ADS1253 ADS 125 3 SBAS199B - MAY 2001 - REVISED SEPTEMBER 2007 24-Bit, 20kHz, Low-Power ANALOG-TO-DIGITAL CONVERTER FEATURES DESCRIPTION 24 BITS--NO MISSING CODES 19 BITS EFFECTIVE RESOLUTION UP TO 20kHz DATA RATE LOW NOISE: 1.8ppm FOUR DIFFERENTIAL INPUTS INL: 15ppm (max) EXTERNAL REFERENCE (0.5V to 5V) POWER-DOWN MODE SYNC MODE LOW POWER: 8mW at 20kHz 5mW at 10kHz The ADS1253 is a precision, wide dynamic range, deltasigma, Analog-to-Digital (A/D) converter with 24-bit resolution operating from a single +5V supply. The delta-sigma architecture is used for wide dynamic range and 24 bits of no missing code performance. An effective resolution of 19 bits (1.8ppm of rms noise) is achieved for conversion rates up to 20kHz. The ADS1253 is designed for high-resolution measurement applications in cardiac diagnostics, smart transmitters, industrial process control, weigh scales, chromatography, and portable instrumentation. The converter includes a flexible, 2-wire synchronous serial interface for low-cost isolation. The ADS1253 is a 4-channel converter and is offered in an SSOP-16 package. APPLICATIONS CARDIAC DIAGNOSTICS DIRECT THERMOCOUPLE INTERFACES BLOOD ANALYSIS INFRARED PYROMETERS LIQUID/GAS CHROMATOGRAPHY PRECISION PROCESS CONTROL ADS1253 VREF CH1+ CH1- CLK CH2+ CH2- MUX CH3+ 4th-Order Modulator Digital Filter Serial Interface CH3- SCLK DOUT/DRDY +VDD CH4+ GND CH4- Control CHSEL0 CHSEL1 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. Copyright (c) 2001-2007, Texas Instruments Incorporated PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. www.ti.com ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) Analog Input: Current (Momentary) .............................................. 100mA (Continuous) ............................................... 10mA Voltage ................................... GND - 0.3V to VDD + 0.3V VDD to GND ............................................................................ -0.3V to 6V VREF Voltage to GND ............................................... -0.3V to VDD + 0.3V Digital Input Voltage to GND ................................... -0.3V to VDD + 0.3V Digital Output Voltage to GND ................................. -0.3V to VDD + 0.3V Lead Temperature (soldering, 10s) .............................................. +300C Power Dissipation (any package) ................................................. 500mW This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. NOTE: (1) Stresses above those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. Exposure to absolute maximum conditions for extended periods may affect device reliability. PACKAGE/ORDERING INFORMATION(1) PRODUCT PACKAGE-LEAD PACKAGE DESIGNATOR SPECIFIED TEMPERATURE RANGE PACKAGE MARKING SSOP-16 DBQ -40C to +85C ADS1253E ADS1253E Rails, 100 " " " " ADS1253E/2K5 Tape and Reel, 2500 ADS1253 " ORDERING NUMBER TRANSPORT MEDIA, QUANTITY NOTE: (1) For the most current package and ordering information, see the Package Option Addendum at the end of this document, or see the TI web site at www.ti.com. PRODUCT FAMILY PRODUCT ADS1250 ADS1251 ADS1252 ADS1253 ADS1254 # OF INPUTS 1 1 1 4 4 MAXIMUM DATA RATE Differential Differential Differential Differential Differential COMMENTS 25.0kHz 26.8kHz 41.7kHz 20.8kHz 20.8kHz Includes PGA from 1 to 8 Includes Separate Analog and Digital Supplies ELECTRICAL CHARACTERISTICS All specifications at TMIN to TMAX, VDD = +5V, CLK = 8MHz, and VREF = 4.096V, unless otherwise specified. ADS1253E PARAMETER ANALOG INPUT Full-Scale Input Voltage Absolute Input Voltage Input Impedance Input Capacitance Input Leakage DYNAMIC CHARACTERISTICS Data Rate Bandwidth Serial Clock (SCLK) System Clock Input (CLK) ACCURACY Integral Nonlinearity(1) THD Noise Resolution No Missing Codes Common-Mode Rejection Gain Error Offset Error Gain Sensitivity to VREF Power-Supply Rejection Ratio CONDITIONS MIN CHx+ or CHx- to GND CLK = 3840Hz CLK = 1MHz CLK = 8MHz GND - 0.3 TYP MAX VREF VDD + 0.3 430 1.7 210 6 5 At +25C At TMIN to TMAX 50 1 20.8 -3dB 4.24 16 8 0.0002 105 1.8 1kHz Input; 0.1dB below FS 24 24 90 60Hz, AC 70 PERFORMANCE OVER TEMPERATURE Offset Drift Gain Drift 102 0.1 20 1:1 88 0.0015 2.7 1 100 0.5 4.096 32 V V M M k pF pA nA kHz kHz MHz MHz % of FSR dB ppm of FSR, rms Bits Bits dB % of FSR ppm of FSR dB 0.07 0.4 VOLTAGE REFERENCE VREF Load Current UNITS ppm/C ppm/C VDD V A NOTE: (1) Applies to full-differential signals. 