LTC2453 Easy-to-Use, Ultra-Tiny, Differential, 16-Bit ΔΣ ADC With I2C Interface DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ±VCC Differential Input Range 16-Bit Resolution (Including Sign), No Missing Codes 2LSB Offset Error 4LSB Full-Scale Error 60 Conversions Per Second Single Conversion Settling Time for Multiplexed Applications Single-Cycle Operation with Auto Shutdown 800μA Supply Current 0.2μA Sleep Current Internal Oscillator—No External Components Required 2-Wire I2C Interface Ultra-Tiny 3mm × 2mm DFN Package APPLICATIONS ■ ■ ■ ■ ■ ■ ■ System Monitoring Environmental Monitoring Direct Temperature Measurements Instrumentation Industrial Process Control Data Acquisition Embedded ADC Upgrades , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 6208279, 6411242, 7088280, 7164378. The LTC®2453 is an ultra-tiny, fully differential, 16-bit, analog-to-digital converter. The LTC2453 uses a single 2.7V to 5.5V supply and communicates through an I2C interface. The ADC is available in an 8-pin, 3mm × 2mm DFN package. It includes an integrated oscillator that does not require any external components. It uses a delta-sigma modulator as a converter core and has no latency for multiplexed applications. The LTC2453 includes a proprietary input sampling scheme that reduces the average input sampling current several orders of magnitude lower than conventional delta-sigma converters. Additionally, due to its architecture, there is negligible current leakage between the input pins. The LTC2453 can sample at 60 conversions per second, and due to the very large oversampling ratio, has extremely relaxed antialiasing requirements. The LTC2453 includes continuous internal offset and full-scale calibration algorithms which are transparent to the user, ensuring accuracy over time and over the operating temperature range. The converter has external REF+ and REF– pins and the differential input voltage range can extend up to ±(VREF+ – VREF–). Following a single conversion, the LTC2453 can automatically enter a sleep mode and reduce its power to less than 0.2μA. If the user reads the ADC once a second, the LTC2453 consumes an average of less than 50μW from a 2.7V supply. Integral Nonlinearity, VCC = 3V TYPICAL APPLICATION 2.0 1.5 2.7V TO 5.5V REF+ 0.1μF IN+ 10k VCC SCL 10k LTC2453 IN– SDA 1.0 10μF 2-WIRE I2C INTERFACE INL (LSB) 0.1μF VCC = 3V VREF+ = 3V VREF– = 0V 0.5 TA = –45°C, 25°C, 90°C 0 –0.5 –1.0 10k R 0.1μF REF– GND –1.5 –2.0 2453 TA01 –3 –2 –1 1 2 0 DIFFERENTIAL INPUT VOLTAGE (V) 3 2453 G02 2453f 1 LTC2453 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Notes 1, 2) Supply Voltage (VCC) ................................... –0.3V to 6V Analog Input Voltage (VIN+, VIN–) .. –0.3V to (VCC + 0.3V) Reference Voltage (VREF+, VREF–) .. –0.3V to (VCC + 0.3V) Digital Voltage (SDA, SCL) ............ –0.3V to (VCC + 0.3V) Storage Temperature Range................... –65°C to 150°C Operating Temperature Range LTC2453C ................................................ 0°C to 70°C LTC2453I ............................................. –40°C to 85°C TOP VIEW GND 1 8 SDA REF– 2 7 SCL 6 IN+ 5 IN– 9 REF+ 3 VCC 4 DD8 PACKAGE 8-LEAD (3mm × 2mm) PLASTIC DFN C/I GRADE TJMAX = 125°C, θJA = 76°C/W (NOTE 4) EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION Lead Free Finish TAPE AND REEL (MINI) TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2453CDDB#TRMPBF LTC2453CDDB#TRPBF LDBQ 8-Lead Plastic (3mm × 2mm) DFN LTC2453IDDB#TRMPBF LTC2453IDDB#TRPBF LDBQ 8-Lead Plastic (3mm × 2mm) DFN TRM = 500 pieces. *Temperature grades are identified by a label on the shipping container. Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 0°C to 70°C –40°C to 85°C ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 2) PARAMETER CONDITIONS Resolution (No Missing Codes) (Note 3) ● Integral Nonlinearity (Note 4) ● Offset Error MIN ● Offset Error Drift Gain Error TYP MAX 16 Bits 2 10 2 10 0.02 ● UNITS 0.01 LSB LSB LSB/°C 0.02 % of FS Gain Error Drift 0.02 LSB/°C Transition Noise 1.4 μVRMS Power Supply Rejection DC 80 dB 2453f 2 LTC2453 ANALOG INPUTS AND REFERENCES The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. SYMBOL PARAMETER + Positive Input Voltage Range – Negative Input Voltage Range VIN CONDITIONS MIN TYP MAX UNITS ● 0 VCC V VCC V ● 0 VREF+ – VREF– ≥ 2.5V VREF+ – VREF– ≥ 2.5V VREF = 5V, VIN– = 2.5V (See Figure 2) VREF = 5V, VIN+ = 2.5V (See Figure 2) ● VCC – 2.5 VCC V ● 0 VCC – 2.5 V IDC_LEAK(IN+) IN+ DC Leakage Current VIN = GND (Note 8) VIN = VCC (Note 8) ● ● –10 –10 1 1 10 10 nA nA IDC_LEAK(IN–) IN– DC Leakage Current VIN = GND (Note 8) VIN = VCC (Note 8) ● ● –10 –10 1 1 10 10 nA nA VREF = 3V (Note 8) ● –10 1 10 nA VIN VREF+ VREF– VOR+, VUR+ VOR–, VUR– Positive Reference Voltage Range CIN IN+, IN– Sampling Capacitance Negative Reference Voltage Range Overrange/Underrange Voltage, IN+ Overrange/Underrange Voltage, IN– IDC_LEAK(REF+, REF–) REF+, REF– DC Leakage Current ICONV 8 LSB 8 LSB 0.35 Input Sampling Current (Note 5) pF 50 nA POWER REQUIREMENTS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. SYMBOL PARAMETER CONDITIONS MIN VCC Supply Voltage ● ICC Supply Current Conversion Sleep ● ● TYP 2.7 800 0.2 MAX UNITS 5.5 V 1200 0.6 μA μA I2C INPUTS AND OUTPUTS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 2, 7) SYMBOL PARAMETER VIH High Level Input Voltage CONDITIONS ● MIN TYP MAX VIL Low Level Input Voltage ● II Digital Input Current ● –10 VHYS Hysteresis of Schmidt Trigger Inputs (Note 3) ● 0.05VCC VOL Low Level Output Voltage (SDA) I = 3mA ● 0.4 V IIN Input Leakage 0.1VCC ≤ VIN ≤ VCC ● 1 μA CI Capacitance for Each I/O Pin ● CB Capacitance Load for Each Bus Line ● 0.7VCC UNITS V 0.3VCC V 10 μA V 10 pF 400 pF 2453f 3 LTC2453 I2C TIMING CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 2, 7) SYMBOL PARAMETER CONDITIONS MIN TYP 16.6 MAX UNITS tCONV Conversion Time ● 13 fSCL SCL Clock Frequency ● 0 tHD(SDA) Hold Time (Repeated) START Condition ● 0.6 μs tLOW LOW Period of the SCL Pin ● 1.3 μs tHIGH HIGH Period of the SCL Pin ● 0.6 μs tSU(STA) Set-Up Time for a Repeated START Condition ● 0.6 μs tHD(DAT) Data Hold Time ● 0 tSU(DAT) Data Set-Up Time ● 100 tr Rise Time for SDA/SCL Signals (Note 6) ● 20 + 0.1CB 300 ns tf Fall Time for SDA/SCL Signals (Note 6) ● 20 + 0.1CB 300 ns 23 ms 400 kHz μs 0.9 ns tSU(STO) Set-Up Time for STOP Condition ● 0.6 μs tBUF Bus Free Time Between a Stop and Start Condition ● 1.3 μs tOF Output Fall Time VIHMIN to VILMAX ● 20 + 0.1CB tSP Input Spike Suppression Bus Load CB 10pF to 400pF (Note 6) ● TYPICAL PERFORMANCE CHARACTERISTICS VCC = 5V VREF+ = 5V VREF– = 0V 1.5 0.5 VCC = 3V VREF+ = 3V VREF– = 0V TA = –45°C, 25°C, 90°C 0 –0.5 0.5 0.5 –1.5 –1.5 5 2453 G01 1.0 –0.5 –1.0 –5 –4 –3 –2 –1 0 1 2 3 4 DIFFERENTIAL INPUT VOLTAGE (V) TA = –45°C, 25°C, 90°C 0 –1.0 –2.0 VCC = VREF+ = 5V, 4.1V, 3V 1.5 1.0 INL (LSB) INL (LSB) 1.0 ns Maximum INL vs Temperature 2.0 INL (LSB) 1.5 2.0 50 (TA = 25°C, unless otherwise noted) Integral Nonlinearity, VCC = 3V Integral Nonlinearity, VCC = 5V ns Note 4. Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. Guaranteed by design and test correlation. Note 5. Input sampling current is the average input current drawn from the input sampling network while the LTC2453 is converting. Note 6. CB = capacitance of one bus line in pF. Note 7. All values refer to VIH(MIN) and VIL(MAX) levels. Note 8. A positive current is flowing into the DUT pin. Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2. All voltage values are with respect to GND. VCC = 2.7V to 5.5V unless otherwise specified. VREF = VREF+ – VREF–, VREFCM = (VREF+ + VREF–)/2, FS = VREF+ – VREF–; VIN = VIN+ – VIN–, –VREF ≤ VIN ≤ VREF; VINCM = (VIN+ + VIN–)/2. Note 3. Guaranteed by design, not subject to test. 2.0 250 –2.0 –3 –2 –1 1 2 0 DIFFERENTIAL INPUT VOLTAGE (V) 3 2453 G02 0 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 2453 G03 2453f 4 LTC2453 TYPICAL PERFORMANCE CHARACTERISTICS Offset Error vs Temperature Gain Error vs Temperature 5 Transition Noise vs Temperature 3.0 TRANSITION NOISE RMS (μV) 5 4 4 3 GAIN ERROR (LSB) OFFSET ERROR (LSB) (TA = 25°C, unless otherwise noted) VCC = VREF+ = 3V VCC = VREF+ = 4.1V 2 1 VCC = VREF+ = 5V VCC = VREF+ = 3V 3 VCC = VREF+ = 4.1V 2 1 0 VCC = VREF+ = 5V –1 –50 –25 0 25 50 TEMPERATURE (°C) 75 0 –50 100 –25 0 25 50 TEMPERATURE (°C) 75 2453 G04 VCC = 4.1V 1.5 1.0 VCC = 5V 0 –50 100 VCC = 3V 0.5 –25 0 25 50 TEMPERATURE (°C) 75 3.0 1200 2.5 1000 100 2453 G06 Conversion Mode Power Supply Current vs Temperature Sleep Mode Power Supply Current vs Temperature 250 60Hz OUTPUT SAMPLE RATE CONVERSION CURRENT (μA) 2.0 1.5 VCC = VREF+ = 5V 1.0 800 –16384 0 16384 OUTPUT CODE VCC = 3V 600 VCC = 4.1V 400 200 0.5 0 –32768 200 VCC = 5V 0 –50 32768 –25 0 25 50 TEMPERATURE (°C) 75 VCC = 5V 150 100 VCC = 4.1V 50 VCC = 3V 0 –50 100 25Hz OUTPUT SAMPLE RATE REJECTIOIN (dB) 100 Conversion Time vs Temperature VCC = 4.1V VREF+ = 2.7V VREF– = 0V VIN+ = 1V VIN– = 2V –20 1Hz OUTPUT SAMPLE RATE 75 21 0 10000 100 0 25 50 TEMPERATURE (°C) 2453 G09 Power Supply Rejection vs Frequency at VCC Average Power Dissipation vs Temperature, VCC = 3V 10Hz OUTPUT SAMPLE RATE –25 2453 G08 2453 G07 1000 SLEEP CURRENT (nA) VCC = VREF+ = 3V 20 CONVERSION TIME (ms) TRANSITION NOISE RMS (μV) 2.0 2453 G05 Transition Noise vs Output Code AVERAGE POWER DISSIPATION (μW) 2.5 –40 –60 10 VCC = 3V 19 VCC = 4.1V 18 VCC = 5V 17 16 –80 15 1 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 2453 G10 –100 1 10 100 1k 10k 100k FREQUENCY AT VCC (Hz) 1M 10M 2453 G11 14 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 2453 G12 2453f 5 LTC2453 PIN FUNCTIONS GND (Pin 1): Ground. Connect to a ground plane through a low impedance connection. REF– (Pin 2), 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+, remains more positive than the negative reference input, REF–, by at least 2.5V. The differential reference voltage (VREF = REF+ to REF–) sets the full-scale range. VCC (Pin 4): Positive Supply Voltage. Bypass to GND (Pin 1) with a 10μF capacitor in parallel with a low-series-inductance 0.1μF capacitor located as close to the part as possible. IN– (Pin 5), IN+ (Pin 6): Differential Analog Input. SCL (Pin 7): Serial Clock Input of the I2C Interface. The LTC2453 can only act as a slave and the SCL pin only accepts external serial clock. Data is shifted into the SDA pin on the rising edges of SCL and output through the SDA pin on the falling edges of SCL. SDA (Pin 8): Bidirectional Serial Data Line of the I2C Interface. The conversion result is output through the SDA pin. The pin is high impedance unless the LTC2453 is in the data output mode. While the LTC2453 is in the data output mode, SDA is an open drain pull down (which requires an external 1.7k pull-up resistor to VCC). Exposed Pad (Pin 9): Ground. Must be soldered to PCB ground. BLOCK DIAGRAM 3 6 IN+ 4 REF+ 16-BIT ΔΣ A/D CONVERTER I2C INTERFACE – 5 IN– VCC REF– SDA 7 8 DECIMATING SINC FILTER 16-BIT ΔΣ A/D CONVERTER 2 SCL INTERNAL OSCILLATOR 1 GND 2453 BD 2453f 6 LTC2453 APPLICATIONS INFORMATION CONVERTER OPERATION Converter Operation Cycle The LTC2453 is a low-power, fully differential, delta-sigma analog-to-digital converter with an I2C interface. Its operation, as shown in Figure 1, is composed of three successive states: CONVERSION, SLEEP and DATA OUTPUT. Initially, at power up, the LTC2453 performs a conversion. Once the conversion is complete, the device enters the sleep state. While in this sleep state, power consumption is reduced by several orders of magnitude. The part remains in the sleep state as long it is not addressed for a read operation. The conversion result is held indefinitely in a static shift register while the part is in the sleep state. POWER-ON RESET CONVERSION SLEEP NO READ ACKNOWLEDGE YES DATA OUTPUT NO STOP OR READ 16-BITS YES 2453 F01 Figure 1. LTC2453 State Diagram The device will not acknowledge an external request during the conversion state. After a conversion is finished, the device is ready to accept a read request. The LTC2453’s address is hard-wired at 0010100. Once the LTC2453 is addressed for a read operation, the device begins outputting the conversion result under the control of the serial clock (SCL). There is no latency in the conversion result. The data output is 16 bits long and contains a 15-bit plus sign conversion result. Data is updated on the falling edges of SCL, allowing the user to reliably latch data on the rising edge of SCL. A new conversion is initiated by a stop condition following a valid read operation, or by the