ADS1252 ADS 125 2 www.ti.com 24-Bit, 40kHz ANALOG-TO-DIGITAL CONVERTER FEATURES DESCRIPTION ● 24 BITS—NO MISSING CODES ● 19 BITS EFFECTIVE RESOLUTION UP TO 40kHz DATA RATE ● LOW NOISE: 2.5ppm ● DIFFERENTIAL INPUTS ● INL: 0.0015% (max) ● EXTERNAL REFERENCE (0.5V to 5V) ● POWER-DOWN MODE ● SYNC MODE The ADS1252 is a precision, wide dynamic range, delta-sigma, 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 to guarantee 24 bits of no missing codes performance. An effective resolution of 19 bits (2.5ppm of rms noise) is achieved for conversion rates up to 40kHz. The ADS1252 is designed for high-resolution measurement applications in cardiac diagnostics, smart transmitters, industrial process control, weight scales, chromatography, and portable instrumentation. The converter includes a flexible, two-wire synchronous serial interface for low-cost isolation. The ADS1252 is a single-channel converter and is offered in an SO-8 package. APPLICATIONS ● ● ● ● ● ● CARDIAC DIAGNOSTICS DIRECT THERMOCOUPLE INTERFACES BLOOD ANALYSIS INFRARED PYROMETERS LIQUID/GAS CHROMATOGRAPHY PRECISION PROCESS CONTROL ADS1252 VREF CLK +VIN –VIN 4th-Order ∆Σ Modulator Digital Filter Serial Interface SCLK DOUT/DRDY +VDD GND Control Copyright © 2000, Texas Instruments Incorporated SBAS127A Printed in U.S.A. November, 2000 SPECIFICATIONS All specifications at TMIN to TMAX, VDD = +5V, CLK = 16MHz, and VREF = 4.096, unless otherwise specified. ADS1252U PARAMETER ANALOG INPUT Input Voltage Range Input Impedance Input Capacitance Input Leakage CONDITIONS PERFORMANCE OVER TEMPERATURE Offset Drift Gain Drift POWER-SUPPLY REQUIREMENTS Operation Quiescent Current Operating Power Power-Down Current TEMPERATURE RANGE Operating Storage 79 300 19 20 5 At +25°C At TMIN to TMAX MAX UNITS ±VREF V MΩ kΩ kΩ pF pA nA 50 1 41.7 –3dB 9 16 16 ±0.0003 97 2.5 1kHz Input; 0.1dB below FS at DC 24 24 90 VREF = 4.096V ±0.1V 60 CLK CLK CLK CLK = = = < ±0.0015 3.8 100 0.4 ±100 1:1 80 1 ±200 0.5 kHz kHz MHz MHz % of FSR dB ppm of FSR, rms Bits Bits dB % of FSR ppm of FSR dB 0.07 7.5 5.2 3.9 3.4 16MHz 14MHz 12MHz 10MHz VOLTAGE REFERENCE VREF Load Current DIGITAL INPUT/OUTPUT Logic Family Logic Level: VIH VIL VOH VOL Input (SCLK, CLK) Hysteresis Data Format TYP 0 CLK = 3,840Hz CLK = 1MHz CLK = 16MHz DYNAMIC CHARACTERISTICS Data Rate Bandwidth Serial Clock (SCLK) System Clock Input (CLK) ACCURACY Integral Non-Linearity(1) THD Noise Resolution No Missing Codes Common-Mode Rejection(2) Gain Error Offset Error Gain Sensitivity to VREF Power-Supply Rejection Ratio MIN ppm/°C ppm/°C ppm/°C ppm/°C ppm/°C 4.096 220 VDD V µA +VDD + 0.3 +0.8 V V V V V CMOS IOH = –500µA IOL = 500µA +4.0 –0.3 +4.5 0.4 0.6 Offset Binary Two’s Complement +4.75 VDD = +5VDC +5 8 40 1 –40 –60 +5.25 10 50 10 VDC mA mW µA +85 +100 °C °C NOTES: (1) Applies to full-differential signals. (2) The common-mode rejection test is performed with a 100mV differential input. 2 ADS1252 SBAS127A PIN CONFIGURATION PIN DESCRIPTIONS Top View +VIN –VIN SO-8 8 1 2 PIN NAME 1 +VIN Analog Input: Positive Input of the Differential Analog Input. 2 –VIN Analog Input: Negative Input of the Differential Analog Input. 3 +VDD Input: Power Supply Voltage, +5V. 4 CLK Digital Input: Device System Clock. The system clock is in the form of a CMOScompatible clock. This is a Schmitt-Trigger input. 5 DOUT/DRDY Digital Output: Serial Data Output/Data Ready. A logic LOW on this output indicates that a new output word is available from the ADS1252 data output register. The serial data is clocked out of the serial data output shift register using SCLK. 6 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. 7 GND Input: Ground. 