2 ADS1253 www.ti.com SBAS199B ELECTRICAL CHARACTERISTICS (Cont.) All specifications at TMIN to TMAX, VDD = +5V, CLK = 8MHz, and VREF = 4.096V, unless otherwise specified. ADS1253E PARAMETER CONDITIONS DIGITAL INPUT/OUTPUT Logic Family Logic Level: VIH VIL VOH VOL Input (SCLK, CLK, CHSEL0, CHSEL1) Hysteresis Data Format MIN TYP MAX UNITS +VDD + 0.3 +0.8 V V V V V CMOS +4.0 -0.3 +4.5 IOH = -500A IOL = 500A 0.4 0.6 Offset Binary Two's Complement POWER-SUPPLY REQUIREMENTS Operation Quiescent Current Operating Power Power-Down Current +4.75 TEMPERATURE RANGE Operating Storage +5 1.5 7.5 0.4 -40 -60 +5.25 2 10 1 VDC mA mW A +85 +100 C C NOTE: (1) Applies to full-differential signals. PIN DESCRIPTIONS PIN CONFIGURATION Top View CH1+ SSOP 16 1 CH4+ CH1- 2 15 CH2+ 3 14 VREF CH2- 4 13 GND PIN NAME PIN DESCRIPTION 1 CH1+ Analog Input: Positive Input of the Differential Analog Input 2 CH1- Analog Input: Negative Input of the Differential Analog Input 3 CH2+ Analog Input: Positive Input of the Differential Analog Input 4 CH2- Analog Input: Negative Input of the Differential Analog Input 5 CH3+ Analog Input: Positive Input of the Differential Analog Input 6 CH3- Analog Input: Negative Input of the Differential Analog Input 7 +VDD Input: Power-Supply Voltage, +5V 8 CLK Digital Input: Device System Clock. The system clock is in the form of a CMOScompatible clock. This is a Schmitt-Trigger input. 9 DOUT/DRDY Digital Output: Serial Data Output/Data Ready. This output indicates that a new output word is available from the ADS1253 data output register. The serial data is clocked out of the serial data output shift register using SCLK. 10 SCLK Digital Input: Serial Clock. The serial clock is in the form of a CMOS-compatible clock. The serial clock operates independently from the system clock, therefore, it is possible to run SCLK at a higher frequency than CLK. The normal state of SCLK is LOW. Holding SCLK HIGH will either initiate a modulator reset for synchronizing multiple converters or enter power-down mode. This is a Schmitt-Trigger input. 11 CHSEL1 Digital Input: Used to select analog input channel. This is a Schmitt-Trigger input. 12 CHSEL0 Digital Input: Used to select analog input channel. This is a Schmitt-Trigger input. 13 GND Input: Ground 14 15 VREF CH4- 16 CH4+ Analog Input: Reference Voltage Input Analog Input: Negative Input of the Differential Analog Input Analog Input: Positive Input of the Differential Analog Input CH4- ADS1253E CH3+ 5 12 CHSEL0 CH3- 6 11 CHSEL1 +VDD 7 10 SCLK CLK 8 9 DOUT/DRDY ADS1253 SBAS199B www.ti.com 3 TYPICAL CHARACTERISTICS At TA = +25C, VDD = +5V, CLK = 8MHz, and VREF = 4.096V, unless otherwise specified. EFFECTIVE RESOLUTION vs DATA OUTPUT RATE RMS NOISE vs DATA OUTPUT RATE 2.0 20.0 Effective Resolution (Bits) RMS Noise (ppm of FS) 19.8 1.8 1.6 1.4 1.2 19.6 19.4 19.2 19.0 18.8 18.6 18.4 18.2 1.0 100 18.0 1k 10k 100k 100 1k Data Output Rate (Hz) 20.0 1.8 19.8 1.6 19.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 19.4 19.2 19.0 18.8 18.6 18.4 18.2 0 -40 18.0 -20 0 20 40 60 80 100 -40 -20 0 Temperature (C) 20 40 60 80 100 Temperature (C) RMS NOISE vs VREF VOLTAGE RMS NOISE vs VREF VOLTAGE 18 14 16 12 RMS Noise (ppm of FS) 14 RMS Noise (V) 100k EFFECTIVE RESOLUTION vs TEMPERATURE 2.0 Effective Resolution (Bits) RMS Noise (ppm of FS) RMS NOISE vs TEMPERATURE 12 10 8 6 4 10 8 6 4 2 2 0 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VREF Voltage (V) VREF Voltage (V) 4 10k Data Output Rate (Hz) ADS1253 www.ti.com SBAS199B TYPICAL CHARACTERISTICS (Cont.) At TA = +25C, VDD = +5V, CLK = 8MHz, and VREF = 4.096V, unless otherwise specified. INTEGRAL NONLINEARITY vs TEMPERATURE 5 1.8 4 INL (ppm of FS) RMS Noise (ppm of FS) RMS NOISE vs INPUT VOLTAGE (VREF = 5.0V) 2.0 1.6 1.4 3 2 1.2 1 1.0 0 -5 -4 -3 -2 -1 0 1 Input Voltage (V) 2 3 4 5 -40 -20 0 20 40 60 80 100 80 100 Temperature (C) INTEGRAL NONLINEARITY vs DATA OUTPUT RATE OFFSET vs TEMPERATURE 20 5 18 DC Offset (ppm of FS) INL (ppm of FS) 4 3 2 16 14 12 10 8 6 4 1 2 0 -40 0 100 1k 10k 100k -20 0 Data Output Rate (Hz) 60 POWER-SUPPLY REJECTION RATIO vs CLK FREQUENCY GAIN ERROR vs TEMPERATURE 570 0 560 -20 550 -40 PSRR (dB) Gain Error (ppm of FS) 20 40 Temperature (C) 540 530 -60 -80 520 -100 510 -120 500 -40 -20 0 20 40 60 80 0 100 ADS1253 SBAS199B 2 4 6 8 Clock Frequency (MHz) Temperature (C) www.ti.com 5 TYPICAL CHARACTERISTICS (Cont.) At TA = +25C, VDD = +5V, CLK = 8MHz, and VREF = 4.096V, unless otherwise specified. CMR AT 60Hz vs CLK FREQUENCY CMR vs COMMON-MODE FREQUENCY -60 -70 -65 -75 -70 -80 CMR (dB) CMR at 60Hz (dB) -75 -80 -85 -90 -95 -85 -90 -95 -100 -100 -105 -110 -105 0 1 2 3 4 5 Clock Frequency (MHz) 6 7 8 10 100 1k 10k Common-Mode Signal Frequency (Hz) POWER DISSIPATION vs CLK FREQUENCY 1.64 9 1.62 8 1.60 7 Power Dissipation (mW) Current (mA) CURRENT vs TEMPERATURE 1.58 1.56 1.54 1.52 1.50 1.48 5 4 3 2 0 -40 -20 0 20 40 60 80 100 0 1 2 3 4 5 6 7 Temperature (C) Clock Frequency (MHz) VREF CURRENT vs CLK FREQUENCY TYPICAL FFT (1kHz input at 0.1dB less than full-scale) 35 0 30 -20 Relative Magnitude (dB) VREF Current (A) 6 1 1.46 25 20 15 10 5 8 -40 -60 -80 -100 -120 -140 0 -160 0 6 100k 1 2 3 4 5 6 Clock Frequency (MHz) 7 8 9 0 1 2 3 4 5 6 7 8 9 10 11 Input Signal Frequency (kHz) ADS1253 www.ti.com SBAS199B THEORY OF OPERATION