ADS1254E

ADS1254 ADS 125 4 SBAS213 – JUNE 2001 24-Bit, 20kHz, Low Power ANALOG-TO-DIGITAL CONVERTER FEATURES q 24 BITS—NO MISSING CODES q 19 BITS EFFECTIVE RESOLUTION UP TO 20kHz DATA RATE q LOW NOISE: 1.8ppm q FOUR DIFFERENTIAL INPUTS q INL: 15ppm (max) q EXTERNAL REFERENCE (0.5V to 5V) q POWER-DOWN MODE q SYNC MODE q LOW POWER: 4mW at 20kHz q SEPARATE DIGITAL INTERFACE SUPPLY 1.8V to 3.6V DESCRIPTION The ADS1254 is a precision, wide dynamic range, deltasigma, Analog-to-Digital (A/D) converter with 24-bit resolution. The delta-sigma architecture is used for wide dynamic range and to ensure 24 bits of no missing codes performance. An effective resolution of 19 bits (1.8ppm of rms noise) is achieved for conversion rates up to 20kHz. The ADS1254 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 ADS1254 is a multi-channel converter and is offered in an SSOP-20 package. APPLICATIONS q q q q q q CARDIAC DIAGNOSTICS DIRECT THERMOCOUPLE INTERFACES BLOOD ANALYSIS INFRARED PYROMETERS LIQUID/GAS CHROMATOGRAPHY PRECISION PROCESS CONTROL ADS1254 CH1+ CH1– CH2+ CH2– Mux CH3+ CH3– CH4+ CH4– Control 4th-Order ∆Σ Modulator Digital Filter Serial Interface SCLK DOUT/DRDY AVDD AGND DVDD DGND VREF CLK 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. 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. Copyright © 2001, Texas Instruments Incorporated www.ti.com ABSOLUTE MAXIMUM RATINGS Analog Input: Current (Momentary) .............................................. ±100mA (Continuous) ............................................... ±10mA Voltage ................................... GND – 0.3V to VDD + 0.3V AVDD to AGND ....................................................................... –0.3V to 6V DVDD to AVDD .......................................................................... –6V to +6V DVDD to DGND ....................................................................... –0.3V to 6V VREF Voltage to AGND ............................................. –0.3V to VDD + 0.3V Digital Input Voltage to DGND ................................. –0.3V to VDD + 0.3V Digital Output Voltage to DGND .............................. –0.3V to VDD + 0.3V Lead Temperature (soldering, 10s) .............................................. +300°C Power Dissipation (any package) ................................................. 500mW ELECTROSTATIC DISCHARGE SENSITIVITY 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. PACKAGE/ORDERING INFORMATION PACKAGE DRAWING NUMBER 349 SPECIFIED TEMPERATURE RANGE –40°C to +85°C PACKAGE MARKING ADS1254E ORDERING NUMBER(1) ADS1254E ADS1254E/2K5 TRANSPORT MEDIA Rails Tape and Reel PRODUCT ADS1254E PACKAGE SSOP-20 " " " " " 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 “ADS1254E/2K5” will get a single 2500-piece Tape and Reel. ELECTRICAL CHARACTERISTICS All specifications at TMIN to TMAX, AVDD = +5V, DVDD = +1.8V. CLK = 8MHz, and VREF = 4.096, unless otherwise specified. ADS1254E PARAMETER ANALOG INPUT Input Voltage Range Input Impedance CONDITIONS MIN AGND CLK = 3,840Hz CLK = 1MHz CLK = 8MHz At +25°C At TMIN to TMAX 260 1 125 6 5 TYP MAX ±VREF UNITS V MΩ MΩ kΩ pF pA nA kHz kHz MHz MHz % of FSR dB ppm of FSR, rms Bits Bits dB % of FSR ppm of FSR dB ppm/°C ppm/°C VDD V µA Input Capacitance Input Leakage 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 Gain Error Offset Error Gain Sensitivity to VREF Power-Supply Rejection Ratio PERFORMANCE OVER TEMPERATURE Offset Drift Gain Drift VOLTAGE REFERENCE VREF Load Current NOTE: (1) Applies to full-differential signals. 