LTC2485 24-Bit ΔΣ ADC with Easy Drive Input Current Cancellation and I2C Interface DESCRIPTION FEATURES n n n n n n n n n n n n n n Easy Drive™ Technology Enables Rail-to-Rail Inputs with Zero Differential Input Current Directly Digitizes High Impedance Sensors with Full Accuracy Integrated Temperature Sensor GND to VCC Input/Reference Common Mode Range 2-Wire I2C Interface Programmable 50Hz, 60Hz or Simultaneous 50Hz/60Hz Rejection Mode 2ppm (0.
LTC2485 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Notes 1, 2) TOP VIEW Supply Voltage (VCC) to GND ...................... – 0.3V to 6V Analog Input Voltage to GND ....... –0.3V to (VCC + 0.3V) Reference Input Voltage to GND .. –0.3V to (VCC + 0.3V) Digital Input Voltage to GND ....... – 0.3V to (VCC + 0.3V) Digital Output Voltage to GND...... –0.3V to (VCC + 0.3V) Operating Temperature Range LTC2485C .................................................... 0°C to 70°C LTC2485I .............................
LTC2485 ELECTRICAL CHARACTERISTICS (2X SPEED) The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) PARAMETER CONDITIONS Resolution (No Missing Codes) 0.1 ≤ VREF ≤ VCC, –FS ≤ VIN ≤ +FS (Note 5) ● Integral Nonlinearity 5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6) 2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6) ● 2 1 10 ppm of VREF Offset Error 2.
LTC2485 ANALOG INPUT AND REFERENCE The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
LTC2485 TIMING CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL PARAMETER MAX UNITS fEOSC External Oscillator Frequency Range CONDITIONS ● MIN 10 TYP 4000 kHz tHEO External Oscillator High Period ● 0.125 100 μs tLEO External Oscillator Low Period ● 0.
LTC2485 TYPICAL PERFORMANCE CHARACTERISTICS Integral Nonlinearity (VCC = 5V, VREF = 5V) –45°C 1 2 INL (ppm OF VREF) 2 INL (ppm OF VREF) 3 VCC = 5V VREF = 5V VIN(CM) = 2.5V 25°C 0 85°C –1 Integral Nonlinearity (VCC = 2.7V, VREF = 2.5V) 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V 1 –45°C, 25°C, 90°C 0 –1 –2 –2 –3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) 2 1 –45°C, 25°C, 90°C 0 –1 –2 –3 –1.25 2.5 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.
LTC2485 TYPICAL PERFORMANCE CHARACTERISTICS RMS Noise vs Input Differential Voltage VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C 0.8 0.7 0.6 0.5 RMS Noise vs Temperature (TA) 1.0 VCC = 5V VREF = 5V VIN = 0V VIN(CM) = GND TA = 25°C 0.9 RMS NOISE (μV) RMS NOISE (ppm OF VREF) 0.9 RMS Noise vs VIN(CM) 1.0 0.8 0.7 0.6 –1 0 2 1 3 5 4 RMS NOISE (μV) RMS NOISE (μV) VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C 0.9 0.8 0.7 0.6 0.5 0.5 0.4 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 1 2 3 VREF (V) –0.
LTC2485 TYPICAL PERFORMANCE CHARACTERISTICS Temperature Sensor vs Temperature 5 VCC = 5V VREF = 1.4V 0.25 308 3 TEMPERATURE ERROR (°C) VPTAT/VREF (V) 0.30 310 VCC = 5V 4 0.35 On-Chip Oscillator Frequency vs Temperature 2 FREQUENCY (kHz) 0.40 Temperature Sensor Error vs Temperature VREF = 1.4V 1 0 –1 –2 306 304 302 –3 –4 0.
LTC2485 TYPICAL PERFORMANCE CHARACTERISTICS Conversion Current vs Output Data Rate Integral Nonlinearity (2x Speed Mode; VCC = 5V, VREF = 5V) 3 2 VCC = 5V 350 300 VCC = 3V 250 25°C, 90°C 0 –1 100 90°C 0 –45°C, 25°C –1 –2 –3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2485 G28 NUMBER OF READINGS (%) 90°C 0 –45°C, 25°C –2 –0.25 0.25 0.75 INPUT VOLTAGE (V) 0.8 8 6 4 0.6 0.4 VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C 0.
