Datasheet

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LTC2492

2492fd

TEMPERATURE (°C)

–55 –30 –5

ABSOLUTE ERROR (°C)

–4

–3

–2

–1

12095704520

2492 TA01b

–5

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

24-Bit 2-/4-Channel ΔΣ

ADC with Easy Drive Input

Current Cancellation

The LTC

2492 is a 4-channel (2-channel differential),

24-bit, No Latency Δ∑™ ADC with Easy Drive technology.

The patented 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 LTC2492 includes a high accuracy temperature sensor

and an integrated oscillator. This device can be conﬁ gured

to measure an external signal (from combinations of 4

analog input channels operating in single-ended or dif-

ferential modes) or its internal temperature sensor. It can

be programmed to reject line frequencies of 50Hz, 60Hz,

or simultaneous 50Hz/60Hz and conﬁ gured to double

its output rate. The integrated temperature sensor offers

1/30th°C resolution and 2°C absolute accuracy.

The LTC2492 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 ﬁ rst conversion after a new channel

selection is valid.

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

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

trademarks are the property of their respective owners.

Data Acquisition System with Temperature Compensation

■

Up to 2 Differential or 4 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/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 DFN 4mm × 3mm Package

■

Direct Sensor Digitizer

■

Direct Temperature Measurement

■

Instrumentation

■

Industrial Process Control

Absolute Temperature Error

SDI

SCK

SDO

REF

2.7V TO 5.5V

10μF

COM

REF

–

24-BIT Δ∑ ADC

WITH EASY-DRIVE

4-CHANNEL

MUX

TEMPERATURE

SENSOR

–

2492 TA01a

4-WIRE

SPI INTERFACE

CH0

CH1

CH3

CH2

0.1μF

OSC

Summary of content (36 pages)

