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LTC2495

2495fd

Features

applications

Description

16-Bit 8-/16-Channel

ADC with PGA, Easy Drive

and I

C Interface

The LTC

2495 is a 16-channel (eight differential), 16-bit,

No Latency DS

ADC with Easy Drive technology and a

2-wire, I

C interface. The patented sampling scheme elimi-

nates dynamic input current errors and the shortcomings

of on-chip buffering through automatic cancellation of

differential input current. This allows large external source

impedances and rail-to-rail input signals to be directly

digitized while maintaining exceptional DC accuracy.

The LTC2495 includes programmable gain, a high accuracy

temperature sensor, and an integrated oscillator. This device

can be conﬁgured to measure an external signal (from com-

binations of 16 analog input channels operating in single-

ended or differential modes) or its internal temperature

sensor. The integrated temperature sensor offers 1/2°C

resolution and 2°C absolute accuracy. The LTC2495 can

be conﬁgured to provide a programmable gain from 1 to

256 in 8 steps.

The LTC2495 allows a wide common mode input range

(0V to V

), independent of the reference voltage. Any

combination of single-ended or differential inputs can

be selected and the ﬁrst conversion, after a new channel

is selected, is valid. Access to the multiplexer output en-

ables optional external ampliﬁers to be shared between all

analog inputs and auto calibration continuously removes

their associated offset and drift.

Data Acquisition System with Temperature Compensation

Up to Eight Differential or 16 Single-Ended Inputs

Easy Drive

Technology Enables Rail-to-Rail

Inputs with Zero Differential Input Current

Directly Digitizes High Impedance Sensors with

Full Accuracy

2-Wire I

C Interface with 27 Addresses Plus One

Global Address for Synchronization

600nV RMS Noise

Programmable Gain from 1 to 256

Integrated High Accuracy Temperature Sensor

GND to V

Input/Reference Common Mode Range

Programmable 50Hz, 60Hz, or Simultaneous

50Hz/60Hz Rejection Mode

2ppm INL, No Missing Codes

1ppm Offset and 15ppm Full-Scale Error

2x Speed/Reduced Power Mode (15Hz Using Internal

Oscillator and 80µA at 7.5Hz Output)

No Latency: Digital Filter Settles in a Single Cycle,

Even After a New Channel is Selected

Single Supply 2.7V to 5.5V Operation (0.8mW)

Internal Oscillator

Tiny 5mm × 7mm QFN Package

Direct Sensor Digitizer

Direct Temperature Measurement

Instrumentation

Industrial Process Control

Built-In High Performance Temperature Sensor

SCL

SDA

REF

MUXOUT/

ADCIN

MUXOUT/

ADCIN

2.7V TO 5.5V

10µF

1.7k

COM

REF

–

16-BIT $3 ADC

WITH EASY DRIVE

16-CHANNEL

MUX

TEMPERATURE

SENSOR

–

2495 TA01

2-WIRE

C INTERFACE

CH0

CH1

•

CH7

CH8

CH15

0.1µF

OSC

TEMPERATURE (°C)

–55 –30 –5

ABSOLUTE ERROR (°C)

–4

–3

–2

–1

12095704520

2495 TA02

–5

typical application

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

No Latency DS and Easy Drive are trademarks of Linear Technology Corporation. All other

trademarks are the property of their respective owners.

Summary of content (34 pages)