conclusion of a complete read cycle (all 16 bits read out of the device). Power-Up Sequence When the power supply voltage (VCC) applied to the converter is below approximately 2.1V, the ADC performs a power-on reset. This feature guarantees the integrity of the conversion result. When VCC rises above this threshold, the converter generates an internal power-on reset (POR) signal for approximately 0.5ms. The POR signal clears all internal registers. Following the POR signal, the LTC2453 starts a conversion cycle and follows the succession of states described in Figure 1. The first conversion result following POR is accurate within the specifications of the device if the power supply voltage VCC is restored within the operating range (2.7V to 5.5V) before the end of the POR time interval. Ease of Use The LTC2453 data output has no latency, filter settling delay or redundant results associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog input voltages requires no special actions. The LTC2453 performs offset calibrations every conversion. This calibration is transparent to the user and has no effect upon the cyclic operation described previously. The advantage of continuous calibration is extreme stability of the ADC performance with respect to time and temperature. The LTC2453 includes a proprietary input sampling scheme that reduces the average input current by several orders of magnitude when compared to traditional delta-sigma architectures. This allows external filter networks to interface directly to the LTC2453. Since the average input sampling current is 50nA, an external RC lowpass filter using a 1kΩ and 0.1μF results in <1LSB additional error. Additionally, there is negligible leakage current between IN+ and IN–. 2453f 7 LTC2453 APPLICATIONS INFORMATION Reference Voltage Range I2C INTERFACE This converter accepts a truly differential external reference voltage. The absolute/common mode voltage range for REF+ and REF– pins covers the entire operating range of the device (GND to VCC). For correct converter operation, VREF+ must be >(2.5V + VREF–). The LTC2453 communicates through an I2C interface. The I2C interface is a 2-wire open-drain interface supporting multiple devices and masters on a single bus. The connected devices can only pull the data line (SDA) LOW and never drive it HIGH. SDA must be externally connected to the supply through a pull-up resistor. When the data line is free, it is HIGH. Data on the I2C bus can be transferred at rates up to 100kbits/s in the Standard-Mode and up to 400kbits/s in the Fast-Mode. The LTC2453 differential reference input range is 2.5V to VCC. For the simplest operation, REF+ can be shorted to VCC and REF– can be shorted to GND. Input Voltage Range For most applications, VREF– ≤ (VIN+, VIN–) ≤ VREF+. Under these conditions the output code is given (see Data Format section) as 32768 • (VIN+ – VIN–)/(VREF+ – VREF–). The output of the LTC2453 is clamped at a maximum value of 32767 and clamped at a minimum value of –32768. The LTC2453 includes a proprietary system that can, typically, correctly digitize each input 8LSB above VREF+ and below VREF–, if the LTC2453’s output is not clamped. As an example (Figure 2), if the user desires to measure a signal slightly below ground, the user could set VIN– = VREF– = GND, and VREF+ = 5V. If VIN+ = GND, the output code would be approximately 0. If VIN+ = GND – 8LSB = –1.22 mV, the output code would be approximately –8. 16 A START (S) 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. When the data transfer is finished, a STOP (P) condition is generated by transitioning SDA from LOW to HIGH while SCL is HIGH. The bus is free after a STOP is generated. START and STOP conditions are always generated by the master. 