INPUT MULTIPLEXER The ADS1253 is a precision, high-dynamic range, 24-bit, delta-sigma, A/D converter capable of achieving very highresolution digital results at high data rates. The analog-input signal is sampled at a rate determined by the frequency of the system clock (CLK). The sampled analog input is modulated by the delta-sigma A/D modulator, which is followed by a digital filter. A Sinc5 digital low-pass filter processes the output of the delta-sigma modulator and writes the result into the data-output register. The DOUT/DRDY pin is pulled LOW, indicating that new data is available to be read by the external microcontroller/microprocessor. As shown in the block diagram on the front page, the main functional blocks of the ADS1253 are the 4th-order delta-sigma modulator, a digital filter, control logic, input multiplexer, and a serial interface. Each of these functional blocks is described in the following sections. ANALOG INPUT The ADS1253 contains a fully differential analog input. In order to provide low system noise, common-mode rejection of 98dB, and excellent power-supply rejection, the design topology is based on a fully differential switched-capacitor architecture. The bipolar input voltage range is from -4.096 to +4.096V, when the reference input voltage equals +4.096V. The bipolar range is with respect to -VIN, and not with respect to GND. The input impedance of the analog input changes with the ADS1253 system clock frequency (CLK). The relationship is: AIN Impedance () = (8MHz/CLK) * 210,000 See application note Understanding the ADS1251, ADS1253, and ADS1254 Input Circuitry (SBAA086), available for download from TI's web site www.ti.com. With regard to the analog-input signal, the overall analog performance of the device is affected by three items: first, the input impedance can affect accuracy. If the source impedance of the input signal is significant, or if there is passive filtering prior to the ADS1253, a significant portion of the signal can be lost across this external impedance. The magnitude of the effect is dependent on the desired system performance. The CHS1 and CHS0 pins are used to select the analog input channel, as shown in Table I. The recommended method for changing channels is to change the channel after the conversion from the previous channel has been completed and read. When a channel is changed, internal logic senses the change on the falling edge of CLK and resets the conversion process. The conversion data from the new channel is valid on the first DRDY after the channel change. CHSEL1 CHSEL0 CHANNEL 0 0 1 1 0 1 0 1 CH1 CH2 CH3 CH4 TABLE I. Channel Selection. When multiplexing inputs, it is possible to achieve sample rates close to 4kHz. This is due to the fact that it requires five internal conversion cycles for the data to fully settle, the data also must be read before the channel is changed. The DRDY signal indicates a valid result after the five cycles have occurred. BIPOLAR INPUT Each of the differential inputs of the ADS1253 must stay between AGND - 0.3V and VDD + 0.3V. With a reference voltage at less than half of VDD, one input can be tied to the reference voltage, and the other input can range from 0V to 2 * VREF. By using a three op amp circuit featuring a single amplifier and four external resistors, the ADS1253 can be configured to accept bipolar inputs referenced to ground. The conventional 2.5V, 5V, and 10V input ranges can be interfaced to the ADS1253 using the resistor values shown in Figure 1. R1 10k Second, the current into or out of the analog inputs must be limited. Under no conditions should the current into or out of the analog inputs exceed 10mA. +IN OPA4350 20k Bipolar Input -IN ADS1253 VREF R2 Third, to prevent aliasing of the input signal, the analog-input signal must be band limited. The bandwidth of the A/D converter is a function of the system clock frequency. With a system clock frequency of 8MHz, the data-output rate is 20.8kHz with a -3dB frequency of 4.24kHz. The -3dB frequency scales with the system clock frequency. OPA4350 OPA4350 To ensure the best linearity of the ADS1253, a fully differential signal is recommended, and the capacitance to ground must be equal on both sides. BIPOLAR INPUT 10V 5V 2.5V For more information about the ADS1253's input structure, please refer to application note SBAA086 located at www.ti.com. R1 R2 2.5k 5k 10k 5k 10k 20k REF 2.5V FIGURE 1. Level-Shift Circuit for Bipolar Input Ranges. ADS1253 SBAS199B www.ti.com 7 DELTA-SIGMA MODULATOR REFERENCE INPUT The ADS1253 operates from a nominal system clock frequency of 8MHz. The modulator frequency is fixed in relation to the system clock frequency. The system clock frequency is divided by 6 to derive the modulator frequency. Therefore, with a system clock frequency of 8MHz, the modulator frequency is 1.333MHz. Furthermore, the