PAGE 1
LTC2495 16-Bit 8-/16-Channel DS ADC with PGA, Easy Drive and I2C Interface Description Features n n n n n n n n n n n n n n n Up to Eight Differential or 16 Single-Ended Inputs Easy DriveTM Technology Enables Rail-to-Rail Inputs with Zero Differential Input Current Directly Digitizes High Impedance Sensors with Full Accuracy 2-Wire I2C Interface with 27 Addresses Plus One Global Address for Synchronization 600nV RMS Noise Programmable Gain from 1 to 256 Integrated High Accuracy Temperature Sensor GN
PAGE 2
LTC2495 Absolute Maximum Ratings Pin Configuration (Notes 1, 2) GND GND GND fO CA0 CA2 CA1 TOP VIEW Supply Voltage (VCC).................................... –0.3V to 6V Analog Input Voltage (CH0-CH15, COM).....................–0.3V to (VCC + 0.3V) REF +, REF –................................–0.3V to (VCC + 0.3V) ADCINN, ADCINP, MUXOUTP, MUXOUTN.................................–0.3V to (VCC + 0.3V) Digital Input Voltage......................–0.3V to (VCC + 0.3V) Digital Output Voltage..............
PAGE 3
LTC2495 Electrical Characteristics (Normal 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 MIN TYP Total Unadjusted Error 5V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V 5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V 2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V 15 15 15 ppm of VREF ppm of VREF ppm of VREF Output Noise 2.7V < VCC < 5.5V, 2.
PAGE 4
LTC2495 Analog Input and Reference The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) VIN Input Differential Voltage Range (IN+ – IN–) l –FS FS Full-Scale of the Differential Input (IN+ – IN–) l 0.5VREF/Gain +FS V V LSB Least Significant Bit of the Output Code l FS/216 REF+ Absolute/Common Mode REF+ Voltage l 0.
PAGE 5
LTC2495 Digital Inputs And Digital Outputs The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL PARAMETER CONDITIONS fEOSC External Oscillator Frequency Range (Note 16) MAX UNITS l tHEO tLEO tCONV_1 Conversion Time for 1x Speed Mode tCONV_2 Conversion Time for 2x Speed Mode MIN 10 TYP 4000 kHz External Oscillator High Period l 0.125 100 µs External Oscillator Low Period l 0.
PAGE 6
LTC2495 Typical Performance Characteristics –45°C 2 25°C 0 85°C –1 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 1 2 –45°C, 25°C, 85°C 0 –1 –2 2 –3 –1.25 2.5 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 2495 G01 25°C 4 0 12 85°C –45°C –4 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 85°C 25°C –45°C 0 –4 2.5 –12 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.25 2495 G03 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.
PAGE 7
LTC2495 Typical Performance Characteristics RMS Noise vs Input Differential Voltage 5 VCC = 5V TA = 25°C VREF = 5V RMS NOISE = 0.60µV VIN = 0V GAIN = 256 VIN(CM) = 2.5V 4 3 2 1 0 –1 –2 –3 0 10 30 40 20 TIME (HOURS) 50 0.7 0.6 2495 G09 RMS NOISE (µV) 0.6 0.5 1.0 VREF = 2.5V VIN = 0V VIN(CM) = GND TA = 25°C fO = GND GAIN = 256 0.8 0.7 0.6 75 0.4 2.7 90 3.1 3.5 3.9 4.3 VCC (V) 5.1 4.7 2495 G12 0.1 0 6 VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C fO = GND GAIN = 256 0.8 0.7 0.6 0.4 5.
PAGE 8
LTC2495 Typical Performance Characteristics 0.2 0.1 0.3 REF+ = 2.5V REF– = GND VIN = 0V VIN(CM) = GND TA = 25°C fO = GND 0 0.1 –0.2 0 3.1 3.9 4.3 VCC (V) 3.5 4.7 5.1 –0.3 5.5 0 1 2 3 VREF (V) 4 On-Chip Oscillator Frequency vs VCC VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 3.0 3.5 4.0 VCC (V) 4.5 5.0 5.5 –20 –40 –60 –80 –120 –120 10 10k 100k 1k 100 FREQUENCY AT VCC (Hz) 2495 G20 CONVERSION CURRENT (µA) REJECTION (dB) 200 VCC = 4.1V DC p0.7V V = 2.
PAGE 9
