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LTC2498

2498ff

Features

applications

Description

24-Bit 8-/16-Channel ΔS

ADC with Easy Drive Input

Current Cancellation

The LTC

2498 is a 16-channel (8-differential) 24-bit No

Latency ΔS™ ADC with Easy Drive technology. The pat-

ented sampling scheme eliminates 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 LTC2498 includes a high accuracy temperature sensor

and an integrated oscillator. This device can be configured

to measure an external signal (from combinations of 16

analog input channels operating in single ended or dif-

ferential modes) or its internal temperature sensor. The

integrated temperature sensor offers 1/30th °C resolution

and 2°C absolute accuracy.

The LTC2498 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 enables

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 8 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

600nV RMS Noise

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 Mode (15Hz Using Internal Oscillator)

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 QFN 5mm × 7mm Package

Direct Sensor Digitizer

Direct Temperature Measurement

Instrumentation

Industrial Process Control

Internal Sensor

Absolute Temperature Error

REF

MUXOUT/

ADCIN

MUXOUT/

ADCIN

2.7V TO 5.5V

10µF

COM

REF

–

24-BIT ∆Σ ADC

WITH EASY DRIVE

16-CHANNEL

MUX

TEMPERATURE

SENSOR

–

2498 TA01a

CH0

CH1

•

CH7

CH8

CH15

0.1µF

OSC

SDI

SCK

SDO

4-WIRE

SPI INTERFACE

TEMPERATURE (°C)

–55 –30 –5

ABSOLUTE ERROR (°C)

–4

–3

–2

–1

12095704520

2498 TA01b

–5

typical application

L, LT, LT C, LT M , Linear Technology and the Linear logo are registered trademarks and

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

trademarks are the property of their respective owners.

Summary of content (38 pages)