oversampling ratio of the modulator is fixed in relation to the modulator frequency. The oversampling ratio of the modulator is 64, and with the modulator frequency running at 1.333MHz, the data rate is 20.8kHz. Using a slower system clock frequency will result in a lower data output rate, as shown in Table II. The reference input takes an average current of 32A with a 8MHz system clock. This current will be proportional to the system clock. A buffered reference is recommended for the ADS1253. The recommended reference circuit is shown in Figure 2. CLK (MHz) DATA OUTPUT RATE (Hz) 8(1) 20,833 19,200 16,000 15,625 12,800 9600 8000 6400 4800 2400 1200 1000 500 100 60 50 30 25 20 16.67 15 12.50 10 7.372800(1) 6.144000(1) 6.000000(1) 4.915200(1) 3.686400(1) 3.072000(1) 2.457600(1) 1.843200(1) 0.921600 0.460800 0.384000 0.192000 0.038400 0.023040 0.019200 0.011520 0.009600 0.007680 0.006400 0.005760 0.004800 0.003840 Reference voltages higher than 4.096V will increase the fullscale range, while the absolute internal circuit noise of the converter remains the same. This will decrease the noise in terms of ppm of full-scale, which increases the effective resolution (see typical characteristic curve, RMS Noise vs VREF Voltage). DIGITAL FILTER The digital filter of the ADS1253, referred to as a sinc5 filter, computes the digital result based on the most recent outputs from the delta-sigma modulator. At the most basic level, the digital filter can be thought of as simply averaging the modulator results in a weighted form and presenting this average as the digital output. The digital output rate, or data rate, scales directly with the system clock frequency. This allows the data output rate to be changed over a very wide range (five orders of magnitude) by changing the system clock frequency. However, it is important to note that the -3dB point of the filter is 0.2035 times the data output rate, so the data output rate should allow for sufficient margin to prevent attenuation of the signal of interest. As the conversion result is essentially an average, the data-output rate determines the location of the resulting notches in the digital filter (see Figure 3). Note that the first notch is located at the data-output rate frequency, and subsequent notches are located at integer multiples of the data-output rate to allow for rejection of not only the fundamental frequency, but also harmonic frequencies. In this manner, the data-output rate can be used to set specific notch frequencies in the digital-filter response. NOTE: (1) Standard Clock Oscillator. TABLE II. CLK Rate versus Data Output Rate. For example, if the rejection of power-line frequencies is desired, then the data-output rate can simply be set to the power-line frequency. For 50Hz rejection, the system clock +5V +5V 0.10F 7 0.1F 2 1 REF3040 2 3 OPA350 + + 3 To VREF Pin 14 of the ADS1253 6 10k 0.1F 10F 0.10F 10F 0.1F 4 FIGURE 2. Recommended External Voltage Reference Circuit for Best Low-Noise Operation with the ADS1253. 8 ADS1253 www.ti.com SBAS199B frequency must be 19.200kHz, and this sets the data-output rate to 50Hz (see Table I and Figure 4). For 60Hz rejection, the system CLK frequency must be 23.040kHz, and this sets the data-output rate to 60Hz (see Table I and Figure 5). If both 50Hz and 60Hz rejection is required, then the system CLK must be 3.840kHz; this sets the data-output rate to 10Hz and rejects both 50Hz and 60Hz (see Table I and Figure 6). There is an additional benefit in using a lower data-output rate. It provides better rejection of signals in the frequency band of interest. For example, with a 50Hz data-output rate, a significant signal at 75Hz may alias back into the passband at 25Hz. This is due to the fact that rejection at 75Hz may only be 66dB in the stopband--frequencies higher than the first-notch frequency (see Figure 4). However, setting the data-output rate to 10Hz provides 135dB rejection at 75Hz (see Figure 6). A similar benefit is gained at frequencies near the data-output rate (see Figures 7, 8, 9, and 10). For example, with a 50Hz data-output rate, rejection at 55Hz may only be 105dB (see Figure 7). With a 10Hz data-output rate, however, rejection at 55Hz will be 122dB (see Figure 8). If a slower data-output rate does not meet the system requirements, then the analog front-end can be designed to provide the needed attenuation to prevent aliasing. Additionally, the data-output rate may be increased and additional digital filtering may be done in the processor or controller. Application note A Spreadsheet to Calculate the Frequency Response of the ADS1250-54 (SBAA103) available for download from TI's web site www.ti.com provides a simple tool for calculating the ADS1250's frequency response for any CLK frequency. The digital filter is described by the following transfer function: * f * 64 sin fMOD