the microcontroller’s serial clock and sampling edge for the TSM1285. The conversion commences on a high-to-low transition of the CS pin. The DOUT pin transitions from a highimpedance state to a logic low, indicating a conversion is in progress. Two consecutive 1-byte data reads are required to transfer the full 12-bit result from the ADC. DOUT output data transitions occur on the SCLK’s low-to-high transition and are transferred into the downstream microcontroller on the SCLK’s low-to-high transition. The first byte contains three leading 0s and then five bits of the conversion result. The second byte contains the remaining seven bits of the conversion result and one trailing zero. Refer to Figure 11 for the circuit connections and to Figure 12 for all timing details. Figure 11: TSM1285 Circuit Connections to Industry-Standard Serial Interfaces. according to the tDOV (SCLK Rise to DOUT Valid) timing specification. Valid output data can then be transferred into µP or µCs on SCLK low-to-high transitions. 3) At or after the 15th SCLK low-to-high transition, the CS pin can be toggled high to halt the transfer process. If the CS pin remains low and the SCLK is still active, trailing zeros are transferred out after the LSB. Page 12 QSPI Details Using a QSPI microcontroller, setting [CPOL:CPHA] = [0:1] configures the microcontroller’s serial clock and sampling edge for the TSM1285. Unlike the SPI, which requires two 1-byte reads to transfer all 12 bits of data from the ADC, the QSPI allows a minimum number of clock cycles necessary to transfer data from the ADC to the microcontroller. Thus, the TSM1285 requires 15 SCLK clock cycles from the microcontroller to transfer the 12 bits of data with no trailing zeros. As shown in Figure 13, the conversion results contain two leading 0s followed by the MSB-first-formatted, 12-bit data stream. TSM1285 Rev. 1.0 TSM1285 Figure 12: SPI/MICROWIRE-TSM1285 Serial Interface Timing Details with [CPOL:CPHA] = [0:0]. Figure 13: QSPI-TSM1285 Serial Interface Timing Details with [CPOL:CPHA] = [0:1]. PCB Layout, Ground Plane Management, and Capacitive Bypassing For best performance, printed circuit boards should always be used and wire-wrap boards are not recommended. Good PC board layout techniques ensure that digital and analog signal lines are kept separate from each other, analog and digital Figure 14: Recommended Power Supply Bypassing and Star Ground Configuration. TSM1285 Rev. 1.0 (especially clock) lines are not routed parallel to one another, and high-speed digital lines are not routed underneath the ADC package. A recommended system ground connection is illustrated in Figure 14. A single-point analog ground (star ground point) should be created at the ADC’s GND and separate from the logic ground. All analog grounds as well as the ADC’s GND pin should be connected to the star ground. No other digital system ground should be connected to this ground. For lowest-noise operation, the ground return to the star ground’s power supply should be low impedance and as short as possible. High-frequency noise on the VDD power supply may affect the ADC’s high-speed comparator. Therefore, it is necessary to bypass the VDD supply pin to the star ground with 0.1μF and 1μF capacitors in parallel and placed close to the ADC’s Pin 1. Component lead lengths should be very short for optimal supply-noise rejection. If the power supply is very noisy, an optional 10-Ω resistor can be used in conjunction with the bypass capacitors to form a low-pass filter as shown in Figure 14. Page 13 TSM1285 PACKAGE OUTLINE DRAWING 8-Pin SOIC Package Outline Drawing (N.B., Drawings are not to scale) Patent Notice Silicon Labs invests in research and development to help our customers differentiate in the market with innovative low-power, small size, analog-intensive mixed-signal solutions. Silicon Labs' extensive patent portfolio is a testament to our unique approach and world-class engineering team. The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice. 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