12 8 OUTPUT CODE The LTC2453 can only be addressed as a slave. It can only transmit the last conversion result. The serial clock line, SCL, is always an input to the LTC2453 and the serial data line SDA is bidirectional. Figure 3 shows the definition of the I2C timing. The START and STOP Conditions 20 4 0 –4 SIGNALS BELOW GND –8 –12 –16 –20 –0.001 Each device on the I2C bus is recognized by a unique address stored in that device and can operate either as a transmitter or receiver, depending on the function of the device. In addition to transmitters and receivers, devices can also be considered as masters or slaves when performing data transfers. A master is the device which initiates a data transfer on the bus and generates the clock signals to permit that transfer. Devices addressed by the master are considered a slave. The address of the LTC2453 is 0010100. –0.005 0.005 0 VIN+/VREF+ 0.001 0.0015 2453 F02 Figure 2. Output Code vs VIN+ with VIN– = 0 and VREF– = 0 When the bus is in use, it stays busy if a repeated START (Sr) is generated instead of a STOP condition. The repeated START timing is functionally identical to the START and is used for reading from the device before the initiation of a new conversion. 2453f 8 LTC2453 APPLICATIONS INFORMATION SDA tf tSU(DAT) tr tLOW tf tHD(SDA) tr tSP tBUF SCL tHD(STA) tHD(DAT) S tSU(STA) tHIGH tSU(STO) Sr P S 2453 F03 Figure 3. Definition of Timing for Fast/Standard Mode Devices on the I2C Bus 1 7 8 9 1 2 3 8 D15 D14 D13 D8 SGN MSB 9 1 2 D7 D6 3 8 9 SCL SDA 7-BIT ADDRESS START BY MASTER R ACK BY LTC2453 SLEEP D5 D0 LSB ACK BY MASTER DATA OUTPUT NACK BY MASTER CONVERSION 2453 F04 Figure 4. Read Sequence Timing Diagram Data Transferring After the START condition, the I2C bus is busy and data transfer can begin between the master and the addressed slave. Data is transferred over the bus in groups of nine bits, one byte followed by one acknowledge (ACK) bit. The master releases the SDA line during the ninth SCL clock cycle. The slave device can issue an ACK by pulling SDA LOW or issue a Not Acknowledge (NAK) by leaving the SDA line HIGH impedance (the external pull-up resistor will hold the line HIGH). Change of data only occurs while the clock line (SCL) is LOW. Following the ACK, the LTC2453 can output data. The data output stream is 16 bits long and is shifted out on the falling edges of SCL (see Figure 4). The first bit output by the LTC2453 is the sign, which is 1 for VIN+ ≥ VIN– and 0 for VIN+ < VIN–. The next bit is the MSB (D14) and is followed by successively less significant bits (D13, D12…) until the LSB is output by the LTC2453. This sequence is shown in Figure 5. OPERATION SEQUENCE Continuous Read Data Format After a START condition, the master sends a 7-bit address followed by a read request (R) bit. The bit R is 1 for a Read Request. If the 7-bit address matches the LTC2453’s address (hard-wired at 0010100) the ADC is selected. When the device is addressed during the conversion state, it does not accept the request and issues a NAK by leaving the SDA line HIGH. If the conversion is complete, the LTC2453 issues an ACK by pulling the SDA line LOW. Conversions from the LTC2453 can be continuously read, see Figure 6. At the end of a read operation, a new conversion automatically begins. At the conclusion of the conversion cycle, the next result may be read using the method described above. If the conversion cycle is not complete and a valid address selects the device, the LTC2453 generates a NAK signal indicating the conversion cycle is in progress. 2453f 9 LTC2453 APPLICATIONS INFORMATION S 7-BIT ADDRESS (0010100) CONVERSION R ACK READ P DATA OUTPUT SLEEP CONVERSION 2453 F05 Figure 5. The LTC2453 Coversion Sequence S CONVERSION 7-BIT ADDRESS (0010100) R ACK READ P DATA OUTPUT SLEEP S 7-BIT ADDRESS (0010100) CONVERSION R ACK READ DATA OUTPUT SLEEP P CONVERSION 2453 F06 Figure 6. Consecutive Reading at the Same Configuration S CONVERSION 7-BIT ADDRESS (0010100) SLEEP R ACK READ (OPTIONAL) DATA OUTPUT P CONVERSION 2453 F07 Figure 7. Start a New Conversion without Reading Old Conversion Result Discarding a Conversion Result and Initiating a New Conversion It is possible to start a new conversion without reading the old result, as shown in Figure 7. Following a valid 7-bit address, a read request (R) bit, and a valid ACK, a STOP command will start a new conversion. PRESERVING THE CONVERTER ACCURACY The LTC2453 is designed to dramatically reduce the conversion result’s sensitivity to device decoupling, PCB layout, antialiasing circuits, line and frequency perturbations. Nevertheless, in