H(f) = *f 64 * sin fMOD 5 ( ) CONTROL LOGIC The control logic is used for communications and control of the ADS1253. Power-Up Sequence Prior to power-up, all digital and analog-input pins must be LOW. At the time of power-up, these signal inputs can be biased to a voltage other than 0V, however, they should never exceed +VDD. Once the ADS1253 powers up, the DOUT/DRDY line will pulse LOW on the first conversion for which the data is valid from the analog input signal. DOUT/DRDY The DOUT/DRDY output signal alternates between two modes of operation. The first mode of operation is the Data Ready mode (DRDY) to indicate that new data has been loaded into the data-output register and is ready to be read. The second mode of operation is the Data Output (DOUT) mode and is used to serially shift data out of the Data Output Register (DOR). See Figure 11 for the time domain partitioning of the DRDY and DOUT function. See Figure 13 for the basic timing of DOUT/DRDY. During the time defined by t2, t3, and t4, the DOUT/DRDY pin functions in DRDY mode. The state of the DOUT/DRDY pin or 1 - z -64 H(z) = 64 * 1 - z -1 The digital filter requires five conversions to fully settle. The modulator has an oversampling ratio of 64, therefore, it requires 5 * 64, or 320 modulator results (or clocks) to fully settle. As the modulator clock is derived from CLK (modulator clock = CLK / 6), the number of system clocks required for the digital filter to fully settle is 5 * 64 * 6, or 1920 CLKs. This means that any significant step change at the analog input requires five full conversions to settle. However, if the step change at the analog input occurs asynchronously to the DOUT/DRDY pulse, six conversions are required to ensure full settling. 5 ADS1253 SBAS199B www.ti.com 9 DIGITAL FILTER RESPONSE 0 -20 -20 -40 -40 -60 -60 -80 -80 Gain (dB) Gain (dB) NORMALIZED DIGITAL FILTER RESPONSE 0 -100 -120 -100 -120 -140 -140 -160 -160 -180 -180 -200 -200 0 1 2 3 4 5 6 7 8 9 10 0 50 100 Frequency (Hz) FIGURE 3. Normalized Digital Filter Response. 0 -20 -20 -40 -40 -60 -60 -80 Gain (dB) Gain (dB) 250 300 DIGITAL FILTER RESPONSE 0 -100 -120 -80 -100 -120 -140 -140 -160 -160 -180 -180 -200 -200 0 50 100 150 200 250 300 0 10 20 30 Frequency (Hz) 40 50 60 70 80 90 100 63 64 65 Frequency (Hz) FIGURE 5. Digital Filter Response (60Hz). FIGURE 6. Digital Filter Response (10Hz). DIGITAL FILTER RESPONSE DIGITAL FILTER RESPONSE 0 0 -20 -20 -40 -40 -60 -60 -80 Gain (dB) Gain (dB) 200 FIGURE 4. Digital Filter Response (50Hz). DIGITAL FILTER RESPONSE -100 -120 -80 -100 -120 -140 -140 -160 -160 -180 -180 -200 -200 45 46 47 48 49 50 51 52 53 54 55 55 Frequency (Hz) 56 57 58 59 60 61 62 Frequency (Hz) FIGURE 7. Expanded Digital Filter Response (50Hz with a 50Hz data output rate). 10 150 Frequency (Hz) FIGURE 8. Expanded Digital Filter Response (50Hz with a 10Hz data output rate). ADS1253 www.ti.com SBAS199B DIGITAL FILTER RESPONSE 0 -20 -20 -40 -40 -60 -60 -80 Gain (dB) Gain (dB) DIGITAL FILTER RESPONSE 0 -100 -120 -80 -100 -120 -140 -140 -160 -160 -180 -180 -200 -200 55 56 57 58 59 60 61 62 63 64 55 65 56 57 58 59 60 61 62 63 64 65 Frequency (Hz) Frequency (Hz) FIGURE 9. Expanded Digital Filter Response (60Hz with a 60Hz data output rate). FIGURE 10. Expanded Digital Filter Response (60Hz with a 10Hz data output rate). is HIGH prior to the internal transfer of new data to the DOR. The result of the A/D conversion is written to the DOR from the Most Significant Bit (MSB) to the Least Significant Bit (LSB) in the time defined by t1 (see Figures 11 and 13). The DOUT/DRDY line then pulses LOW for the time defined by t2, and then drives the line HIGH for the time defined by t3 to indicate that new data is available to be read. At this point, the function of the DOUT/DRDY pin changes to DOUT mode. Data is shifted out on the pin after t7. If the MSB is high (because of a negative result) the DOUT/DRDY signal will stay HIGH after the end of time t3. The device communicating with the ADS1253 can provide SCLKs to the ADS1253 after the time defined by t6. The normal mode of reading data from the ADS1253 is for the device reading the ADS1253 to latch the data on the rising edge of SCLK (because data is shifted out of the ADS1253 on the falling edge of SCLK). In order to retrieve valid data, the entire DOR must be read before the DOUT/DRDY pin reverts back to DRDY mode. The internal data pointer for shifting data out on DOUT/DRDY is reset on the falling edge of the time defined by t1 and t4. This ensures that the first bit of data shifted out of the ADS1253 after DRDY mode is always the MSB of new data. If SCLKs are not provided to the ADS1253 during the DOUT mode, the MSB of the DOR is present on the DOUT/DRDY line until the beginning of the time defined by t4. If an incomplete read of the ADS1253 takes place while in DOUT mode (that is, fewer