50 1 20.8 –3dB 4.24 8 8 ±0.0002 105 1.8 24 24 102 0.1 ±30 1:1 88 0.07 0.4 0.5 4.096 32 ±0.0015 2.7 1kHz Input; 0.1dB below FS 60Hz, AC 90 1 ±100 70 2 ADS1254 SBAS213 ELECTRICAL CHARACTERISTICS (Cont.) All specifications at TMIN to TMAX, AVDD = +5V, DVDD = +1.8V. CLK = 8MHz, and VREF = 4.096, unless otherwise specified. ADS1254E PARAMETER DIGITAL INPUT/OUTPUT Logic Family Logic Level: VIH VIL VOH VOL Input (SCLK, CLK, CHSEL0, CHSEL1) Hysteresis Data Format POWER-SUPPLY REQUIREMENTS Power Supply Voltage Quiescent Current Operating Power Power-Down Current TEMPERATURE RANGE Operating Storage –40 –60 CONDITIONS MIN TYP MAX UNITS CMOS 0.65 • DVDD –0.3 DVDD –0.4 DVDD + 0.3 0.35 • DVDD 0.4 0.6 Offset Binary Two’s Complement DVDD AVDD AVDD = +5V DVDD = +1.8V 1.8 4.75 3.6 5.25 1.15 0.4 6.5 1 +85 +100 V V V V V IOH = –500µA IOL = 500µA 5 0.8 0.2 4.3 0.4 VDC VDC mA mA mW µA °C °C PIN CONFIGURATION Top View SSOP-20 PIN DESCRIPTIONS PIN 1 2 3 4 5 NAME CH1+ CH1– CH2+ CH2– CH3+ CH3– AVDD CLK PIN DESCRIPTION Analog Input: Positive Input of the Differential Analog Input Analog Input: Negative Input of the Differential Analog Input Analog Input: Positive Input of the Differential Analog Input Analog Input: Negative Input of the Differential Analog Input Analog Input: Positive Input of the Differential Analog Input Analog Input: Negative Input of the Differential Analog Input Input: Analog Power Supply Voltage, +5V Digital Input: Device System Clock. The system clock is in the form of a CMOScompatible clock. This is a Schmitt-Trigger input Input: Digital Power Supply Voltage No Connection No Connection Input: Digital Ground Digital Output: Serial Data Output/Data Ready. This output indicates that a new output word is available from the ADS1254 data output register. The serial data is clocked out of the serial data output shift register using 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. Digital Input: Used to select analog input channel. This is a Schmitt-Trigger Input Digital Input: Used to select analog input channel. This is a Schmitt-Trigger Input Input: Analog Ground Analog Input: Reference Voltage Input Analog Input: Negative Input of the Differential Analog Input Analog Input: Positive Input of the Differential Analog Input CH1+ CH1– CH2+ CH2– CH3+ CH3– AVDD CLK 1 2 3 4 5 6 7 8 20 19 CH4+ CH4– 6 7 8 18 VREF 17 AGND 16 15 CHSEL0 CHSEL1 ADS1254E 9 10 11 12 13 DVDD NC NC DGND DOUT/DRDY 14 SCLK 14 13 DOUT/DRDY 12 DGND 11 NC SCLK DVDD 9 NC 10 15 16 17 18 19 20 CHSEL1 CHSEL0 AGND VREF CH4– CH4+ ADS1254 SBAS213 3 TYPICAL CHARACTERISTICS At TA = +25°C, AVDD = +5V, DVDD = +1.8V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified. RMS NOISE vs DATA OUTPUT RATE 2.0 1.9 20.0 19.8 EFFECTIVE RESOLUTION vs DATA OUTPUT RATE RMS Noise (ppm of FS) 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 100 1k 10k 100k Data Ouput Rate (Hz) Effective Resolution (Bits) 1.8 19.6 19.4 19.2 19.0 18.8 18.6 18.4 18.2 18.0 100 1k 10k 100k Data Ouput Rate (Hz) RMS NOISE vs TEMPERATURE 2.0 1.8 20 19.8 EFFECTIVE RESOLUTION vs TEMPERATURE 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 –40 –20 0 20 40 60 80 100 Temperature (°C) Effective Resolution (Bits) RMS Noise (ppm of FS) 1.6 19.6 19.4 19.2 19.0 18.8 18.6 18.4 18.2 18.0 –40 –20 0 20 40 60 80 100 Temperature (°C) RMS NOISE vs VREF Voltage 10 9 RMS Noise (ppm of FS) RMS NOISE vs VREF Voltage 12 10 8 6 4 2 0 8 RMS Noise (µV) 7 6 5 4 3 2 1 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VREF Voltage (V) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5 VREF Voltage (V) 4 ADS1254 SBAS213 TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, AVDD = +5V, DVDD = +1.8V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified. RMS NOISE vs INPUT VOLTAGE 2 INTEGRAL NONLINEARITY vs TEMPERATURE 2.5 RMS Noise (ppm of FS) 1.5 2.0 INL (ppm of FS) –5 –4 –3 –2 –1 0 1 2 3 4 5 1.5 1 1.0 0.5 0.5 0 Input Voltage (V) 0 –40 –20 0 20 40 60 80 100 Temperature (°C) INTEGRAL NON-LINEARITY vs DATA OUTPUT RATE 6 5 DC Offset (ppm of FS) OFFSET vs TEMPERATURE 20 18 16 14 12 10 8 6 4 2 0 –40 –20 0 20 40 60 80 100 INL (ppm of FS) 4 3 2 1 0 100 1k 10k 100k Data Output Rate (Hz) Temperature (°C) GAIN ERROR vs TEMPERATURE 600 580 Gain Error (ppm of FS) PSR vs CLK FREQUENCY –0 –10 –20 –30 560 540 520 500 480 –60 –40 –20 0 20 40 60 80 100 Temperature (°C) PSR (dB) –40 –50 –60 –70 –80 –90 –100 1 2 3 4 5 6 7 8 Clock Frequency (MHz) ADS1254 SBAS213 5 TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, AVDD = +5V, DVDD = +1.8V, CLK = 8MHz, and VREF = 4.096, unless otherwise specified. CMR AT 60Hz vs CLK FREQUENCY –50 –60 CMR vs COMMON-MODE FREQUENCY –70 –75 CMR (dB) CMR at 60Hz (dB) –80 –85 –90 –95 –70 –80 –90 –100 –110 0 1 2 3 4 5 6 7 8 Clock Frequency (MHz) –100 –105 10 100 1k 10k 100k Common-Mode Signal Frequency (Hz) CURRENT vs TEMPERATURE 0.9 0.8 Power Dissipation (mW) POWER DISSIPATION vs CLK FREQUENCY 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 Analog (5V) Digital (3.3V) Digital (1.8V) 0.7 Current (mA) 0.6 0.5 0.4 0.3 0.2 0.1 0 –40 –20 0 20 40 60 80 100 AVDD (5V) DVDD (1.8V) 0 1 2 3 4 5 6 7 8 Temperature (°C) Clock Frequency (MHz) VREF CURRENT vs CLK FREQUENCY 35 30 Relative Magnitude (dB) TYPICAL FFT (1kHz input at 0.1dB less than full-scale) 0 –20 –40 –60 –80 –100 –120 –140 –160 VREF Current (µA) 25 20 15 10 5 0 0 1 2 3 4 5 6 7 8 Clock Frequency (MHz) 0 1 2 3 4 5 6 7 8 9 10 11 Input Signal Frequency (kHz) 6 ADS1254 SBAS213 THEORY OF OPERATION The ADS1254 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 ADS1254 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 ADS1254 contains a fully differential analog input. In order to provide low system noise, common-mode rejection of 102dB, and excellent power-supply rejection, the design topology is based on a fully differential switched-capacitor architecture. The bipolar input voltage range is from –4.096V 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 ADS1254. The impedance is directly related to the sampling frequency of the input capacitor that is set by the CLK rate. Higher CLK rates result in lower impedance, and lower CLK rates result in higher impedance. RSW (1300Ω typical) AIN 20.8kHz with a –3dB frequency of 4.24kHz. The –3dB frequency scales with the system clock frequency. To ensure the best linearity of the ADS1254, a fully differential signal is recommended. INPUT MULTIPLEXER The CHSEL1 and CHSEL0 pins are used to select the analog input channel, as shown in Table I. The recommended method for changing channels is to change them 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. 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 CHSEL1 0 0 1 1 CHSEL0 0 1 0 1 CHANNEL CH1 CH2 CH3 CH4 TABLE I. Channel Selection. Each of the differential inputs of the ADS1254 must stay between AGND and AVDD. With a reference voltage at less than half of AVDD, one input can be tied to the reference voltage, and the other input can range from AGND to 2 • VREF. By using a three op-amp circuit featuring a single amplifier and four external resistors, the ADS1254 can be configured to accept bipolar inputs referenced to ground. The conventional ±2.5V, ±5V, and ±10V input