PAGE 1
LTC2492 24-Bit 2-/4-Channel ΔΣ ADC with Easy Drive Input Current Cancellation DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®2492 is a 4-channel (2-channel differential), 24-bit, No Latency Δ∑™ ADC with Easy Drive technology. The patented sampling scheme eliminates dynamic input current errors and the shortcomings of on-chip buffering through automatic cancellation of differential input current.
PAGE 2
LTC2492 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Notes 1, 2) Supply Voltage (VCC) ...................................– 0.3V to 6V Analog Input Voltage (CH0 to CH3, COM) ................. – 0.3V to (VCC + 0.3V) REF +, REF – .................................. – 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 LTC2492C ................................................
PAGE 3
LTC2492 ELECTRICAL CHARACTERISTICS (2X SPEED) The ● 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) 0.1V ≤ VREF ≤ VCC , –FS ≤ VIN ≤ +FS (Note 5) MIN TYP MAX UNITS Integral Nonlinearity 5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6) 2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.2V (Note 6) ● ● Offset Error 2.
PAGE 4
LTC2492 ANALOG INPUT AND REFERENCE The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. (Note 3) SYMBOL PARAMETER IN + Absolute/Common Mode IN + Voltage (IN+ Corresponds to the Selected Positive Input Channel) CONDITIONS GND – 0.3V MIN TYP VCC + 0.3V MAX UNITS V IN – Absolute/Common Mode IN – Voltage (IN– Corresponds to the Selected Negative Input Channel) GND – 0.3V VCC + 0.
PAGE 5
LTC2492 DIGITAL INPUTS AND DIGITAL OUTPUTS The ● 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) tHEO tLEO tCONV_1 Conversion Time for 1x Speed Mode tCONV_2 Conversion Time for 2x Speed Mode fISCK MAX UNITS ● MIN 10 4000 kHz External Oscillator High Period ● 0.125 100 μs External Oscillator Low Period ● 0.
PAGE 6
LTC2492 TYPICAL PERFORMANCE CHARACTERISTICS Integral Nonlinearity (VCC = 5V, VREF = 5V) 25°C 0 85°C –1 –2 1 –45°C, 25°C, 90°C 0 –1 2 –3 –1.25 2.5 –0.75 Total Unadjusted Error (VCC = 5V, VREF = 2.5V) VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 8 85°C 25°C TUE (ppm OF VREF) TUE (ppm OF VREF) 12 4 0 –45°C –4 –8 2 12 85°C 8 –45°C 0 –4 Noise Histogram (6.8sps) 12 12 NUMBER OF READINGS (%) 4 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) –4 1.2 1.8 2492 G07 –0.75 –0.25 0.25 0.
PAGE 7
LTC2492 TYPICAL PERFORMANCE CHARACTERISTICS RMS Noise vs Input Differential Voltage VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C RMS NOISE (μV) VCC = 5V VREF = 5V VIN = 0V TA = 25°C 0.9 0.8 0.7 0.6 RMS Noise vs Temperature (TA) 1.0 0.8 0.7 0.6 0.5 –1 0 2 1 3 5 4 0.8 RMS NOISE (μV) RMS NOISE (μV) VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C 0.9 0.7 0.6 0.5 0.5 3.5 3.9 4.3 VCC (V) 4.7 5.1 0 5.5 1 2 3 VREF (V) –0.1 –0.2 0 15 30 45 60 TEMPERATURE (°C) –0.1 –0.
PAGE 8
LTC2492 TYPICAL PERFORMANCE CHARACTERISTICS On-Chip Oscillator Frequency vs VCC 310 310 306 304 VCC = 4.1V VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND 300 –45 –30 –15 306 304 300 0 15 30 45 60 TEMPERATURE (°C) 75 90 2.5 3.0 3.5 4.0 VCC (V) 4.5 5.
PAGE 9
LTC2492 TYPICAL PERFORMANCE CHARACTERISTICS Integral Nonlinearity (2x Speed Mode; VCC = 5V, VREF = 2.5V) 2 INL (ppm OF VREF) 2 INL (ppm OF VREF) 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 1 90°C 0 –45°C, 25°C –1 –2 16 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V fO = GND 1 90°C 0 –45°C, 25°C –1 –2 –3 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) –3 –1.25 1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 196 OFFSET ERROR (μV) RMS NOISE (μV) 0.8 0.6 0.
PAGE 10
LTC2492 TYPICAL PERFORMANCE CHARACTERISTICS Offset Error vs VCC (2x Speed Mode) Offset Error vs VREF (2x Speed Mode) VCC = 5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 230 OFFSET ERROR (μV) OFFSET ERROR (μV) 200 0 240 VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 150 100 220 –40 210 200 190 2 2.5 3 4 3.5 VCC (V) 4.5 5.5 5 160 –140 0 1 2 4 3 VREF (V) 2492 G34 –40 –60 10k 100k 1k 100 FREQUENCY AT VCC (Hz) 1M 2492 G36 0 VCC = 4.1V DC ±1.4V REF + = 2.