PAGE 1
LTC2498 24-Bit 8-/16-Channel ΔS ADC with Easy Drive Input Current Cancellation Description Features n n n n n n n n n n n n n n Up to 8 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 600nV RMS Noise Integrated High Accuracy Temperature Sensor GND to VCC Input/Reference Common Mode Range Programmable 50Hz, 60Hz or Simultaneous 50Hz/60Hz Rejection Mode 2ppm INL, No
PAGE 2
LTC2498 Pin Configuration Supply Voltage (VCC).................................... –0.3V to 6V Analog Input Voltage (CH0 to CH15, COM)..................–0.3V to (VCC + 0.3V) Reference Input Voltage ADCINN, ADCINP, MUXOUTP, MUXOUTN.................................–0.3V to (VCC + 0.3V) Digital Input Voltage......................–0.3V to (VCC + 0.3V) Digital Output Voltage....................–0.3V to (VCC + 0.3V) Operating Temperature Range LTC2498C.................................................
PAGE 3
LTC2498 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 5.5V < VCC < 2.7V, 2.
PAGE 4
LTC2498 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) SYMBOL PARAMETER VIN Input Differential Voltage Range (IN+ – IN–) CONDITIONS MIN l –FS FS Full Scale of the Differential Input (IN+ – IN–) l 0.5VREF FS/224 TYP MAX UNITS +FS V V LSB Least Significant Bit of the Output Code l REF+ Absolute/Common Mode REF+ Voltage l 0.
PAGE 5
LTC2498 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 (Note 16) MIN TYP MAX UNITS fEOSC External Oscillator Frequency Range l 10 4000 kHz tHEO External Oscillator High Period l 0.125 50 µs tLEO External Oscillator Low Period l 0.
PAGE 6
LTC2498 Typical Performance Characteristics 3 –45°C 1 25°C 0 85°C –1 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 2 INL (ppm OF VREF) 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 –0.75 0 VCC = 5V VREF = 5V VIN(CM) = 1.25V fO = GND 8 85°C –45°C –4 2 12 85°C –4 Noise Histogram (6.8sps) 12 12 NUMBER OF READINGS (%) 8 6 4 –0.75 –4 –0.25 0.25 0.75 INPUT VOLTAGE (V) 2498 G06 VCC = 5V, VREF = 5V, VIN = 0V, VIN(CM) = 2.
PAGE 7
LTC2498 Typical Performance Characteristics RMS Noise vs Input Differential Voltage RMS NOISE (ppm OF VREF) 0.9 RMS Noise vs VIN(CM) 1.0 VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C VCC = 5V VREF = 5V VIN = 0V VIN(CM) = GND TA = 25°C 0.9 RMS NOISE (µV) 0.8 0.7 0.6 0.5 RMS Noise vs Temperature (TA) 1.0 0.8 0.7 0.6 0.4 2.5 –1 0 2 1 3 5 4 RMS NOISE (µV) RMS NOISE (µV) RMS Noise vs VREF 0.6 0.3 VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C 0.9 0.7 0.8 0.7 0.6 0.5 0.5 3.1 3.5 3.9 4.
PAGE 8
LTC2498 Typical Performance Characteristics 310 310 VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 308 FREQUENCY (kHz) 306 304 VCC = 4.1V VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND 300 –45 –30 –15 306 304 75 300 90 2.5 3.0 3.5 4.0 VCC (V) 4.5 5.0 –80 –100 –140 200 VCC = 4.1V DC ±0.7V = 2.5V V –20 INREF + = GND – = GND IN –40 fO = GND TA = 25°C –60 –80 –140 30600 30650 30700 30750 FREQUENCY AT VCC (Hz) 2498 G22 SUPPLY CURRENT (µA) 0.8 VCC = 2.7V 0.
PAGE 9
LTC2498 Typical Performance Characteristics 1 0 –45°C, 25°C –2 1 90°C 0 –45°C, 25°C –1 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) –3 –1.25 1.25 6 4 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 200 OFFSET ERROR (µV) 0.4 VCC = 5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 1 0 240 194 192 190 188 186 180 5 –1 240 1 0 3 VIN(CM) (V) 2 4 5 220 0 4.5 5 5.5 –40 210 200 190 2498 G34 160 90 VCC = 4.1V DC REF+ = 2.
PAGE 10
LTC2498 Typical Performance Characteristics PSRR vs Frequency at VCC (2x Speed Mode) PSRR vs Frequency at VCC (2x Speed Mode) RREJECTION (dB) –20 –40 –60 0 VCC = 4.1V DC ±1.4V REF+ = 2.5V REF– = GND IN+ = GND IN– = GND fO = GND TA = 25°C –80 –100 –80 –100 –120 –140 VCC = 4.1V DC ±0.7V REF+ = 2.
PAGE 11
LTC2498 Pin Functions SDI (Pin 34): Serial Data Input. This pin is used to select the line frequency rejection mode, 1x or 2x mode, temperature sensor, as well as the input channel. The serial data input is applied under control of the serial clock (SCK) during the data output/input operation. The first conversion following a new input or mode change is valid. fO (Pin 35): Frequency Control Pin. Digital input that controls the internal conversion clock rate.
PAGE 12
LTC2498 Functional Block Diagram TEMP SENSOR VCC INTERNAL OSCILLATOR MUXOUTP ADCINP GND CH0 CH1 CH15 COM – • • • fO (INT/EXT) AUTOCALIBRATION AND CONTROL REF+ REF– + DIFFERENTIAL 3RD ORDER ΔΣ MODULATOR MUX SDI SCK SDO CS SERIAL INTERFACE DECIMATING FIR ADDRESS 2498 BD MUXOUTN ADCINN Figure 1. Functional Block Diagram Test Circuits VCC SDO 1.69k 1.