LTC2485 TYPICAL PERFORMANCE CHARACTERISTICS Offset Error vs VCC (2x Speed Mode) Offset Error vs VREF (2x Speed Mode) 250 VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C 230 150 100 220 –40 210 200 190 160 2.7 3 4 4.5 VCC (V) 3.5 5.5 5 –140 0 1 2 4 3 VREF (V) PSRR vs Frequency at VCC (2x Speed Mode) –40 10 10k 100k 1k 100 FREQUENCY AT VCC (Hz) 1M 2485 G38 0 VCC = 4.1V DC ±1.4V REF+ = 2.5V REF– = GND IN+ = GND IN– = GND TA = 25°C VCC = 4.1V DC ±0.7V REF+ = 2.
LTC2485 PIN FUNCTIONS REF+ (Pin 1), REF– (Pin 3): Differential Reference Input. The voltage on these pins can have any value between GND and VCC as long as the reference positive input, REF+, is more positive than the reference negative input, REF –, by at least 0.1V. VCC (Pin 2): Positive Supply Voltage. Bypass to GND (Pin 8) with a 1μF tantalum capacitor in parallel with 0.1μF ceramic capacitor as close to the part as possible. IN+ (Pin 4), IN– (Pin 5): Differential Analog Input.
LTC2485 APPLICATIONS INFORMATION CONVERTER OPERATION Converter Operation Cycle The LTC2485 is a low power, ΔΣ analog-to-digital converter with an I2C interface. After power on reset, its operation is made up of three states. The converter operating cycle begins with the conversion, followed by the low power sleep state and ends with the data output/input (see Figure 1).
LTC2485 APPLICATIONS INFORMATION is an input only and the data line SDA is bidirectional. The device supports the Standard-mode and the Fast-mode for data transfer speeds up to 400kbit/s. Figure 2 shows the definition of timing for Fast/Standard-mode devices on the I2C-bus. The START and STOP Conditions A START condition is generated by transitioning SDA from HIGH to LOW while SCL is HIGH. The bus is considered to be busy after the START condition.
LTC2485 APPLICATIONS INFORMATION The LTC2485 powers up in a default mode commonly used for most measurements. The device will remain in this mode until a valid write cycle is performed. In this default mode, the measured input is external, the digital filter simultaneously rejects 50Hz and 60Hz line frequency noise, and the speed mode is 1x (offset automatically, continuously calibrated). The I2C serial interface grants access to any or all special functions contained within the LTC2485.
LTC2485 APPLICATIONS INFORMATION LTC2485 Data Format After a START condition, the master sends a 7-bit address followed by a R/W bit. The bit R/W is 1 for a Read request and 0 for a Write request. If the 7-bit address agrees with an LTC2485’s address, that device is selected. When the device is in the conversion state, it does not accept the request and issues a Not-Acknowledge (NAK) by leaving SDA HIGH. A write operation will also generate an NAK signal.
LTC2485 APPLICATIONS INFORMATION Initiating a New Conversion LTC2485 Address When the LTC2485 finishes a conversion, it automatically enters the sleep state. Once in the sleep state, the device is ready for a Read operation. After the device acknowledges a Read request, the device exits the sleep state and enters the data output state. The data output state concludes and the LTC2485 starts a new conversion once a STOP condition is issued by the master or all 32 bits of data are read out of the device.
LTC2485 APPLICATIONS INFORMATION OPERATION SEQUENCE The LTC2485 acts as a transmitter or receiver. The device may be programmed to perform several functions. These include measuring an external differential input signal or an integrated temperature sensor, selecting line frequency rejection (50Hz, 60Hz, or simultaneous 50Hz and 60Hz), and a 2x speed up mode. may be addressed for a read operation. At the end of a read operation, a new conversion begins.
LTC2485 APPLICATIONS INFORMATION on all the LTC2485s without changing the configuration registers. Discarding a Conversion Result and Initiating a New Conversion with Optional Configuration Updating At the conclusion of a conversion cycle, a Write cycle can be initiated. Once the Write cycle is acknowledged, a stop (P) command initiates a new conversion. If a new configuration is required, this data can be written into the device and a stop command initiates a new conversion, see Figure 8.
LTC2485 APPLICATIONS INFORMATION Frequency Rejection Selection (CA0/f0) The LTC2485 internal oscillator provides better than 110dB normal mode rejection at the line frequency and all its harmonics (up to the 255th) for 50Hz ±2% or 60Hz ±2%, or better than 87dB normal mode rejection from 48Hz to 62.4Hz. The rejection mode is selected by writing to the on-chip configuration register (the default mode at powerup is simultaneous 50Hz/60Hz rejection). ± 4% and its harmonics.