8 VREF Analog Input: Reference Voltage Input. VREF 7 GND ADS1252U +VDD 3 6 SCLK CLK 4 5 DOUT/DRDY PIN DESCRIPTION ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS Analog Input: Current ............................................... ±100mA, Momentary ±10mA, Continuous 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) .............................................. +300°C Power Dissipation (any package) ................................................. 500mW This integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. BurrBrown Corporation recommends that all integrated circuits be handled and stored using appropriate ESD protection methods. PACKAGE/ORDERING INFORMATION PRODUCT PACKAGE PACKAGE DRAWING NUMBER ADS1252U SO-8 182 –40°C to +85°C ADS1252U " " " " " SPECIFIED TEMPERATURE RANGE PACKAGE MARKING ORDERING NUMBER(1) TRANSPORT MEDIA ADS1252U ADS1252U/2K5 Rails Tape and Reel NOTE: (1) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “ADS1252U/2K5” will get a single 2500-piece Tape and Reel. ADS1252 SBAS127A 3 TYPICAL PERFORMANCE CURVES At TA = +25°C, VDD = +5V, CLK = 16MHz, and VREF = 4.096, unless otherwise specified. RMS NOISE vs DATA OUTPUT RATE EFFECTIVE RESOLUTION vs DATA OUTPUT RATE 3.0 19.5 Effective Resolution (Bits) RMS Noise (ppm of FS) 2.5 2.0 1.5 1.0 0.5 19.25 19 18.75 0 18.5 100 1k 10k 100 100k 1k 10k 100k Data Output Rate (Hz) Data Output Rate (Hz) RMS NOISE vs TEMPERATURE EFFECTIVE RESOLUTION vs TEMPERATURE 3.0 19.0 18.9 Effective Resolution (Bits) RMS Noise (ppm of FS) 2.5 2.0 1.5 1.0 0.5 18.8 18.7 18.6 18.5 18.4 18.3 18.2 18.1 0 18.0 –40 –20 0 20 40 60 –40 80 100 –20 40 60 80 100 25 2.5 20 RMS Noise (µV) RMS Noise (ppm of FS) 20 RMS NOISE µV vs VREF RMS NOISE vs INPUT VOLTAGE 3.0 2.0 1.5 1.0 15 10 5 0.5 0 0 –40 –20 0 20 40 Differential Input Voltage (V) 4 0 Temperature (°C) Temperature (°C) 60 80 0 1 2 3 4 5 6 VREF (V) ADS1252 SBAS127A TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = +5V, CLK = 16MHz, and VREF = 4.096, unless otherwise specified. EFFECTIVE RESOLUTION vs VREF RMS NOISE vs VREF 19 10 8 Effective Resolution (Bits) RMS Noise (ppm of FS) 9 7 6 5 4 3 2 18.5 18 17.5 17 1 0 16.5 0 1 2 3 4 5 6 0 1 2 3 VREF (V) 4 5 6 VREF (V) INTEGRAL NON-LINEARITY vs TEMPERATURE INTEGRAL NON-LINEARITY vs DATA OUTPUT RATE 4 4 3.5 3 INL (ppm of FS) INL (ppm of FS) 3 2.5 2 1.5 2 1 1 0.5 0 –40 0 –20 0 20 40 60 80 100 100 1k Temperature (°C) 10k GAIN DRIFT vs TEMPERATURE OFFSET DRIFT vs TEMPERATURE 800 10 8 CLK = 16MHz 600 6 4 400 Drift (ppm) Offset Drift (ppm of FS) 100k Data Output Rate (Hz) 2 0 –2 CLK = 14.3MHz 200 CLK < 10MHz 0 –4 CLK < 10MHz –6 –200 –8 CLK = 16MHz –10 –40 –20 0 20 40 Temperature (°C) ADS1252 SBAS127A 60 80 100 –400 –40 –20 0 20 40 60 80 100 Temperature (°C) 5 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = +5V, CLK = 16MHz, and VREF = 4.096, unless otherwise specified. POWER SUPPLY REJECTION RATIO vs CLK FREQUENCY COMMON-MODE REJECTION RATIO vs CLK FREQUENCY 100 110 95 105 85 CMRR (dB) PSRR (dB) 90 80 75 100 95 90 70 85 65 60 80 0 5 10 15 20 0 5 10 CLK Frequency (MHz) 20 POWER DISSIPATION vs CLOCK FREQUENCY CURRENT vs TEMPERATURE 9.5 45 9.0 40 8.5 35 Power Dissipation (mW) Supply Current (mA) 15 CLK Frequency (MHz) 8.0 7.5 7.0 6.5 6.0 30 25 20 15 10 5 5.5 0 5.0 –40 –20 0 20 40 60 80 0 100 5 10 15 20 CLK Frequency (MHz) Temperature (°C) TYPICAL FFT ANALYSIS OF THE 1kHz fS INPUT SIGNAL 0 Dynamic Range (dB) –20 –40 –60 –80 –100 –120 –140 –160 –180 0 2000 4000 6000 8000 10000 12000 14000 Imput Signal Frequency (Hz) 6 ADS1252 SBAS127A THEORY OF OPERATION The ADS1252 is a precision, high-dynamic range, 24-bit, delta-sigma, A/D converter capable of achieving very high-resolution 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, the main functional blocks of the ADS1252 are the fourth-order delta-sigma modulator, a digital filter, control logic, and a serial