ranges can be interfaced to the ADS1254 using the resistor values shown in Figure 2. R1 10kΩ 20kΩ Bipolar Input R2 Internal Circuitry CINT (6pF typical) VCM Modulator Frequency = fMOD FIGURE 1. Analog-Input Structure. The input impedance of the analog input changes with the ADS1254 system clock frequency (CLK). The relationship is: AIN Impedance (Ω) = (8MHz/CLK) • 125,000 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 ADS1254, 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 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 OPA4350 +IN –IN ADS1254 VREF OPA4350 OPA4350 BIPOLAR INPUT ±10V ±5V ±2.5V R1 2.5kΩ 5kΩ 10kΩ R2 5kΩ 10kΩ 20kΩ REF 2.5V FIGURE 2. Level Shift Circuit for Bipolar Input Ranges. 7 ADS1254 SBAS213 DELTA-SIGMA MODULATOR The ADS1254 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. CLK (MHz) 8(1) 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. DATA OUTPUT RATE (Hz) 20,833 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 REFERENCE INPUT Reference input takes an average current of 32µA with a 8MHz system clock. This current will be proportional to the system clock. A buffered reference is recommended for the ADS1254. 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 Characteristic “RMS Noise vs VREF Voltage”). DIGITAL FILTER The digital filter of the ADS1254, 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 0.2035 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 TABLE II. CLK Rate versus Data Output Rate. +5V +5V 0.10µF 7 4.99kΩ 2 6 10kΩ OPA350 + + LM404-4.1 10µF 0.10µF 4 10µF 0.1µF To VREF Pin 18 of the ADS1254 1 3 FIGURE 3. Recommended External Voltage Reference Circuit for Best Low-Noise Operation with the ADS1254. 8 ADS1254 SBAS213 NORMALIZED DIGITAL FILTER RESPONSE 0 –20 –40 –60 Gain (dB) DIGITAL FILTER RESPONSE 0 –20 –40 –60 –80 –100 –120 –140 –160 –180 –200 0 1 2 3 4 5 6 7 8 9 10 Frequency (Hz) Gain (dB) –80 –100 –120 –140 –160 –180 –200 0 50 100 150 Frequency (Hz) 200 250 300 FIGURE 4. Normalized Digital Filter Response. FIGURE 5. Digital Filter Response (50Hz). DIGITAL FILTER RESPONSE 0 –20 –40 –60 0 –20 –40 –60 Gain (dB) –80 –100 –120 –140 –160 –180 –200 DIGITAL FILTER RESPONSE Gain (dB) –80 –100 –120 –140 –160 –180 –200 0 50 100 150 Frequency (Hz) 200 250 300 0 10 20 30 40 50 60 70 80 90 100 Frequency (Hz) FIGURE 6. Digital Filter Response (60Hz). FIGURE 7. Digital Filter Response (10Hz). DIGITAL FILTER RESPONSE 0 –20 –40 –60 0 –20 –40 –60 Gain (dB) DIGITAL FILTER RESPONSE Gain (dB) –80 –100 –120 –140 –160 –180 –200 45 46 47 48 49 50 51 52 53 54 55 Frequency (Hz) –80 –100 –120 –140 –160 –180 –200 45 46 47 48 49 50 51 52 53 54 55 Frequency (Hz) FIGURE 8. Expanded Digital Filter Response (50Hz with a 50Hz Data Output Rate). FIGURE 9. Expanded Digital Filter Response (50Hz with a 10Hz Data Output Rate). ADS1254 SBAS213 9 DIGITAL FILTER RESPONSE 0 –20 –40 –60 Gain (dB) 0 –20 –40 –60 DIGITAL FILTER RESPONSE Gain (dB) –80 –100 –120 –140 –160 –180 –200 55 56 57 58 59 60 61 62 63 64 65 Frequency (Hz) –80 –100 –120 –140 –160 –180 –200 55 56 57 58 59 60 61 62 63 64 65 Frequency (Hz) FIGURE 10. Expanded Digital Filter Response (60Hz with a 60Hz Data Output Rate). 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 23.040kHz, 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 firstnotch frequency (see Figure 5). However, setting the dataoutput 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. FIGURE 11. Expanded Digital Filter Response (60Hz with a 10Hz Data Output Rate). 