PAGE 13
LTC2498 timing diagrams Timing Diagram Using Internal SCK (SCK HIGH with CS↓) CS t1 t2 SDO tKQMIN t3 tKQMAX SCK t7 t8 SDI 2498 TD01 SLEEP DATA IN/OUT CONVERSION Timing Diagram Using External SCK (SCK LOW with CS↓) CS t1 t2 SDO t5 SCK tKQMIN t6 t4 t7 tKQMAX t8 SDI 2498 TD02 SLEEP DATA IN/OUT CONVERSION 2498ff 13
PAGE 14
LTC2498 Applications Information Converter Operation Converter Operation Cycle The LTC2498 is a multi-channel, low power, delta-sigma analog-to-digital converter with an easy to use 4-wire interface and automatic differential input current cancellation. Its operation is made up of three states (See Figure 2). The converter operating cycle begins with the conversion, followed by the sleep state and ends with the data input/ output cycle.
PAGE 15
LTC2498 Applications Information enables external RC networks and high impedance sensors to directly interface to the LTC2498 without external amplifiers. The remaining common mode input current is eliminated by either balancing the differential input impedances or setting the common mode input equal to the common mode reference (see the Automatic Differential Input Current Cancellation section).
PAGE 16
LTC2498 Applications Information Serial Interface Pins The LTC2498 transmits the conversion result, reads the input configuration, and receives a start of conversion command through a synchronous 3- or 4-wire interface. During the conversion and sleep states, this interface can be used to access the converter status. During the data output state, it is used to read the conversion result, program the input channel, rejection frequency, speed multiplier, and select the temperature sensor.
PAGE 17
LTC2498 Applications Information +FS. If both Bit 29 and Bit 28 are LOW, the differential input voltage is below –FS. The function of these bits is summarized in Table 1. Table 1. LTC2498 Status Bits Input Range VIN ≥ 0.5 • VREF Bit 31 EOC Bit 30 DMY Bit 29 SIG Bit 28 MSB 0 0 1 1 0V ≤ VIN < 0.5 • VREF 0 0 1/0 0 –0.5 • VREF ≤ VIN < 0V 0 0 0 1 VIN < –0.5 • VREF 0 0 0 0 Bits 28 to 5 are the 24-bit conversion result MSB first. Bit 5 is the least significant bit (LSB24).
PAGE 18
LTC2498 Applications Information Table 2. Output Data Format Differential Input Voltage VIN* Bit 31 EOC Bit 30 DMY Bit 29 SIG Bit 28 MSB Bit 27 Bit 26 Bit 25 … Bit 0 VIN* ≥ 0.5 • VREF** 0 0 1 1 0 0 0 … 0 0.5 • VREF** – 1LSB 0 0 1 0 1 1 1 … 1 0.25 • VREF** 0 0 1 0 1 0 0 … 0 0.25 • VREF** – 1LSB 0 0 1/0*** 0 0 1 1 … 1 0 0 0 1 0 0 0 0 … 0 –1LSB 0 0 0 1 1 1 1 … 1 –0.25 • VREF** 0 0 0 1 1 0 0 … 0 –0.
PAGE 19
LTC2498 Applications Information Table 3.
PAGE 20
LTC2498 Applications Information Table 4.
PAGE 21
LTC2498 Applications Information Slope calibration is not required if the reference voltage (VREF) is known. A 5V reference has a slope of 314 LSBs24/°C.
PAGE 22
LTC2498 Applications Information External Serial Clock, Single Cycle Operation When the device is in the sleep state, its conversion result is held in an internal static shift register. The device remains in the sleep state until the first rising edge of SCK is seen while CS is LOW. The input data is then shifted in via the SDI pin on each rising edge of SCK (including the first rising edge). The channel selection and converter configuration mode will be used for the following conversion cycle.
PAGE 23
LTC2498 Applications Information Typically, CS remains LOW during the data output/input state. However, the data output state may be aborted by pulling CS HIGH any time between the 1st falling edge and the 32nd falling edge of SCK, see Figure 7. On the rising edge of CS, the device aborts the data output state and immediately initiates a new conversion. In order to program a new input channel, 8 SCK clock pulses are required.
PAGE 24
LTC2498 Applications Information 10µF 2.7V TO 5.5V 28 0.1µF VCC fO = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 35 LTC2498 REFERENCE VOLTAGE 0.
PAGE 25
LTC2498 Applications Information 10µF 2.7V TO 5.5V 28 VCC = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 35 fO LTC2498 0.1µF REFERENCE VOLTAGE 0.
PAGE 26
LTC2498 Applications Information 2.7V TO 5.5V 10µF 28 VCC fO = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 35 LTC2498 0.1µF REFERENCE VOLTAGE 0.
PAGE 27
LTC2498 Applications Information The input data is shifted through the SDI pin on the rising edge of SCK (including the first rising edge) and the output data is shifted out the SDO pin on the falling edge of SCK. The data input/output cycle is concluded and a new conversion automatically begins after the 32nd rising edge of SCK. During the next conversion, SCK and SDO remain HIGH until the conversion is complete.