interface. Each of these functional blocks is described below. ANALOG INPUT The ADS1252 contains a fully differential analog input. In order to provide low system noise, common-mode rejection of 100dB 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. Figure 1 shows the basic input structure of the ADS1252. The impedance is directly related to the sampling frequency of the input capacitor which is set by the CLK rate. Higher CLK rates result in lower impedance, and lower CLK rates result in higher impedance. BIPOLAR INPUT The differential inputs of the ADS1252 were designed to accept differential inputs. However, each analog input voltage must stay between –0.3V and VDD. 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 0 to 2 • VREF. By using a single op amp circuit featuring a single amplifier and four external resistors, the ADS1252 can be configured to accept bipolar inputs referenced to ground. The conventional ±2.5V, ±5V, and ±10V input ranges can be interfaced to the ADS1252 using the resistor values shown in Figure 2. R1 RSW (300Ω typical) Internal Circuitry AIN Modulator Frequency = fMOD 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 ADS1252, 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. 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. Third, to prevent aliasing of the input signal, the bandwidth of the analog-input signal must be band limited; the bandwidth is a function of the system clock frequency. With a system clock frequency of 16MHz, the data-output rate is 41.667kHz with a –3dB frequency of 9kHz. The –3dB frequency scales with the system clock frequency. To guarantee the best linearity of the ADS1252, a fully differential signal is recommended. CINT (20pF typical) 10kΩ +IN OPA2350 20kΩ Bipolar Input ADS1252 –IN VCM VREF R2 FIGURE 1. Analog-Input Structure. OPA2350 The input impedance of the analog input changes with ADS1252 system clock frequency (CLK). The relationship is: AIN Impedance (Ω) = (16MHz/CLK) • 19,000 BIPOLAR INPUT R1 R2 ±10V ±5V ±2.5V 2.5kΩ 5kΩ 10kΩ 5kΩ 10kΩ 20kΩ REF 2.5V FIGURE 2. Level Shift Circuit for Bipolar Input Ranges. ADS1252 SBAS127A 7 DELTA-SIGMA MODULATOR REFERENCE INPUT The ADS1252 operates from a nominal system clock frequency of 16MHz. 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 16MHz, the modulator frequency is 2.667MHz. 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 2.667MHz, the data rate is 41.667kHz. Using a slower system clock frequency will result in a lower data output rate, as shown in Table I. Reference input takes an average current of 220µA with a 16MHz system clock. This current will be proportional to the system clock. A buffered reference is recommended for ADS1252. The recommended reference circuit is shown in Figure 3. Reference voltages higher than 4.096V will increase the full-scale 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 the Typical Performance Curve “RMS Noise vs VREF”). CLK (MHz) DIGITAL FILTER The digital filter of the ADS1252, 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 CLK frequency. This allows the data output rate to be changed over a very wide range (five orders of magnitude) by changing the system CLK frequency. However, it is important to note that the –3dB point of the filter is 0.216 times the data output rate, so the data output rate should allow for sufficient margin to prevent attenuation of the signal of interest. Since the conversion result is essentially an average, the data-output rate determines the location of the resulting notches in the digital filter (see Figure 4). 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. 