LTC2485 APPLICATIONS INFORMATION Ease of Use The LTC2485 data output has no latency, filter settling delay or redundant data associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog voltages is easy. The LTC2485 performs offset and full-scale calibrations every conversion cycle. This calibration is transparent to the user and has no effect on the cyclic operation described above.
LTC2485 APPLICATIONS INFORMATION Reference Voltage Range output codes. Since the differential input current cancellation does not rely on an on-chip buffer, current cancellation and DC performance is maintained rail-to-rail. The LTC2485 external reference voltage range is 0.1V to VCC. The converter output noise is determined by the thermal noise of the front-end circuits, and as such, its value in nanovolts is nearly constant with reference voltage.
LTC2485 APPLICATIONS INFORMATION For a simple approximation, the source impedance RS driving an analog input pin (IN+, IN–, REF+ or REF–) can be considered to form, together with RSW and CEQ (see Figure 12), a first order passive network with a time constant τ = (RS + RSW) • CEQ. The converter is able to sample the input signal with better than 1ppm accuracy if the sampling period is at least 14 times greater than the input circuit time constant τ.
LTC2485 APPLICATIONS INFORMATION voltage). Table 6 summarizes the effects of mismatched source impedance and differences in reference/input common mode voltages. RSOURCE VINCM + 0.5VIN IN+ CPAR 20pF CEXT LTC2485 Table 6. Suggested Input Configuration for LTC2485 RSOURCE UNBALANCED INPUT RESISTANCES CEXT > 1nF at Both IN+ and IN–. Can Take Large Source Resistance. Unbalanced Resistance Results in an Offset Which Can be Calibrated CEXT > 1nF at Both Varying VIN(CM) – VREF(CM) IN+ and IN–.
LTC2485 APPLICATIONS INFORMATION Reference Current In a similar fashion, the LTC2485 samples the differential reference pins REF+ and REF– transferring small amount of charge to and from the external driving circuits thus producing a dynamic reference current. This current does not change the converter offset, but it may degrade the gain and INL performance. The effect of this current can be analyzed in two distinct situations.
LTC2485 APPLICATIONS INFORMATION 90 VCC = 5V VREF = 5V VIN+ = 3.75V VIN– = 1.25V TA = 25°C 80 70 +FS ERROR (ppm) In addition to the reference sampling charge, the reference pins ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally 1nA (±10nA max), results in a small gain error. A 100Ω source resistance will create a 0.05μV typical and 0.5μV maximum full-scale error. Output Data Rate 60 50 CREF = 0.01μF CREF = 0.
LTC2485 APPLICATIONS INFORMATION with an external conversion clock (CA0/f0 connected to an external oscillator), the LTC2485 output data rate can be increased as desired. The duration of the conversion phase is 41036/fEOSC. If fEOSC = 307.2kHz, the converter behaves as if the internal oscillator is used and the notch is set at 60Hz. An increase in fEOSC over the nominal 307.2kHz will translate into a proportional increase in the maximum output data rate.
LTC2485 APPLICATIONS INFORMATION 0 24 –500 22 –1000 20 TA = 25°C RESOLUTION (BITS) –FS ERROR (ppm OF VREF) TA = 85°C TA = 25°C –1500 TA = 85°C –2000 –2500 –3000 –3500 VIN(CM) = VREF(CM) VCC = VREF = 5V CA0/f0 = EXT CLOCK 18 16 14 12 10 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) VIN(CM) = VREF(CM) VCC = VREF = 5V VIN = 0V CA0/f0 = EXT CLOCK RES = LOG 2 (VREF/NOISERMS) 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2485 F24 2485 F23 Figure 23.
LTC2485 APPLICATIONS INFORMATION Due to the complex filtering and calibration algorithms utilized, the converter input bandwidth is not modeled very accurately by a first order filter with the pole located at the 3dB frequency. When the internal oscillator is used, the shape of the LTC2485 input bandwidth is shown in Figure 29.
LTC2485 APPLICATIONS INFORMATION 100 –1 50Hz AND 60Hz MODE –2 50Hz MODE –3 60Hz MODE –4 –5 –6 INPUT REFERRED NOISE EQUIVALENT BANDWIDTH (Hz) INPUT SIGNAL ATTENUATION (dB) 0 50Hz MODE 1 0.1 0.
LTC2485 APPLICATIONS INFORMATION The user can expect to achieve this level of performance using the internal oscillator as it is demonstrated by Figures 35, 36 and 37. Typical measured values of the normal mode rejection of the LTC2485 operating with an internal oscillator and a 60Hz notch setting are shown in Figure 35 superimposed over the theoretical calculated curve.