LTC2495 Typical Performance Characteristics 2.0 500 fO = GND VREF = VCC IN+ = GND IN– = GND fO = EXT OSC TA = 25°C 450 1.6 SUPPLY CURRENT (µA) SLEEP MODE CURRENT (µA) 1.8 Conversion Current vs Output Data Rate 1.4 1.2 VCC = 5V 1.0 0.8 0.6 VCC = 2.7V 0.4 400 350 3 VCC = 5V VCC = 3V 0 –45 –30 –15 0 15 30 45 60 TEMPERATURE (°C) 75 100 90 –1 0 0 –1 –45°C, 25°C 16 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V fO = GND 85°C 0 –45°C, 25°C –1 –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.25 –3 –1.
PAGE 10
LTC2495 Typical Performance Characteristics Offset Error vs VCC (2x Speed Mode) 240 220 210 200 190 180 240 VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 200 OFFSET ERROR (µV) 230 OFFSET ERROR (µV) 250 VCC = 5V VREF = 5V VIN = 0V VIN(CM) = GND fO = GND Offset Error vs VREF (2x Speed Mode) 150 100 210 200 190 170 0 15 30 45 60 TEMPERATURE (°C) 75 0 90 2 2.5 3 4 3.5 VCC (V) 4.
PAGE 11
LTC2495 Pin Functions GND (Pins 1, 4, 6, 31, 32, 33, 34): Ground. Multiple ground pins internally connected for optimum ground current flow and VCC decoupling. Connect each one of these pins to a common ground plane through a low impedance connection. All seven pins must be connected to ground for proper operation. SCL (Pin 2): Serial Clock Pin of the I2C Interface. The LTC2495 can only act as a slave and the SCL pin only accepts an external serial clock.
PAGE 12
LTC2495 FUNCTIONAL Block Diagram TEMP SENSOR VCC GND INTERNAL OSCILLATOR CH0 CH1 CH15 COM – • • • MUX fO (INT/EXT) AUTOCALIBRATION AND CONTROL MUXOUTP ADCINP REF+ REF– + DIFFERENTIAL 3RD ORDER $3 MODULATOR I2C 2-WIRE INTERFACE SDA SCL DECIMATING FIR ADDRESS 2495 BD MUXOUTN ADCINN 2495fd 12
PAGE 13
LTC2495 Applications Information CONVERTER OPERATION Converter Operation Cycle The LTC2495 is a multichannel, low power, delta-sigma, analog-to-digital converter with a 2-wire, I2C interface. Its operation is made up of four states (see Figure 1). The converter operating cycle begins with the conversion, followed by the sleep state, and ends with the data input/output cycle. Initially, at power-up, the LTC2495 performs a conversion. Once the conversion is complete, the device enters the sleep state.
PAGE 14
LTC2495 Applications Information cancellation does not interfere with the transparent offset and full-scale auto-calibration and the absolute accuracy (full-scale + offset + linearity + drift) is maintained even with external RC networks. Power-Up Sequence The LTC2495 automatically enters an internal reset state when the power supply voltage, VCC, drops below a threshold of approximately 2.0V. This feature guarantees the integrity of the conversion result and input channel selection.
PAGE 15
LTC2495 Applications Information I2C INTERFACE The START and STOP Conditions The LTC2495 communicates through an I2C interface. The I2C interface is a 2-wire, open-drain interface supporting multiple devices and multiple masters on a single bus. The connected devices can only pull the data line (SDA) LOW and can never drive it HIGH. SDA is required to be externally connected to the supply through a pull-up resistor. When the data line is not being driven, it is HIGH.
PAGE 16
LTC2495 Applications Information DATA FORMAT The data output stream is 24 bits long and is shifted out on the falling edges of SCL (see Figure 3a). The first bit is the conversion result sign bit (SIG) (see Tables 1 and 2). This bit is HIGH if VIN ≥ 0 and LOW if VIN < 0 (where VIN corresponds to the selected input signal IN+ – IN–). The second bit is the most significant bit (MSB) of the result. The first two bits (SIG and MSB) can be used to indicate over and underrange conditions (see Table 2).
PAGE 17