PAGE 28
LTC2498 Applications Information Driving the Input and Reference terminals in order to filter unwanted noise (anti-aliasing) results in incomplete settling. The input and reference pins of the LTC2498 are connected directly to a switched capacitor network. Depending on the relationship between the differential input voltage and the differential reference voltage, these capacitors are switched between these four pins.
PAGE 29
LTC2498 Applications Information independent of external settling errors. This allows direct digitization of high impedance sensors without the need of buffers. stability of the internal sampling capacitors and the accuracy of the internal oscillator, a one-time calibration will remove this error. The switching algorithm forces the average input current on the positive input (IIN+) to be equal to the average input current in the negative input (IIN–).
PAGE 30
LTC2498 Applications Information Reference Current Similar to the analog inputs, the LTC2498 samples the differential reference pins (REF+ and REF–) transferring small amounts of charge to and from these pins, thus producing a dynamic reference current. If incomplete settling occurs (as a function the reference source resistance and reference bypass capacitance) linearity and gain errors are introduced.
PAGE 31
LTC2498 Applications Information 500 VCC = 5V VREF = 5V VIN+ = 3.75V VIN– = 1.25V fO = GND TA = 25°C +FS ERROR (ppm) 400 300 CREF = 1µF, 10µF CREF = 0.1µF 200 CREF = 0.01µF 100 0 200 0 600 400 RSOURCE (Ω) 800 1000 2498 F16 Figure 16. +FS Error vs RSOURCE at VREF (Large CREF) The SINC4 digital filter provides excellent normal mode rejection at all frequencies except DC and integer multiples of the modulator sampling frequency (fS), see Figures 19 and 20.
PAGE 32
LTC2498 Applications Information noise to 15dB (see Figure 22), if noise sources are present at these frequencies anti-aliasing will reduce their effects. Traditional high order delta-sigma modulators suffer from potential instabilities at large input signal levels. The proprietary architecture used for the LTC2498 third order modulator resolves this problem and guarantees stability with input signals 150% of full-scale.
PAGE 33
LTC2498 Applications Information Using the 2x speed mode of the LTC2498 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 28 and 29. 1x type frequency rejection can be achieved using the 2x mode by performing a running average of the conversion results, see Figure 30. Output Data Rate When using its internal oscillator, the LTC2498 produces up to 7.
PAGE 34
LTC2498 Applications Information time required for complete settling (580ns for fEOSC = 307.2kHz) is reduced, proportionally. mode rejection of line frequencies. The common mode rejection of line frequencies remains unchanged, thus fully differential input signals with a high degree of symmetry on both the IN+ and IN– pins will continue to reject line frequency noise.
PAGE 35
LTC2498 Applications Information 24 22 10 5 VCC = VREF = 5V 0 –5 –10 22 VCC = VREF = 5V VCC = 5V, VREF = 2.5V 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2480 F36 Figure 36. Offset Error vs Output Data Rate and Reference Voltage 20 20 VCC = 5V, VREF = 2.5V 18 16 14 VIN(CM) = VREF(CM) VIN = 0V fO = EXT CLOCK 12 T = 25°C A RES = LOG 2 (VREF/NOISERMS) 10 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2498 F37 Figure 37.
PAGE 36
LTC2498 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 ± 0.05 5.50 ± 0.05 5.15 ± 0.05 4.10 ± 0.05 3.00 REF 3.15 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 5.5 REF 6.10 ± 0.05 7.50 ± 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 ± 0.10 0.75 ± 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER 3.
PAGE 37
LTC2498 Revision History (Revision history begins at Rev D) REV DATE DESCRIPTION D 11/09 Update Tables 1 and 2 E 07/10 Revised Typical Application drawing F 02/12 PAGE NUMBER 17, 18 1 Added Note 18 to Digital Inputs and Digital Outputs section 4, 5 Updated title of TA01b 1 2, 4 Added H-Grade 2498ff Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 38
LTC2498 Typical Application External Buffers Provide High Impedance Inputs and Amplifier Offsets Are Automatically Cancelled LTC2498 ΔΣ ADC WITH EASY DRIVE INPUTS MUXOUTN INPUT MUX MUXOUTP ANALOG 17 INPUTS SDI SCK SDO CS 2 – 1/2 LT6078 3 + 6 – 5 1 1/2 LT6078 1k 0.1µF 7 1k 0.1µF + 2498 TA02 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.