than 24 SCLKs were provided), the state of the last bit read is present on the DOUT/DRDY line until the beginning of the time defined by t4. If more than 24 SCLKs are provided during DOUT mode, the DOUT/DRDY line stays LOW until the time defined by t4. SYNCHRONIZING MULTIPLE CONVERTERS The normal state of SCLK is LOW; however, by holding SCLK HIGH, multiple ADS1253s can be synchronized. This is accomplished by holding SCLK HIGH for at least four, but less than 20, consecutive DOUT/DRDY cycles (see Figure 13). After the ADS1253 circuitry detects that SCLK has been held HIGH for four consecutive DOUT/DRDY cycles, the DOUT/DRDY pin pulses LOW for one CLK cycle and then is held HIGH, and the modulator is held in a reset state. The modulator will be released from reset and synchronization occurs on the falling edge of SCLK. With multiple converters, the falling edge transition of SCLK must occur simultaneously on all devices. It is important to note that prior to synchronization, the DOUT/DRDY pulse of multiple ADS1253s in the system could have a difference in timing up to one DRDY period. Therefore, to ensure synchronization, the SCLK must be held HIGH for at least five DRDY cycles. The first DOUT/DRDY pulse after the falling edge of SCLK occurs at t14. The first DOUT/DRDY pulse indicates valid data. ADS1253 SBAS199B www.ti.com 11 POWER-DOWN MODE SERIAL INTERFACE The normal state of SCLK is LOW; however, by holding SCLK HIGH, the ADS1253 will enter power-down mode. This is accomplished by holding SCLK HIGH for at least 20 consecutive DOUT/DRDY periods (see Figure 14). After the ADS1253 circuitry detects that SCLK has been held HIGH for four consecutive DOUT/DRDY cycles, the DOUT/DRDY pin pulses LOW for one CLK cycle and then is held HIGH, and the modulator is held in a reset state. If SCLK is held HIGH for an additional 16 DOUT/DRDY periods, the ADS1253 will enter power-down mode. The part will be released from power-down mode on the falling edge of SCLK. It is important to note that the DOUT/DRDY pin is held HIGH after four DOUT/DRDY cycles, but power-down mode is not entered for an additional 16 DOUT/DRDY periods. The first DOUT/DRDY pulse after the falling edge of SCLK occurs at t16 and indicates valid data. Subsequent DOUT/DRDY pulses will occur normally. The ADS1253 includes a simple serial interface that can be connected to microcontrollers and digital signal processors in a variety of ways. Communications with the ADS1253 can commence on the first detection of the DOUT/DRDY pulse after power up. SYMBOL tOSC tDRDY DRDY Mode DOUT Mode t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15 t16 t17 t18 It is important to note that the data from the ADS1253 is a 24-bit result transmitted MSB-first in Offset Binary Two's Complement format, as shown in Table IV. The data must be clocked out before the ADS1253 enters DRDY mode to ensure reception of valid data, as described in the DOUT/DRDY section of this data sheet. DIFFERENTIAL VOLTAGE INPUT DIGITAL OUTPUT (HEX) +Full-Scale Zero -Full-Scale 7FFFFFH 000000H 800000H TABLE IV. ADS1253 Data Format (Offset Binary Two's Complement). DESCRIPTION MIN CLK Period Conversion Cycle DRDY Mode DOUT Mode DOR Write Time DOUT/DRDY LOW Time DOUT/DRDY HIGH Time (Prior to Data Out) DOUT/DRDY HIGH Time (Prior to Data Ready) Rising Edge of CLK to Falling Edge of DOUT/DRDY End of DRDY Mode to Rising Edge of First SCLK End of DRDY Mode to Data Valid (Propagation Delay) Falling Edge of SCLK to Data Valid (Hold Time) Falling Edge of SCLK to Next Data Out Valid (Propagation Delay) SCLK Setup Time for Synchronization or Power Down DOUT/DRDY Pulse for Synchronization or Power Down Rising Edge of SCLK Until Start of Synchronization Synchronization Time Falling Edge of CLK (After SCLK Goes LOW) Until Start of DRDY Mode Rising Edge of SCLK Until Start of Power Down Falling Edge of CLK (After SCLK Goes LOW) Until Start of DRDY Mode Falling Edge of Last DOUT/DRDY to Start of Power Down DOUT/DRDY High Time After MUX Change 125 TYP MAX UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns 384 * tOSC 36 * tOSC 348 * tOSC 6 * tOSC 6 * tOSC 6 * tOSC 24 * tOSC 30 30 30 5 30 30 3 * tOSC 1537 * CLK 0.5 * CLK 7679 * CLK 6143.5 * CLK 2042.5 * CLK 7681 * CLK 2318.5 * tOSC 6144.5 * tOSC 2043.5 * tosc TABLE III. Digital Timing. DRDY Mode DOUT Mode t2 t4 t3 DATA DOUT/DRDY DRDY Mode DOUT Mode DATA DATA t1 FIGURE 11. DOUT/DRDY Partitioning. t18 DOUT/DRDY DATA DATA CHS0, CHS1 MUX Change FIGURE 12. Multiplexer Operation. 