order to preserve the high accuracy capability of this part, some simple precautions are desirable. Digital Signal Levels Due to the nature of CMOS logic, it is advisable to keep input digital signals near GND or VCC. Voltages in the range of 0.5V to VCC – 0.5V may result in additional current leakage from the part. Driving VCC and GND In relation to the VCC and GND pins, the LTC2453 combines internal high frequency decoupling with damping elements, which reduce the ADC performance sensitivity to PCB layout and external components. Nevertheless, the very high accuracy of this converter is best preserved by careful low and high frequency power supply decoupling. A 0.1μF, high quality, ceramic capacitor in parallel with a 10μF ceramic capacitor should be connected between the VCC and GND pins, as close as possible to the package. The 0.1μF capacitor should be placed closest to the ADC package. It is also desirable to avoid any via in the circuit path, starting from the converter VCC pin, passing through these two decoupling capacitors, and returning to the converter GND pin. The area encompassed by this circuit path, as well as the path length, should be minimized. Very low impedance ground and power planes, and star connections at both VCC and GND pins, are preferable. The VCC pin should have three distinct connections: the 2453f 10 LTC2453 APPLICATIONS INFORMATION VCC ILEAK VCC RSW 15k (TYP) RS REF+ ILEAK SIG+ VCC ILEAK ILEAK + – IN+ ILEAK CIN VCC IN+ ILEAK RS IN– ILEAK SIG– VCC ILEAK CEQ 0.35pF (TYP) CPAR RSW 15k (TYP) CEQ 0.35pF (TYP) RSW 15k (TYP) IN– + – ILEAK CIN CPAR RSW 15k (TYP) ICONV RSW 15k (TYP) CEQ 0.35pF (TYP) ICONV 2453 F09 Figure 9. LTC2453 Input Drive Equivalent Circuit ILEAK VCC ILEAK REF– package. The 0.1μF capacitor should be placed closest to the ADC. RSW 15k (TYP) 2453 F08 ILEAK Figure 8. LTC2453 Analog Input/Reference Equivalent Circuit first to the decoupling capacitors described above, the second to the ground return for the input signal source, and the third to the ground return for the power supply voltage source. Driving REF+ and REF– A simplified equivalent circuit for REF+ and REF– is shown in Figure 8. Like all other A/D converters, the LTC2453 is only as accurate as the reference it is using. Therefore, it is important to keep the reference line quiet by careful low and high frequency power supply decoupling. The LT6660 reference is an ideal match for driving the LTC2453’s REF+ pin. The LTC6660 is available in a 2mm × 2mm DFN package with 2.5V, 3V, 3.3V and 5V options. A 0.1μF, high quality, ceramic capacitor in parallel with a 10μF ceramic capacitor should be connected between the REF+/REF– and GND pins, as close as possible to the Driving VIN+ and VIN– The input drive requirements can best be analyzed using the equivalent circuit of Figure 9. The input signal VSIG is connected to the ADC input pins (IN+ and IN–) through an equivalent source resistance RS. This resistor includes both the actual generator source resistance and any additional optional resistors connected to the input pins. Optional input capacitors CIN are also connected to the ADC input pins. This capacitor is placed in parallel with the ADC input parasitic capacitance CPAR. Depending on the PCB layout, CPAR has typical values between 2pF and 15pF. In addition, the equivalent circuit of Figure 9 includes the converter equivalent internal resistor RSW and sampling capacitor CEQ. There are some immediate trade-offs in RS and CIN without needing a full circuit analysis. Increasing RS and CIN can give the following benefits: 1) Due to the LTC2453’s input sampling algorithm, the input current drawn by either VIN+ or VIN– over a conversion cycle is 50nA. A high RS • CIN attenuates the high frequency components of the input current, and RS values up to 1k result in <1LSB error. 2453f 11 LTC2453 APPLICATIONS INFORMATION 2) The bandwidth from VSIG is reduced at the input pins (IN+, IN–). This bandwidth reduction isolates the ADC from high frequency signals, and as such provides simple antialiasing and input noise reduction. 3) Switching transients generated by the ADC are attenuated before they go back to the signal source. 