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 CLK DATA OUTPUT RATE (Hz) 16.000(1) 15.360(1) 15.000(1) 14.745600(1) 14.318180(1) 12.288000(1) 12.000000(1) 11.059220(1) 10.000000(1) 9.600000 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 NOTE: (1) Standard Clock Oscillator. 41,667 40,000 30,063 38,400 37,287 32,000 31,250 28,800 26,042 25,000 19,200 16,000 15,625 12,800 9,600 8,000 6,400 4,800 2,400 1,200 1,000 500 100 60 50 30 25 20 16.67 15 12.50 10 TABLE I. CLK Rate versus Data Output Rate. +5V +5V 0.10µF 7 4.99kΩ 2 To VREF Pin 8 of the ADS1252 6 10kΩ 3 1 OPA350 + + LM404-4.1 10µF 0.10µF 10µF 0.1µF 4 FIGURE 3. Recommended External Voltage Reference Circuit for Best Low-Noise Operation with the ADS1252. 8 ADS1252 SBAS127A frequency should be 19.200kHz, this will set the data-output rate to 50Hz (see Table I and Figure 5). For 60Hz rejection, the system CLK frequency should be 20.040kHz, this will set the data-output rate to 60Hz (see Table I and Figure 6). If both 50Hz and 60Hz rejection is required, then the system CLK should be 3.840kHz; this will set the data-output rate to 10Hz and reject both 50Hz and 60Hz (See Table I and Figure 7). 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 5). However, setting the data-output rate to 10Hz will provide 135dB rejection at 75Hz (see Figure 7). A similar benefit is gained at frequencies near the data-output rate (see Figures 8, 9, 10, and 11). For example, with a 50Hz data-output rate, rejection at 55Hz may only be 105dB (see Figure 8). However, with a 10Hz data-output rate, rejection at 55Hz will be 122dB (see Figure 9). 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. The digital filter is described by the following transfer function: π • f • 64 sin fMOD H( f ) = π•f 64 • sin fMOD or 1 – z –64 H( z ) = 64 • 1 – z –1 ( ADS1252 SBAS127A ) 5 5 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. Since the modulator clock is derived from the system clock (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 analog input change occurs asynchronously to the DOUT/DRDY pulse, six conversions are required to ensure full settling. CONTROL LOGIC The control logic is used for communications and control of the ADS1252. 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 ADS1252 powers up, the DOUT/DRDY line will pulse LOW on the first conversion. This data will not be valid. The sixth pulse of DOUT/DRDY will be valid data 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 12 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 would be HIGH prior to the internal transfer of new data to the DOR. The result of the A/D 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 4. 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 Frequency (Hz) FIGURE 6. Digital Filter Response (60Hz). FIGURE 7. Digital Filter Response (10Hz Multiples). DIGITAL FILTER RESPONSE DIGITAL FILTER RESPONSE 0 0 –20 –20 –40 –40 –60 –60 –80 Gain (dB) Gain (dB) 200 FIGURE 5. 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 Frequency (Hz) FIGURE 8. Expanded Digital Filter Response (50Hz with a 50Hz Notch). 10 150 Frequency (Hz) 45 46 47 48 49 50 51 52 53 54 55 Frequency (Hz) FIGURE 9. Expanded Digital Filter Response (50Hz with a 10Hz Notch). ADS1252 SBAS127A 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 65 Frequency (Hz) 55 56 57 58 59 60 61 62 63 64 65 Frequency (Hz) FIGURE 10. Expanded Digital Filter Response (60Hz with a 60Hz Notch). FIGURE 11. Expanded Digital Filter Response (60Hz with a 10Hz Notch). conversion would be written to the DOR from MSB to LSB in the time defined by t1 (see Figures 12 and 13). The DOUT/DRDY line would then pulse LOW for the time defined by t2, and then pulse HIGH for the time defined by t3 to indicate that new data was available to be read. At this point, the function of the DOUT/DRDY pin would change to DOUT mode. Data would be shifted out on the pin after