PAGE 11
LTC2492 PIN FUNCTIONS fO (Pin 1): Frequency Control Pin. Digital input that controls the internal conversion clock rate. When fO is connected to GND, the converter uses its internal oscillator running at 307.2kHz. The conversion clock may also be overridden by driving the fO pin with an external clock in order to change the output rate and the digital ﬁlter rejection null. this pin is used as the conversion status output.
PAGE 12
LTC2492 FUNCTIONAL BLOCK DIAGRAM VCC INTERNAL OSCILLATOR TEMP SENSOR GND fO (INT/EXT) AUTOCALIBRATION AND CONTROL REF + REF – CH0 CH1 CH2 CH3 COM – IN + MUX + DIFFERENTIAL 3RD ORDER Δ∑ MODULATOR IN – SERIAL INTERFACE SDI SCK SDO CS DECIMATING FIR ADDRESS 2492 BD Figure 1. Functional Block Diagram TEST CIRCUITS VCC 1.69k SDO SDO 1.
PAGE 13
LTC2492 TIMING DIAGRAMS Timing Diagram Using Internal SCK (SCK HIGH with CS↓) CS t1 t2 SDO Hi-Z Hi-Z tKQMIN t3 tKQMAX SCK t7 t8 SDI SLEEP DATA IN/OUT CONVERSION 2492 TD01 Timing Diagram Using External SCK (SCK LOW with CS↓) CS t1 t2 SDO Hi-Z Hi-Z t5 tKQMIN t4 tKQMAX SCK t7 t8 SDI SLEEP DATA IN/OUT CONVERSION 2492 TD02 2492fd 13
PAGE 14
LTC2492 APPLICATIONS INFORMATION CONVERTER OPERATION Converter Operation Cycle The LTC2492 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 four 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
LTC2492 APPLICATIONS INFORMATION Easy Drive Input Current Cancellation Reference Voltage Range The LTC2492 combines a high precision delta-sigma ADC with an automatic, differential, input current cancellation front end. A proprietary front-end passive sampling network transparently removes the differential input current. This enables external RC networks and high impedance sensors to directly interface to the LTC2492 without external ampliﬁers.
PAGE 16
LTC2492 APPLICATIONS INFORMATION SERIAL INTERFACE PINS The LTC2492 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
LTC2492 APPLICATIONS INFORMATION Bit 28 (fourth output bit) is the most signiﬁcant bit (MSB) of the result. This bit in conjunction with Bit 29 also provides underrange and overrange indication. If both Bit 29 and Bit 28 are HIGH, the differential input voltage is above +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. LTC2492 Status Bits INPUT RANGE BIT 31 EOC BIT 30 DMY BIT 29 SIG BIT 28 MSB VIN ≥ 0.
PAGE 18
LTC2492 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 5 LSB BITS 4 to 0 SUB LSBs VIN* ≥ 0.5 • VREF** 0 0 1 1 0 0 0 … 0 00000 0.5 • VREF** – 1LSB 0 0 1 0 1 1 1 … 1 XXXXX 0.25 • VREF** 0 0 1 0 1 0 0 … 0 XXXXX 0.25 • VREF** – 1LSB 0 0 1/0† 0 0 1 1 … 1 XXXXX 0 0 0 1 0 0 0 0 … 0 XXXXX –1LSB 0 0 0 1 1 1 1 … 1 XXXXX –0.
PAGE 19
LTC2492 APPLICATIONS INFORMATION Table 4.
PAGE 20
LTC2492 APPLICATIONS INFORMATION from the output code (DATAOUT24) for a 5V reference using the following formula: All Kelvin temperature readings can be converted to TC (°C) using the fundamental equation: TK = DATAOUT24/314 in Kelvin TC = TK – 273 If a different value of VREF is used, the temperature output is: TK = DATAOUT24 • VREF/1570 in Kelvin If the value of VREF is not known, the slope is determined by measuring the temperature sensor at a known temperature TN (in °K) and using the following form
PAGE 21
LTC2492 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 22
LTC2492 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 23
LTC2492 APPLICATIONS INFORMATION 2.7V TO 5.5V 12 = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2492 10μF 0.1μF VCC REFERENCE VOLTAGE 0.
PAGE 24
LTC2492 APPLICATIONS INFORMATION 2.7V TO 5.5V 12 VCC = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2492 10μF REFERENCE VOLTAGE 0.1V TO VCC 0.
