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LTC2486

2486fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

16-Bit 2-/4-Channel ΔΣ ADC

with PGA and Easy Drive

Input Current Cancellation

The LTC

2486 is a 4-channel (2-channel differential),

16-bit, No Latency ΔΣ™ 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 LTC2486 includes programmable gain, 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 differential modes) or its internal temperature sen-

sor. It can be programmed to reject line frequencies of 50Hz,

60Hz, or simultaneous 50Hz/60Hz, provide a programmable

gain from 1 to 256 in 8 steps, and conﬁ gured to double its

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

resolution and 2°C absolute accuracy.

The LTC2486 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 of Linear

Technology Corporation. No Latency ΔΣ and Easy Drive 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

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 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 4mm × 3mm DFN 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

–

16-BIT ΔΣ ADC

WITH EASY-DRIVE

4-CHANNEL

MUX

TEMPERATURE

SENSOR

–

2486 TA01a

4-WIRE

SPI INTERFACE

CH0

CH1

CH3

CH2

0.1μF

OSC

TEMPERATURE (°C)

–55 –30 –5

ABSOLUTE ERROR (°C)

–4

–3

–2

–1

12095704520

2486 TA01b

–5

Summary of content (36 pages)

PAGE 1
LTC2486 16-Bit 2-/4-Channel ΔΣ ADC with PGA and Easy Drive Input Current Cancellation DESCRIPTION FEATURES n n n n n n n n n n n n n n n 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 Programmable Gain from 1 to 256 Integrated High Accuracy Temperature Sensor GND to VCC Input/Reference Common Mode Range Programmable 50Hz, 60Hz, or Simultaneous 5
PAGE 2
LTC2486 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 LTC2486C ................................................
PAGE 3
LTC2486 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) 0.1V ≤ VREF ≤ VCC, –FS ≤ VIN ≤ +FS (Note 5) MIN 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) l l Offset Error 2.
PAGE 4
LTC2486 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 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
LTC2486 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) tHEO tLEO tCONV_1 Conversion Time for 1x Speed Mode tCONV_2 Conversion Time for 2x Speed Mode fISCK MAX UNITS l MIN 10 4000 kHz External Oscillator High Period l 0.125 50 μs External Oscillator Low Period l 0.
PAGE 6
LTC2486 TYPICAL PERFORMANCE CHARACTERISTICS Integral Nonlinearity (VCC = 5V, VREF = 5V) –45°C 1 25°C 0 85°C –1 –2 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 2 INL (ppm OF VREF) 2 1 –45°C, 25°C, 85°C 0 –1 –2 –3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) 2 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 2486 G01 12 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 8 85°C 25°C 4 0 –45°C –4 –8 2 12 85°C 8 –45°C 0 –4 12 12 NUMBER OF READINGS (%) 2 –0.75 –4 1.8 2486 G07 –0.
PAGE 7
LTC2486 TYPICAL PERFORMANCE CHARACTERISTICS RMS Noise vs Input Differential Voltage VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C 0.7 0.6 0.5 0.8 0.7 0.6 –1 0 0.8 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 0.6 0.1 0 –0.1 –0.2 2 3 VREF (V) 5 4 –1 –0.2 75 90 2486 G16 0 1 2486 G14 OFFSET ERROR (ppm OF VREF) OFFSET ERROR (ppm OF VREF) –0.1 0 15 30 45 60 TEMPERATURE (°C) 0.2 Offset Error vs VCC 0 –0.
PAGE 8
LTC2486 TYPICAL PERFORMANCE CHARACTERISTICS On-Chip Oscillator Frequency vs VCC 310 308 308 306 304 302 VCC = 4.1V VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND 300 –45 –30 –15 VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 306 304 302 300 0 15 30 45 60 TEMPERATURE (°C) 75 90 –80 2.5 3.0 3.5 4.0 VCC (V) 5.0 4.
PAGE 9
LTC2486 TYPICAL PERFORMANCE CHARACTERISTICS Integral Nonlinearity (2x Speed Mode; VCC = 5V, VREF = 5V) 1 25°C, 85°C 0 2 INL (ppm OF VREF) 2 –1 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 2 1 85°C 0 –45°C, 25°C –1 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V fO = GND 1 85°C 0 –45°C, 25°C –1 –45°C –2 –2 –3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) 2 –2 –3 –1.25 2.5 –0.75 RMS = 0.85μV 10,000 CONSECUTIVE AVERAGE = 0.