LTC2485 APPLICATIONS INFORMATION MEASURED DATA CALCULATED DATA –20 –40 0 VCC = 5V VREF = 5V VIN(CM) = 2.5V VIN(P-P) = 5V TA = 25°C NORMAL MODE REJECTION (dB) NORMAL MODE REJECTION (dB) 0 –60 –80 –100 –120 0 15 30 45 60 75 –20 –40 VCC = 5V VREF = 5V VIN(CM) = 2.5V VIN(P-P) = 5V TA = 25°C –60 –80 –100 –120 90 105 120 135 150 165 180 195 210 225 240 INPUT FREQUENCY (Hz) MEASURED DATA CALCULATED DATA 0 12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.
LTC2485 APPLICATIONS INFORMATION Using the 2x speed mode of the LTC2485, the device bypasses the digital offset calibration operation to double the output data rate. The superior normal mode rejection is maintained as shown in Figures 31 and 32. However, the magnified details near DC and fS = 256fN are different, see Figures 40 and 41. In 2x speed mode, the bandwidth is 11.4Hz for the 50Hz rejection mode, 13.6Hz for the 60Hz rejection mode and 12.4Hz for the 50Hz/60Hz rejection mode.
LTC2485 APPLICATIONS INFORMATION 0 INPUT NORMAL REJECTION (dB) INPUT NORMAL REJECTION (dB) 0 –20 –40 –60 –80 –100 –120 0 fN 2fN 3fN 4fN 5fN 6fN 7fN INPUT SIGNAL FREQUENCY (fN) –20 –40 –60 –80 –100 –120 248 250 252 254 256 258 260 262 264 INPUT SIGNAL FREQUENCY (fN) 8fN 2485 F41 2485 F40 Figure 40. Input Normal Mode Rejection 2x Speed Mode –70 MEASURED DATA VCC = 5V CALCULATED DATA VREF = 5V VINCM = 2.
LTC2485 APPLICATIONS INFORMATION /* LTC248X.h Processor setup and Lots of useful defines for configuring the LTC2481, LTC2483, and LTC2485. */ #include <16F73.h> // Device #use delay(clock=6000000) // 6MHz clock //#fuses NOWDT,HS, PUT, NOPROTECT, NOBROWNOUT // Configuration fuses #rom 0x2007={0x3F3A} // Equivalent and more reliable fuse config. #use I2C(master, sda=PIN_C5, scl=PIN_C3, SLOW)// Set up i2c port #include “PCM73A.h” // Various defines #include “lcd.
LTC2485 APPLICATIONS INFORMATION /* LTC2485.c Basic voltmeter test program for LTC2485 Reads LTC2485, converts result to volts, and prints voltage to a 2 line by 16 character LCD display. Mark Thoren Linear Technology Corporation June 23, 2005 Written for CCS PCM compiler, Version 3.182 */ #include “LTC248X.h” /*** read_LTC2485() ************************************************************ This is the function that actually does all the work of talking to the LTC2485.
LTC2485 APPLICATIONS INFORMATION *******************************************************************************/ signed int32 read_LTC2485(char addr, char config) { struct fourbytes // Define structure of four consecutive bytes { // To allow byte access to a 32 bit int or float. int8 te0; // int8 te1; // The make32() function in this compiler will int8 te2; // also work, but a union of 4 bytes and a 32 bit int int8 te3; // is probably more portable.
LTC2485 APPLICATIONS INFORMATION /*** main() ******************************************************************** Main program initializes microcontroller registers, then reads the LTC2481 repeatedly *******************************************************************************/ void main() { signed int32 x, y; // Integer result from LTC2481 float voltage; // Variable for floating point math int16 timeout; initialize(); while(1) { delay_ms(1); // // // // // // Hardware initialization // Pace the main lo
LTC2485 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) 0.70 p0.05 3.55 p0.05 1.65 p0.05 2.15 p0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.38 p0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p0.10 (4 SIDES) R = 0.125 TYP 6 0.40 p 0.10 10 1.65 p 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 s 45o CHAMFER PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN REV C 0310 5 0.200 REF 1 0.25 p 0.05 0.50 BSC 0.75 p0.05 0.00 – 0.05 2.38 p0.
LTC2485 REVISION HISTORY REV DATE DESCRIPTION B 11/09 Update Tables 2 and 3 C 7/10 (Revision history begins at Rev B) PAGE NUMBER 15 Revised Typical Application drawing 1 Added text to I2C Interface section 12 2485fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
LTC2485 TYPICAL APPLICATION 5V PIC16F73 C8 1μF C7 0.1μF ISOTHERMAL 1.7k 1.