12 ADS1253 www.ti.com SBAS199B ADS1253 SBAS199B www.ti.com 13 t4 t1 t10 t2 t3 tDRDY t10 t2 t3 FIGURE 15. Power-Down Mode. DOUT/DRDY SCLK CLK tDRDY FIGURE 14. Synchronization Mode. DOUT/DRDY SCLK CLK FIGURE 13. DOUT/DRDY Timing. DOUT/DRDY SCLK CLK t2 DOUT Mode DATA DOUT Mode DATA t4 t4 DRDY Mode t5 4 tDRDY 4 tDRDY t12 t3 t15 t11 MSB t7 t6 DATA DATA t8 t9 tDRDY t11 t17 t13 t11 t16 Power-Down Occurs Here t14 t2 t2 Synchronization Begins Here Synchronization Mode Starts Here DOUT Mode LSB t3 t3 DATA DATA tDRDY DOUT Mode tDRDY DOUT Mode t4 t4 SYSTEM CONSIDERATIONS ISOLATION The serial interface of the ADS1253 provides for simple isolation methods. The CLK signal can be local to the ADS1253, which then only requires two signals (SCLK and DOUT/DRDY) to be used for isolated data acquisition. The channel select signals (CHS0, CHS1) also need to be isolated unless a counter is used to auto multiplex the channels. The recommendations for power supplies and grounding will change depending on the requirements and specific design of the overall system. Achieving 24 bits of noise performance is a great deal more difficult than achieving 12 bits of noise performance. In general, a system can be broken up into four different stages: * Analog Processing LAYOUT * Analog Portion of the ADS1253 POWER SUPPLY * Digital Portion of the ADS1253 The power supply must be well regulated and low noise. For designs requiring very high resolution from the ADS1253, power-supply rejection will be a concern. Avoid running digital lines under the device as they may couple noise onto the die. High-frequency noise can capacitively couple into the analog portion of the device and will alias back into the passband of the digital filter, affecting the conversion result. This clock noise will cause an offset error. * Digital Processing GROUNDING The analog and digital sections of the system design should be carefully and cleanly partitioned. Each section should have its own ground plane with no overlap between them. GND should be connected to the analog ground plane, as well as all other analog grounds. Do not join the analog and digital ground planes on the board, but instead connect the two with a moderate signal trace. For multiple converters, connect the two ground planes at one location as central to all of the converters as possible. In some cases, experimentation may be required to find the best point to connect the two planes together. The printed circuit board can be designed to provide different analog/digital ground connections via short jumpers. The initial prototype can be used to establish which connection works best. DECOUPLING Good decoupling practices should be used for the ADS1253 and for all components in the design. All decoupling capacitors, and specifically the 0.1F ceramic capacitors, should be placed as close as possible to the pin being decoupled. A 1F to 10F capacitor, in parallel with a 0.1F ceramic capacitor, should be used to decouple VDD to GND. 14 For the simplest system consisting of minimal analog signal processing (basic filtering and gain), a microcontroller, and one clock source, one can achieve high resolution by powering all components by a common power supply. In addition, all components could share a common ground plane. Thus, there would be no distinctions between analog power and ground, and digital power and ground. The layout should still include a power plane, a ground plane, and careful decoupling. In a more extreme case, the design could include: * Multiple ADS1253s * Extensive Analog Signal Processing * One or More Microcontrollers, Digital Signal Processors, or Microprocessors * Many Different Clock Sources * Interconnections to Various Other Systems High resolution will be very difficult to achieve for this design. The approach would be to break the system into as many different parts as possible. For example, each ADS1253 may have its own analog processing front end. DEFINITION OF TERMS An attempt has been made to be consistent with the terminology used in this data sheet. In that regard, the definition of each term is given as follows: Analog-Input Differential Voltage--for an analog signal that is fully differential, the voltage range can be compared to that of an instrumentation amplifier. For example, if both analog inputs of the ADS1253 are at 2.048V, the differential voltage is 0V. If one analog input is at 0V and the other analog input is at 4.096V, then the differential voltage magnitude is 4.096V. This is the case regardless of which input ADS1253 www.ti.com SBAS199B is at 0V and which is at 4.096V. The digital-output result, however, is quite different. The analog-input differential voltage is given by the following equation: +VIN - (-VIN) A positive digital output is produced whenever the analoginput differential voltage is positive, whereas a negative digital output is produced whenever the differential is negative. For example, a positive full-scale output is produced when the converter is configured with a 4.096V reference, and the analog-input differential is 4.096V. The negative fullscale output is produced when the differential voltage is -4.096V. In each case, the actual input voltages must remain within