PAGE 10
LTC2486 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 2486 G34 1 2 4 3 VREF (V) –40 –60 10k 100k 1k 100 FREQUENCY AT VCC (Hz) 1M 2486 G36 0 VCC = 4.1V DC ±1.4V REF+ = 2.
PAGE 11
LTC2486 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. SDI (Pin 2): Serial Data Input.
PAGE 12
LTC2486 FUNCTIONAL BLOCK DIAGRAM VCC INTERNAL OSCILLATOR TEMP SENSOR GND fO (INT/EXT) AUTOCALIBRATION AND CONTROL REF+ REF– CH0 CH1 CH2 CH3 COM IN+ MUX IN– – + DIFFERENTIAL 3RD ORDER Δ3 MODULATOR SERIAL INTERFACE SDI SCK SDO CS DECIMATING FIR ADDRESS 2486 F01 Figure 1. Functional Block Diagram TEST CIRCUITS VCC 1.69k SDO SDO 1.
PAGE 13
LTC2486 APPLICATIONS INFORMATION CONVERTER OPERATION Converter Operation Cycle The LTC2486 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 14
LTC2486 APPLICATIONS INFORMATION Easy Drive Input Current Cancellation Reference Voltage Range The LTC2486 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 LTC2486 without external ampliﬁers.
PAGE 15
LTC2486 APPLICATIONS INFORMATION SERIAL INTERFACE PINS The LTC2486 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, select the temperature sensor and set the gain.
PAGE 16
LTC2486 APPLICATIONS INFORMATION +FS. If both Bit 21 and Bit 20 are LOW, the differential input voltage is below –FS. The function of these bits is summarized in Table 1. Table 1. LTC2486 Status Bits Input Range Bit 23 EOC Bit 22 DMY Bit 21 SIG Bit 20 MSB VIN ≥ 0.5 • VREF/Gain 0 0 1 1 0V ≤ VIN < 0.5 • VREF/Gain 0 0 1/0 0 –0.5 • VREF/Gain ≤ VIN < 0V 0 0 0 1 VIN < –0.5 • VREF/Gain 0 0 0 0 Bits 20 to 4 are the 16-bit plus sign conversion result MSB ﬁrst.
PAGE 17
LTC2486 APPLICATIONS INFORMATION Table 2. LTC2486 Output Data Format DIFFERENTIAL INPUT VOLTAGE VIN* BIT 23 EOC BIT 22 DMY BIT 21 SIG BIT 20 MSB BIT 19 BIT 18 BIT 17 … BIT 4 BITS 3 TO 0 0 0 1 1 0 0 0 … 0 0000 VIN* ≥ FS** FS** – 1LSB 0 0 1 0 1 1 1 … 1 0000 0.5 • FS** 0 0 1 0 1 0 0 … 0 0000 0.5 • FS** – 1LSB 0 0 1 0 0 1 1 … 1 0000 0 0 0 1/0*** 0 0 0 0 … 0 0000 –1LSB 0 0 0 1 1 1 1 … 1 0000 –0.
PAGE 18
LTC2486 APPLICATIONS INFORMATION Table 4.
PAGE 19
LTC2486 APPLICATIONS INFORMATION Table 5a. Performance vs Gain in Normal Speed Mode (VCC = 5V, VREF = 5V) GAIN 1 4 8 16 32 64 128 256 Input Span ±2.5 ±0.625 ±0.312 ±0.156 ±78m ±39m ±19.5m ±9.76m V LSB 38.1 9.54 4.77 2.38 1.19 0.596 0.298 0.149 μV 65536 65536 65536 65536 65536 65536 32768 16384 Counts Noise Free Resolution* UNIT Gain Error 5 5 5 5 5 5 5 8 Offset Error 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 ppm of FS μV UNIT Table 5b.
PAGE 20
LTC2486 APPLICATIONS INFORMATION 1020 5 VCC = 5V VREF = 5V 960 SLOPE = 2.45 LSB /K 16 4 ABSOLUTE ERROR (°C) 3 DATAOUT16 800 640 480 320 2 1 0 –1 –2 –3 160 0 –4 0 100 –5 –55 400 200 300 TEMPERATURE (K) –30 –5 20 45 70 TEMPERATURE (°C) 2486 F04 95 120 2486 F05 Figure 4. Internal PTAT Digital Output vs Temperature Figure 5. Absolute Temperature Error Table 6.
PAGE 21
LTC2486 APPLICATIONS INFORMATION The external serial clock mode is selected during the powerup sequence and on each falling edge of CS. In order to enter and remain in the external SCK mode of operation, SCK must be driven LOW both at power up and on each CS falling edge. If SCK is HIGH on the falling edge of CS, the device will switch to the internal SCK mode. The serial data output pin (SDO) is Hi-Z as long as CS is HIGH.
PAGE 22
LTC2486 APPLICATIONS INFORMATION 2.7V TO 5.5V 12 VCC = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2486 10μF REFERENCE VOLTAGE 0.1V TO VCC 0.
PAGE 23
LTC2486 APPLICATIONS INFORMATION On the falling edge of EOC, the conversion result is loading into an internal static shift register. The output data can now be shifted out the SDO pin under control of the externally applied SCK signal. Data is updated on the falling edge of SCK. The input data is shifted into the device through the SDI pin on the rising edge of SCK. On the 24th falling edge of SCK, SDO goes HIGH, indicating a new conversion has begun. This data now serves as EOC for the next conversion.