t7. The device communicating with the ADS1252 can provide SCLKs to the ADS1252 after the time defined by t6. The normal mode of reading data from the ADS1252 would be for the device reading the ADS1252 to latch the data on the rising edge of SCLK (since data is shifted out of the ADS1252 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. If SCLKs were not provided to the ADS1252 during the DOUT mode, the MSB of the DOR would be present on the DOUT/DRDY line until the time defined by t4. If an incomplete read of the ADS1252 took place while in DOUT mode (i.e., less than 24 SCLKs were provided), the state of the last bit read would be present on the DOUT/DRDY line until the time defined by t4. If more than 24 SCLKs were provided during DOUT mode, the DOUT/DRDY line would stay LOW until the time defined by t4. 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 ADS1252 after DRDY mode is always the MSB of new data. ADS1252 SBAS127A SYNCHRONIZING MULTIPLE CONVERTERS The normal state of SCLK is LOW, however, by holding SCLK HIGH, multiple ADS1252s can be synchronized. This is accomplished by holding SCLK HIGH for at least four, but less than twenty, consecutive DOUT/DRDY cycles (see Figure 14). After the ADS1252 circuitry detects that SCLK has been held HIGH for four consecutive DOUT/DRDY cycles, the DOUT/DRDY pin will pulse LOW for 3 CLK cycles and then be held HIGH, and the modulator will be held in a reset state. The modulator will be released from reset and synchronization will occur on the falling edge of SCLK. It is important to note that prior to synchronization, the DOUT/DRDY pulse of multiple ADS1252s in the system could have a difference in timing up to one DRDY period. Therefore to ensure synchronization, the SCLK should be held HIGH for at least five DRDY cycles. The first DOUT/DRDY pulse after the falling edge of SCLK will occur at t14. Valid data will not be present until the sixth DOUT/DRDY pulse. 11 POWER-DOWN MODE The normal state of SCLK is LOW, however, by holding SCLK HIGH, the ADS1252 will enter power-down mode. This is accomplished by holding SCLK HIGH for at least twenty consecutive DOUT/DRDY periods (see Figure 15). After the ADS1252 circuitry detects that SCLK has been held HIGH for four consecutive DOUT/DRDY cycles, the DOUT/DRDY pin will pulse LOW for 3 CLK cycles and then be held HIGH, and the modulator will be held in a reset state. If SCLK is held HIGH for an additional sixteen DOUT/DRDY periods, the ADS1252 will enter power-down mode. The part will be released from powerdown mode on the falling edge of SCLK. It is important to note that the DOUT/DRDY pin will be held HIGH after four DOUT/DRDY cycles, but power-down mode will not be entered for an additional sixteen DOUT/DRDY periods. The first DOUT/DRDY pulse after the falling edge of SCLK will occur at t16. Subsequent DOUT/DRDY pulses will occur normally. Valid data will not be present until the sixth DOUT/DRDY pulse. SYMBOL tDRDY DRDY Mode DOUT Mode t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15 t16 t17 SERIAL INTERFACE The ADS1252 includes a simple serial interface which can be connected to microcontrollers and digital signal processors in a variety of ways. Communications with the ADS1252 can commence on the first detection of the DOUT/DRDY pulse after power up, although data will not be valid until the sixth conversion. It is important to note that the data from the ADS1252 is a 24-bit result transmitted MSB-first in Offset Two’s Complement format, as shown in Table III. DIFFERENTIAL VOLTAGE INPUT DIGITAL OUTPUT (HEX) +Fulll Scale Zero –Full Scale 7FFFFFH 000000H 800000H TABLE III. ADS1252 Data Format (Offset Two's Complement). DESCRIPTION MIN 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 (Propogation Delay) Falling Edge of SCLK to Data Valid (Hold Time) Falling Edge of SCLK to Next Data Out Valid (Propogation 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 TYP MAX UNITS 384 • CLK 36 • CLK 348 • CLK 6 • CLK 6 • CLK 6 • CLK 24 • CLK ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns 30 30 30 5 30 30 3 • CLK 1537 • CLK 0.5 • CLK 7679 • CLK 6143.5 • CLK 314.5 • CLK 7681 • CLK 591.5 • CLK 592.5 • CLK 6143.5 • CLK TABLE II. Digital Timing. DOUT Mode DRDY Mode t2 t4 DOUT/DRDY DOUT Mode DATA DRDY Mode t3 DATA DATA t1 FIGURE 12. DOUT/DRDY Partitioning. 