LTC2495 Applications Information and 1x output rate (auto-calibration enabled), and gain = 1. The first conversion automatically begins at power-up using this default configuration. Once the conversion is complete, up to two words may be written into the device. Table 2. LTC2495 Status Bits Bit 23 SIG Bit 22 MSB 1 1 0V ≤ VIN < FS 1/ 0 0 –FS ≤ VIN < 0V 0 1 VIN < –FS 0 0 Input Range VIN ≥ FS The first three bits of the first input word consist of two preamble bits and one enable bit.
PAGE 18
LTC2495 Applications Information Table 3.
PAGE 19
LTC2495 Applications Information one of 16 channels is selected as the positive input. The negative input is COM for all single-ended operations. The remaining four bits (ODD, A2, A1, A0) determine which channel(s) is/are selected and the polarity (for a differential input). Once the first word is written into the device, a second word may be input in order to select a configuration mode. The first bit of the second word is the enable bit for the conversion configuration (EN2).
PAGE 20
LTC2495 Applications Information Rejection Mode (FA, FB) GAIN (GS2, GS1, GS0) The LTC2495 includes a high accuracy on-chip oscillator with no required external components. Coupled with an integrated fourth-order digital lowpass filter, the LTC2495 rejects line frequency noise. In the default mode, the LTC2495 simultaneously rejects 50Hz and 60Hz by at least 87dB. If more rejection is required, the LTC2495 can be configured to reject 50Hz or 60Hz to better than 110dB.
PAGE 21
LTC2495 Applications Information The internal temperature sensor output is 28mV at 27°C (300°K), with a slope of 93.5µV/°C independent of VREF (see Figures 4 and 5). Slope calibration is not required if the reference voltage (VREF) is known. A 5V reference has a slope of 2.45 LSBs16/°C. The temperature is calculated from the output code (where DATAOUT16 is the decimal representation of the 16-bit result) for a 5V reference using the following formula: TK = DATAOUT16 in Kelvin 2 .
PAGE 22
LTC2495 Applications Information Operation Sequence Table 6.
PAGE 23
LTC2495 Applications Information S 7-BIT ADDRESS CONVERSION W ACK SLEEP WRITE Sr 7-BIT ADDRESS DATA INPUT R ACK ADDRESS READ DATA OUTPUT P CONVERSION 2495 F08 Figure 8. Write, Read, START Conversion S 7-BIT ADDRESS CONVERSION W ACK SLEEP WRITE (OPTIONAL) DATA INPUT P CONVERSION 2495 F09 Figure 9.
PAGE 24
LTC2495 Applications Information IIN+ IN+ INPUT MULTIPLEXER 100Ω EXTERNAL CONNECTION MUXOUTP ADCINP INTERNAL SWITCH NETWORK ( ) I IN+ 10k AVG ( ) I REF + IREF+ AVG ≈ AVG = VIN(CM) − VREF(CM) ( 0.5 • REQ 1.5VREF + VREF(CM) – VIN(CM) 0.5 • REQ )– 2 VIN VREF • REQ where : IIN– IN– ( ) = I IN– 100Ω MUXOUTN ADCINN VREF = REF + − REF − 10k  REF + – REF −  VREF(CM) =     2 EXTERNAL CONNECTION REF+ CEQ 12µF 10k  IN+ – IN−  VIN(CM) =     2 REQ = 2.
PAGE 25
LTC2495 Applications Information independent of external settling errors. This allows direct digitization of high impedance sensors without the need for buffers. of 1.5V, the common mode input current is approximately 0.74µA (in simultaneous 50Hz/60Hz rejection mode). This common mode input current does not degrade the accuracy if the source impedances tied to IN+ and IN– are matched. Mismatches in source impedance lead to a fixed offset error but do not effect the linearity or full-scale reading.
PAGE 26
LTC2495 Applications Information current. This current, nominally 1nA (±10nA max), results in a small offset shift. A 1k source resistance will create a 1µV typical and a 10µV maximum offset voltage. Automatic Offset Calibration of External Buffers/ Amplifiers In addition to the Easy Drive input current cancellation, the LTC2495 allows an external amplifier to be inserted between the multiplexer output and the ADC input (see Figure 12).