the -0.3V to +VDD range. Actual Analog-Input Voltage--the voltage at any one analog input relative to GND. Full-Scale Range (FSR)--as with most A/D converters, the full-scale range of the ADS1253 is defined as the input that produces the positive full-scale digital output minus the input that produces the negative full-scale digital output. For example, when the converter is configured with a 4.096V reference, the differential full-scale range is: [4.096V (positive full-scale) - (-4.096V) (negative full-scale)] = 8.192V Least Significant Bit (LSB) Weight--this is the theoretical amount of voltage that the differential voltage at the analog input would have to change in order to observe a change in the output data of one least significant bit. It is computed as follows: LSB Weight = Full-ScaleRange 2 * VREF = N 2N - 1 2 -1 The 2 * VREF figure in each calculation represents the fullscale range of the ADS1253. This means that both units are absolute expressions of resolution--the performance in different configurations can be directly compared, regardless of the units. fMOD--frequency of the modulator and the frequency the input is sampled. fMOD = fDATA--Data output rate. fDATA = fMOD CLK Frequency = 64 384 Noise Reduction--for random noise, the ER can be improved with averaging. The result is the reduction in noise by the factor N, where N is the number of averages, as shown in Table V. This can be used to achieve true 24-bit performance at a lower data rate. To achieve 24 bits of resolution, more than 24 bits must be accumulated. A 36-bit accumulator is required to achieve an ER of 24 bits. The following uses VREF = 4.096V, with the ADS1253 outputting data at 20kHz, a 4096 point average will take 204.8ms. The benefits of averaging will be degraded if the input signal drifts during that 200ms. N (Number of Averages) NOISE REDUCTION FACTOR ER IN Vrms ER IN BITS rms 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 1 1.414 2 2.82 4 5.66 8 11.3 16 22.6 32 45.25 64 14.6V 10.3V 7.3V 5.16V 3.65V 2.58V 1.83V 1.29V 0.91V 0.65V 0.46V 0.32V 0.23V 19.1 19.6 20.1 20.6 21.1 21.6 22.1 22.6 23.1 23.6 24.1 24.6 25.1 where N is the number of bits in the digital output. Conversion Cycle--as used here, a conversion cycle refers to the time period between DOUT/DRDY pulses. Effective Resolution (ER)--of the ADS1253 in a particular configuration can be expressed in two different units: bits rms (referenced to output) and Vrms (referenced to input). Computed directly from the converter's output data, each is a statistical calculation based on a given number of results. Noise occurs randomly; the rms value represents a statistical measure that is one standard deviation. The ER in bits can be computed as follows: CLK Frequency 6 TABLE V. Averaging. 2 * VREF 20 * log Vrms noise ER in bits rms = 6.02 ADS1253 SBAS199B www.ti.com 15 Revision History DATE REVISION PAGE SECTION DESCRIPTION 9/07 B 12 Table II Changed t11 from 1 * CLK to 3 * CLK. 6/06 A 11 DOUT/DRDY Text changes to DOUT/DRDY section. NOTE: Page numbers for previous revisions may differ from page numbers in the current version. 16 ADS1253 www.ti.com SBAS199B PACKAGE OPTION ADDENDUM www.ti.com 16-Aug-2012 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Drawing Pins Package Qty Eco Plan (2) Lead/ Ball Finish MSL Peak Temp (3) ADS1253E ACTIVE SSOP DBQ 16 75 Green (RoHS & no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR ADS1253E/2K5 ACTIVE SSOP DBQ 16 2500 Green (RoHS & no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR ADS1253E/2K5G4 ACTIVE SSOP DBQ 16 2500 Green (RoHS & no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR ADS1253EG4 ACTIVE SSOP DBQ 16 75 Green (RoHS & no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR Samples (Requires Login) (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. TBD: The Pb-Free/Green conversion plan has not been defined. Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above. Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material) (3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. 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Addendum-Page 1 PACKAGE MATERIALS INFORMATION www.ti.com 16-Aug-2012 TAPE AND REEL INFORMATION *All dimensions are nominal Device ADS1253E/2K5 Package Package Pins Type Drawing SSOP DBQ 16 SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) 2500 330.0 12.4 Pack Materials-Page 1 6.4 B0 (mm) K0 (mm) P1 (mm) 5.2 2.1 8.0 W Pin1 (mm) Quadrant 12.0 Q1 PACKAGE MATERIALS INFORMATION www.ti.com 16-Aug-2012 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) ADS1253E/2K5 SSOP DBQ 16 2500 367.0 367.0 35.0 Pack Materials-Page 2 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46C and to discontinue any product or service per JESD48B. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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