12 ADS1252 SBAS127A ADS1252 SBAS127A 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 t11 t17 t13 t16 Power Down Occurs Here t14 Synchronization Begins Here Synchronization Mode Starts Here DOUT Mode LSB t2 t2 t3 t3 DATA DATA tDRDY DOUT Mode tDRDY DOUT Mode t4 t4 The data must be clocked out before the ADS1252 enters DRDY mode to ensure reception of valid data, as described in the DOUT/DRDY section of this data sheet. ISOLATION The serial interface of the ADS1252 provides for simple isolation methods. The CLK signal can be local to the ADS1252, which then only requires two signals (SCLK, and DOUT/DRDY) to be used for isolated data acquisition. LAYOUT POWER SUPPLY The power supply should be well regulated and low noise. For designs requiring very high resolution from the ADS1252, 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. 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 ADS1252 and for all components in the design. All decoupling capacitors, and specifically the 0.1µF ceramic capacitors, should be placed as close as possible to the pin being decoupled. A 1µF to 10µF capacitor, in parallel with a 0.1µF ceramic capacitor, should be used to decouple VDD to GND. 14 SYSTEM CONSIDERATIONS 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 Analog Portion of the ADS1252 Digital Portion of the ADS1252 Digital Processing 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 ADS1252s • 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 ADS1252 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 ADS1252 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 ADS1252 SBAS127A input 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 analog-input differential voltage is positive, while 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 full-scale 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 ADCs, the fullscale range of the ADS1252 is defined as the “input” which produces the positive full-scale digital output minus the “input” which 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−Scale Range 2N 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 ADS1252 in a particular configuration can be expressed in two different units: bits rms (referenced to output) and µVrms (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 which is one standard deviation. The ER in bits can be computed as follows: 2 • VREF 20 • log Vrms noise ER in bits rms = 6.02 The 2 • VREF figure in each calculation represents the full-scale range of the ADS1252. This means that both units are absolute expressions of resolution—the performance in different configurations can be directly compared, regardless of the units. 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 IV. 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 ADS1252 outputting data at 40kHz, a 4096 point average will take 102.4ms. The benefits of averaging will be degraded if the input signal drifts during that 100ms. 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 31.3µV 22.1µV 15.6µV 11.1µV 7.82µV 5.53µV 3.91µV 2.77µV 1.96µV 1.38µV 978nV 692nV 489nV 18 18.5 19 19.5 20 20.5 21 21.5 22 22.5 23 23.5 24 TABLE IV. Averaging. 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