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  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 step change at the analog input occurs asynchronously to the DOUT/DRDY pulse, six conversions are required to ensure full settling. 10 ADS1254 SBAS213 CONTROL LOGIC The control logic is used for communications and control of the ADS1254. Power-Up Sequence Prior to power-up, all digital and analog-input pins must be LOW. During power-up, these signal inputs should never exceed +AVDD or +DVDD. Once the ADS1254 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). The time domain partitioning of the DRDY and DOUT function as shown in Figure 12. 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 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 ADS1254 can provide SCLKs to the ADS1254 after the time defined by t6. The normal mode of reading data from the ADS1254 would be for the device reading the ADS1254 to latch the data on the rising edge of SCLK (since data is shifted out of the ADS1254 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 ADS1254 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 ADS1254 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 ADS1254 after DRDY mode is always the MSB of new data. SYNCHRONIZING MULTIPLE CONVERTERS The normal state of SCLK is LOW, however, by holding SCLK HIGH, multiple ADS1254s 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 ADS1254 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. 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 ADS1254s 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. The first DOUT/DRDY pulse indicates valid data. POWER-DOWN MODE The normal state of SCLK is LOW, however, by holding SCLK HIGH, the ADS1254 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 ADS1254 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 ADS1254 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 and will indicate valid data. Subsequent DOUT/ DRDY pulses will occur normally. DOUT Mode DRDY Mode DOUT Mode DRDY Mode t4 DATA t1 t2 t3 DOUT/DRDY DATA DATA FIGURE 12. DOUT/DRDY Partitioning. ADS1254 SBAS213 11 SERIAL INTERFACE The ADS1254 includes a simple serial interface that can be connected to microcontrollers and digital signal processors in a variety of ways. Communications with the ADS1254 can commence on the first detection of the DOUT/DRDY pulse after power up. It is important to note that the data from the ADS1254 is a 24-bit result transmitted MSB-first in Offset Two’s Complement format, as shown in Table IV. The data must be clocked out before the ADS1254 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 ADS1254 provides for simple isolation methods. The CLK signal can be local to the ADS1254, which then only requires two signals (SCLK and DOUT/DRDY) to be used for isolated data acquisition. The channel select signals (CHSEL0, CHSEL1) will also need to be isolated unless a counter is used to auto multiplex the channels. DIFFERENTIAL VOLTAGE INPUT +Full Scale Zero –Full Scale DIGITAL OUTPUT (HEX) 7FFFFFH 000000H 800000H TABLE IV. ADS1254 Data Format (Offset Two's Complement). 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 DESCRIPTION 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. MIN 125 384 • tOSC 36 • tOSC 348 • tOSC 6 • tOSC 6 • tOSC 6 • tOSC 24 • tOSC 50 30 50 5 50 30 3 • tOSC 1537 • CLK 0.5 • CLK 2042.5 • tOSC 7681 • CLK 2318.5 • tOSC 6144.5 • tOSC 2043.5 • tosc 7679 • CLK 6143.5 • CLK TYP MAX UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns TABLE III. Digital Timing. t18 DOUT/DRDY DATA DATA CHSEL0, CHSEL1 MUX CHANGE FIGURE 13. Multiplexer Operation. 