PAGE 25
LTC2492 APPLICATIONS INFORMATION 2.7V TO 5.5V 12 VCC = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2492 10μF REFERENCE VOLTAGE 0.1V TO VCC 0.
PAGE 26
LTC2492 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 27
LTC2492 APPLICATIONS INFORMATION Driving the Input and Reference The input and reference pins of the LTC2492 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. Each time a capacitor is switched between two of these pins, a small amount of charge is transferred. A simpliﬁed equivalent circuit is shown in Figure 12.
PAGE 28
LTC2492 APPLICATIONS INFORMATION In applications where the input common mode voltage is constant but different from the reference common mode voltage, the differential input current remains zero while the common mode input current is proportional to the difference between VIN(CM) and VREF(CM). For a reference common mode voltage of 2.5V and an input common mode of 1.5V, the common mode input current is approximately 0.74μA.
PAGE 29
LTC2492 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 CREF = 0.01μF 100 0 200 600 400 RSOURCE (Ω) 800 1000 2492 F15 Figure 15. +FS Error vs RSOURCE at VREF (Large CREF) –100 –FS ERROR (ppm) R = 1k 2 R = 500Ω 0 R = 100Ω –2 –4 –6 –8 –10 –0.5 –0.3 0.1 –0.1 VIN/VREF 0.3 0.5 Figure 17.
PAGE 30
LTC2492 APPLICATIONS INFORMATION 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 18 and 19). The modulator sampling frequency is fS = 15,360Hz while operating with its internal oscillator and fS = FEOSC/20 when operating with an external oscillator of frequency FEOSC.
PAGE 31
LTC2492 APPLICATIONS INFORMATION MEASURED DATA CALCULATED DATA –20 –40 VCC = 5V VREF = 5V VIN(CM) = 2.5V VIN(P-P) = 5V TA = 25°C –60 –80 –100 –120 0 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 INPUT FREQUENCY (Hz) 2492 F22 Figure 22. Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 100% (60Hz Notch) NORMAL MODE REJECTION (dB) 0 MEASURED DATA CALCULATED DATA –20 –40 VCC = 5V VREF = 5V VIN(CM) = 2.
PAGE 32
LTC2492 APPLICATIONS INFORMATION Using the 2x speed mode of the LTC2492 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 conversion results (see Figure 29). Output Data Rate When using its internal oscillator, the LTC2492 produces up to 7.
PAGE 33
LTC2492 APPLICATIONS INFORMATION 50 NO AVERAGE –90 WITH RUNNING AVERAGE –100 –110 –120 –130 3500 VIN(CM) = VREF(CM) VCC = VREF = 5V VIN = 0V fO = EXT CLOCK 40 3000 +FS ERROR (ppm OF VREF) –80 OFFSET ERROR (ppm OF VREF) NORMAL MODE REJECTION (dB) –70 30 TA = 85°C 20 10 VIN(CM) = VREF(CM) VCC = VREF = 5V fO = EXT CLOCK 2500 TA = 85°C 2000 1500 TA = 25°C 1000 0 500 TA = 25°C –140 0 –10 60 62 54 56 58 48 50 52 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 0 10 20 30 40 50 60 70 80 90 100 OUTPUT
PAGE 34
LTC2492 APPLICATIONS INFORMATION Easy Drive ADCs Simplify Measurement of High Impedance Sensors Delta-sigma ADCs, with their high accuracy and high noise immunity, are ideal for directly measuring many types of sensors. Nevertheless, input sampling currents can overwhelm high source impedances or low-bandwidth, micropower signal conditioning circuits. The LTC2492 solves this problem by balancing the input currents, thus simplifying or eliminating the need for signal conditioning circuits.
PAGE 35
LTC2492 REVISION HISTORY REV DATE DESCRIPTION C 11/09 Update Tables 1 and 2 D 07/10 (Revision history begins at Rev C) PAGE NUMBER Updated Typical Application Added Note 18 17, 18 1 4, 5 2492fd 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.
PAGE 36
LTC2492 TYPICAL APPLICATION 5V 5V R1 51.1k C4 0.1μF 12 10μF IIN+ = 0 R3 10k TO 100k IIN– = 0 R4 51.1k 14 8 5V 9 10 5V 102k 11 + 0.1μF 10k TO 100k 7 1k LT1494 REF + REF – CH0 2 SDI 3 SCK SDO 3-WIRE SPI INTERFACE 5 CH1 CH2 4 CS CH3 COM GND 6 2492 F38 0.1μF – = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2492 13 0.1μF C3 0.1μF VCC 1k 0.1μF Figure 38.