4) A large CIN gives a better AC ground at the input pins, helping reduce reflections back to the signal source. 5) Increasing RS protects the ADC by limiting the current during an outside-the-rails fault condition. There is a limit to how large RS • CIN should be for a given application. Increasing RS beyond a given point increases the voltage drop across RS due to the input current, to the point that significant measurement errors exist. Additionally, for some applications, increasing the RS • CIN product too much may unacceptably attenuate the signal at frequencies of interest. For most applications, it is desirable to implement CIN as a high-quality 0.1μF ceramic capacitor and RS ≤ 1k. This capacitor should be located as close as possible to the actual VIN package pin. Furthermore, the area encompassed by this circuit path, as well as the path length, should be minimized. In the case of a 2-wire sensor that is not remotely grounded, it is desirable to split RS and place series resistors in the ADC input line as well as in the sensor ground return line, which should be tied to the ADC GND pin using a star connection topology. Figure 10 shows the measured LTC2453 INL vs Input Voltage as a function of RS value with an input capacitor CIN = 0.1μF. In some cases, RS can be increased above these guidelines. The input current is zero when the ADC is either in sleep or I/O modes. Thus, if the time constant of the input RC circuit τ = RS • CIN, is of the same order of magnitude or longer than the time periods between actual conversions, then one can consider the input current to be reduced correspondingly. These considerations need to be balanced out by the input signal bandwidth. The 3dB bandwidth ≈ 1/(2πRSCIN). Finally, if the recommended choice for CIN is unacceptable for the user’s specific application, an alternate strategy is to eliminate CIN and minimize CPAR and RS. In practical terms, this configuration corresponds to a low impedance sensor directly connected to the ADC through minimum length traces. Actual applications include current measurements through low value sense resistors, temperature measurements, low impedance voltage source monitoring, and so on. The resultant INL vs VIN is shown in Figure 11. The measurements of Figure 11 include a capacitor CPAR corresponding to a minimum sized layout pad and a minimum width input trace of about 1 inch length. 10 10 6 CIN = 0 8 V = 5V CC 6 TA = 25°C RS = 10k INL (LSB) 4 2 0 –2 4 RS = 2k RS = 1k INL (LSB) 8 CIN = 0.1μF VCC = 5V TA = 25°C RS = 0 2 0 –2 –4 –4 –6 –6 –8 –8 –10 –10 –5 –4 –3 –2 –1 0 1 2 3 4 DIFFERENTIAL INPUT VOLTAGE (V) 5 2453 F10 Figure 10. Measured INL vs Input Voltage, CIN = 0.1μF, VCC = 5V, TA = 25°C RS = 10k RS = 0 RS = 1k, 2k –5 –4 –3 –2 –1 0 1 2 3 4 DIFFERENTIAL INPUT VOLTAGE (V) 5 2453 F11 Figure 11. Measured INL vs Input Voltage, CIN = 0, VCC = 5V, TA = 25°C 2453f 12 LTC2453 APPLICATIONS INFORMATION 0 INPUT SIGNAL ATTENUATIOIN (dB) INPUT SIGNAL ATTENUATION (dB) 0 –20 –40 –60 –80 –5 –10 –15 –20 –25 –30 –35 –40 –45 –100 0 2.5 5.0 7.5 1.00 1.25 1.50 INPUT SIGNAL FREQUENCY (MHz) –50 0 60 120 180 240 300 360 420 480 540 600 INPUT SIGNAL FREQUENCY (Hz) 2453 F12 Figure 12. LTC2453 Input Signal Attentuation vs Frequency Signal Bandwidth, Transition Noise and Noise Equivalent Input Bandwidth The LTC2453 includes a sinc1 type digital filter with the first notch located at f0 = 60Hz. As such, the 3dB input signal bandwidth is 26.54Hz. The calculated LTC2453 input signal attenuation vs frequency over a wide frequency range is shown in Figure 12. The calculated LTC2453 input signal attenuation vs frequency at low frequencies is shown in Figure 13. The converter noise level is about 1.4μVRMS and can be modeled by a white noise source connected at the input of a noise-free converter. On a related note, the LTC2453 uses two separate A/D converters to digitize the positive and negative inputs. Each of these A/D converters has 1.4μVRMS transition noise. If one of the input voltages is within this small transition noise band, then the output will fluctuate one 2453 F13 Figure 13. LTC2453 Input Signal Attenuation vs Frequency (Low