12 ADS1254 SBAS213 CLK t5 t7 t1 MSB LSB ADS1254 t6 t8 t9 t4 t2 t3 DOUT Mode tDRDY DRDY Mode Synchronization Mode Starts Here Synchronization Begins Here SBAS213 SCLK DOUT/DRDY FIGURE 14. DOUT/DRDY Timing. CLK t10 t12 DATA DATA SCLK DOUT/DRDY t3 t4 4 tDRDY tDRDY DOUT Mode DATA t2 t11 t13 t14 t2 t3 DOUT Mode t4 tDRDY FIGURE 15. Synchronization Mode. Power Down Occurs Here CLK t17 t15 DATA DATA DATA t10 SCLK DOUT/DRDY t3 tDRDY DOUT Mode t2 t4 4 tDRDY t11 t11 t16 t2 t3 DOUT Mode t4 tDRDY 13 FIGURE 16. Power-Down Mode. LAYOUT POWER SUPPLY The power supply should be well regulated and low noise. For designs requiring very high resolution from the ADS1254, 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. 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. AGND 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 ADS1254 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 Supply to ground. could include: • Multiple ADS1254s • 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 ADS1254 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 ADS1254 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 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 +AVDD range. Actual Analog-Input Voltage—the voltage at any one analog input relative to AGND. Full-Scale Range (FSR)—as with most A/D Converters, the full-scale range of the ADS1254 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 – Scale Range 2 • VREF =N 2N – 1 2 –1 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 ADS1254 Digital Portion of the ADS1254 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 14 where N is the number of bits in the digital output. ADS1254 SBAS213 Conversion Cycle—as used here, a conversion cycle refers to the time period between DOUT/DRDY pulses. Effective Resolution (ER)—of the ADS1254 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 that is one standard deviation. The ER in bits can be computed as follows: 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. Table V uses VREF = 4.096V, with the ADS1254 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) 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 NOISE REDUCTION FACTOR 1 1.414 2 2.82 4 5.66 8 11.3 16 22.6 32 45.25 64 ER IN Vrms 14.6µV 10.3µV 7.3µV 5.16µV 3.65µV 2.58µV 1.83µV 1.29µV 0.91µV 0.65µV 0.46µV 0.32µV 0.23µV ER IN BITS rms 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 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 ADS1254. 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 = CLK Frequency 6 TABLE V. Averaging. fDATA—Data output rate. fDATA = fMOD CLK Frequency = 64 384 ADS1254 SBAS213 15 PACKAGE OPTION ADDENDUM www.ti.com 9-Dec-2004 PACKAGING INFORMATION Orderable Device ADS1254E ADS1254E/2K5 (1) Status (1) ACTIVE ACTIVE Package Type SSOP/ QSOP SSOP/ QSOP Package Drawing DBQ DBQ Pins Package Eco Plan (2) Qty 20 20 56 2500 None None Lead/Ball Finish CU SNPB CU SNPB MSL Peak Temp (3) Level-3-250C-168 HR Level-3-250C-168 HR 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 - May not be currently available - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. None: Not yet available Lead (Pb-Free). 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. Green (RoHS & no Sb/Br): TI defines "Green" to mean "Pb-Free" and in addition, uses package materials that do not contain halogens, including bromine (Br) or antimony (Sb) above 0.1% of total product weight. (3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDECindustry standard classifications, and peak solder temperature. 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