PAGE 27
LTC2495 Applications Information –100 CREF = 0.1µF 200 CREF = 0.01µF CREF = 0.01µF –200 CREF = 1µF, 10µF –300 100 –400 0 –500 0 200 600 400 RSOURCE (Ω) 800 1000 INL (ppm OF VREF) 300 10 VCC = 5V VREF = 5V VIN+ = 1.25V VIN– = 3.75V fO = GND TA = 25°C 0 200 2495 F15 Figure 15. +FS Error vs RSOURCE at VREF (Large CREF) CREF = 0.1µF In addition to the reference sampling charge, the reference ESD protection diodes have a temperature dependent leakage current.
PAGE 28
LTC2495 Applications Information 0 fN = fEOSC/5120 INPUT NORMAL MODE REJECTION (dB) INPUT NORMAL MODE REJECTION (dB) 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 0 fN 2fN 3fN 4fN 5fN 6fN 7fN INPUT SIGNAL FREQUENCY (Hz) 8fN fN = fEOSC/5120 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 250fN 252fN 254fN 256fN 258fN 260fN 262fN INPUT SIGNAL FREQUENCY (Hz) 2495 F20 2495 F21 Figure 20. Input Normal Mode Rejection at DC Figure 21.
PAGE 29
LTC2495 Applications Information The user can expect to achieve this level of performance using the internal oscillator, as shown in Figures 22, 23, and 24. Measured values of normal mode rejection are shown superimposed over the theoretical values in all three rejection modes. NORMAL MODE REJECTION (dB) 0 MEASURED DATA CALCULATED DATA –20 –40 VCC = 5V VREF = 5V VIN(CM) = 2.
PAGE 30
LTC2495 Applications Information Using the 2x speed mode of the LTC2495 alters the rejection characteristics around DC and multiples of fS. The device bypasses the offset calibration in order to increase the output rate. The resulting rejection plots are shown in Figures 27 and 28. 1x type frequency rejection can be achieved using the 2x mode by performing a running average of the previous two conversion results (see Figure 29).
PAGE 31
LTC2495 Applications Information 50 OFFSET ERROR (ppm OF VREF) NO AVERAGE –90 WITH RUNNING AVERAGE –100 –110 –120 –130 –140 40 30 3000 TA = 85°C 20 10 –10 0 RESOLUTION (BITS) –2000 TA = 85°C –2500 16 V = VREF(CM) –3000 IN(CM) VCC = VREF = 5V fO = EXT CLOCK –3500 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 10 2495 F32 VIN(CM) = VREF(CM) VCC = VREF = 5V VIN = 0V fO = EXT CLOCK RES = LOG 2 (VREF/NOISERMS) 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) R
PAGE 32
LTC2495 Package Description UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 p 0.05 5.50 p 0.05 5.15 ± 0.05 4.10 p 0.05 3.00 REF 3.15 ± 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 5.5 REF 6.10 p 0.05 7.50 p 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 p 0.10 0.75 p 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 s 45o CHAMFER 3.00 REF 37 0.00 – 0.05 38 0.40 p0.10 PIN 1 TOP MARK (SEE NOTE 6) 1 2 5.15 ± 0.10 5.50 REF 7.
PAGE 33
LTC2495 Revision History (Revision history begins at Rev C) REV DATE DESCRIPTION C 11/09 Update Tables 1 and 2 PAGE NUMBER D 07/10 Revised Typical Application drawing Added fO pin to parameter of VIHA 16 in I2C Inputs and Outputs section Added information to I2C Interface section 1 4 15 2495fd Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 34
LTC2495 Typical Application External Buffers Provide High Impedance Inputs and Amplifier Offsets are Automatically Cancelled $3 ADC WITH EASY DRIVE INPUTS MUXOUTN INPUT MUX MUXOUTP LTC2495 ANALOG 17 INPUTS 2 – 1/2 LTC6078 3 + 6 – 5 1/2 LTC6078 + 1 SDA SCL 1k 0.1µF 7 1k 0.1µF 2495 TA03 Related Parts PART NUMBER DESCRIPTION COMMENTS LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max Initial Accuracy, 5ppm/°C Drift LT1460 Micropower Series Reference 0.