Frequencies) bit, regardless of the value of the other input voltage. If both of the input voltages are within their transition noise bands, the output can fluctuate 2 bits. For a simple system noise analysis, the VIN drive circuit can be modeled as a single-pole equivalent circuit characterized by a pole location fi and a noise spectral density ni. If the converter has an unlimited bandwidth, or at least a bandwidth substantially larger than fi, then the total noise contribution of the external drive circuit would be: Vn = ni π / 2 • fi Then, the total system noise level can be estimated as the square root of the sum of (Vn2) and the square of the LTC2453 noise floor (~1.4μV2). 2453f 13 LTC2453 TYPICAL APPLICATION DC1266A Demo Board Schematic V+ LT6660 3 IN OUT 1 5V C4 1μF GND GND 2 4 JP1 VCC EXT C9 1μF C3 1μF R4 1.0Ω E5 REF+ V+ 1 2 C1 0.1μF 6 SCL SDA VCC 4 C10 0.1μF 7 5 E1 IN+ E2 IN– E3 VCC 3 R1 1k R9 1k C6 0.1μF VCC 6 + IN+ REF 4 VCC SCL C2 0.1μF IN– – REF C8 0.1μF 2 SDA GND GND 1 9 10 8 R6 4.99k 1% R7 4.99k 1% 8 6 7 SCL R8 4.99k 1% VCC 3 2 EXT 1 9 12 14 WP C5 0.1μF 24LC025-I/ST E4 GND E6 REF– 7 LTC2453 5 C7 0.1μF 11 SDA A2 A1 5 VUNREG 5V CS SCK/SCL MOSI/SDA MISO TO CONTROLLER J1 EESCL EEVCC EESDA EEGND NC GND GND GND 3 8 13 2453 TA02 A0 GND JP2 GND 4 2453f 14 LTC2453 PACKAGE DESCRIPTION DDB Package 8-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1702 Rev B) 0.61 ±0.05 (2 SIDES) 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.20 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 5 R = 0.05 TYP 0.40 ± 0.10 8 2.00 ±0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.56 ± 0.05 (2 SIDES) 0.200 REF 0.75 ±0.05 0 – 0.05 4 0.25 ± 0.05 1 PIN 1 R = 0.20 OR 0.25 × 45° CHAMFER (DDB8) DFN 0905 REV B 0.50 BSC 2.15 ±0.05 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 2453f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LTC2453 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max, 5ppm/°C Drift LT1461 Micropower Series Reference, 2.5V 0.04% Max, 3ppm/°C Drift LT1790 Micropower Precision Reference in TSOT-23-6 Package 60μA Max Supply Current, 10ppm/°C Max Drift, 1.25V, 2.048V, 2.5V, 3V, 3.3V, 4.096V and 5V Options LTC1860/LTC1861 12-Bit, 5V, 1-/2-Channel 250ksps SAR ADC in MSOP 850μA at 250ksps, 2μA at 1ksps, SO-8 and MSOP Packages LTC1860L/LTC1861L 12-Bit, 3V, 1-/2-Channel 150ksps SAR ADC 450μA at 150ksps, 10μA at 1ksps, SO-8 and MSOP Packages LTC1864/LTC1865 16-Bit, 5V, 1-/2-Channel 250ksps SAR ADC in MSOP 850μA at 250ksps, 2μA at 1ksps, SO-8 and MSOP Packages LTC1864L/LTC1865L 16-bit, 3V, 1-/2-Channel 150ksps SAR ADC 450μA at 150ksps, 10μA at 1ksps, SO-8 and MSOP Packages LTC2440 24-Bit No Latency ΔΣTMADC 200nVRMS Noise, 8kHz Output Rate, 15ppm INL LTC2480 16-Bit, Differential Input, No Latency ΔΣ ADC, with PGA, Temp. Sensor, SPI Easy-Drive Input Current Cancellation, 600nVRMS Noise, Tiny 10-Lead DFN Package LTC2481 16-Bit, Differential Input, No Latency ΔΣ ADC, with PGA, Temp. Sensor, I2C Easy-Drive Input Current Cancellation, 600nVRMS Noise, Tiny 10-Lead DFN Package LTC2482 16-Bit, Differential Input, No Latency ΔΣ ADC, SPI Easy-Drive Input Current Cancellation, 600nVRMS Noise, Tiny 10-Lead DFN Package LTC2483 16-Bit, Differential Input, No Latency ΔΣ ADC, I2C Easy-Drive Input Current Cancellation, 600nVRMS Noise, Tiny 10-Lead DFN Package LTC2484 24-Bit, Differential Input, No Latency ΔΣ ADC, SPI Easy-Drive Input Current Cancellation, 600nVRMS Noise, Tiny 10-Lead DFN Package LTC2485 24-Bit, Differential Input, No Latency ΔΣ ADC, I2C Easy-Drive Input Current Cancellation, 600nVRMS Noise, Tiny 10-Lead DFN Package LTC6241 Dual, 18MHz, Low Noise, Rail-to-Rail Op Amp 550nVP-P Noise, 125μV Offset Max LT6660 Micropower References in 2mm × 2mm DFN Package, 2.5V, 3V, 3.3V, 5V 20ppm/°C max drift, 0.2% Max LTC2450 Easy-to-Use, Ultra-Tiny 16-Bit ADC 2 LSB INL, 50nA Sleep current, Tiny 2mm × 2mm DFN-6 Package, 30Hz Output Rate LTC2450-1 Easy-to-Use, Ultra-Tiny 16-Bit ADC 2 LSB INL, 50nA Sleep Current, Tiny 2mm × 2mm DFN-6 Package, 60Hz Output Rate No Latency ΔΣ is a trademark of Linear Technology Corporation. 2453f 16 Linear Technology Corporation LT 1007 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007