PAGE 24
LTC2486 APPLICATIONS INFORMATION remains HIGH for the duration of the conversion cycle. Once the conversion is complete, the cycle repeats. Internal Serial Clock, 3-Wire I/O, Continuous Conversion. Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling CS HIGH any time between the 1st rising edge and the 32nd falling edge of SCK (see Figure 10).
PAGE 25
LTC2486 APPLICATIONS INFORMATION 2.7V TO 5.5V 12 = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2486 10μF 0.1μF VCC REFERENCE VOLTAGE 0.
PAGE 26
LTC2486 APPLICATIONS INFORMATION Digital Signal Levels The LTC2486’s digital interface is easy to use. Its digital inputs SDI, fO, CS, and SCK (in external serial clock mode) accept standard CMOS logic levels. Internal hysteresis circuits can tolerate edge transition times as slow as 100μs. The digital input signal range is 0.5V to VCC – 0.5V. During transitions, the CMOS input circuits draw dynamic current.
PAGE 27
LTC2486 APPLICATIONS INFORMATION When using the LTC2486’s internal oscillator, the input capacitor array is switched at 123kHz. The effect of the charge transfer depends on the circuitry driving the input/reference pins. If the total external RC time constant is less than 580ns the errors introduced by the sampling process are negligible since complete settling occurs. Typically, the reference inputs are driven from a low impedance source.
PAGE 28
LTC2486 APPLICATIONS INFORMATION For relatively small values of external reference capacitance (CREF < 1nF), the voltage on the sampling capacitor settles for reference impedances of many kΩ (if CREF = 100pF up to 10kΩ will not degrade the performance) (see Figures 13 and 14). In cases where large bypass capacitors are required on the reference inputs (CREF > 0.01μF) full-scale and linearity errors are proportional to the value of the reference resistance.
PAGE 29
LTC2486 APPLICATIONS INFORMATION A 1V difference in between common mode input and common mode reference results in a 6.7ppm INL error for every 100Ω of reference resistance. In addition to the reference sampling charge, the reference ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally 1nA (±10nA max) results in a small gain error. A 100Ω reference resistance will create a 0.5μV full scale error.
PAGE 30
LTC2486 APPLICATIONS INFORMATION NORMAL MODE REJECTION (dB) 0 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) 2486 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 31
LTC2486 APPLICATIONS INFORMATION Using the 2x speed mode of the LTC2486 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).
PAGE 32
LTC2486 APPLICATIONS INFORMATION 50 NO AVERAGE –90 WITH RUNNING AVERAGE –100 –110 –120 –130 40 30 TA = 85°C 20 10 TA = 85°C 2000 1500 TA = 25°C 1000 0 500 0 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2486 F29 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2486 F31 2486 F30 Figure 29. Input Normal Mode Rejection 2x Speed Mode with and Without Running Averaging Figure 30. Offset Error vs Output Data Rate and Temperature 0 Figure 31.
PAGE 33
LTC2486 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 LTC2486 solves this problem by balancing the input currents, thus simplifying or eliminating the need for signal conditioning circuits.
PAGE 34
LTC2486 PACKAGE DESCRIPTION DE Package 14-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1708 Rev B) 0.70 ±0.05 3.30 ±0.05 3.60 ±0.05 2.20 ±0.05 1.70 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 3.00 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.115 TYP 4.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) 8 0.40 ± 0.10 14 3.30 ±0.10 1.70 ± 0.10 PIN 1 NOTCH R = 0.20 OR 0.
PAGE 35
LTC2486 REVISION HISTORY REV DATE DESCRIPTION C 11/09 Update Tables 1 and 2 D 7/10 (Revision history begins at Rev C) PAGE NUMBER Revised Typical Application drawing Added Note 18 16, 17 1 4, 5 2486fd Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 36
LTC2486 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 2486 F38 0.1μF – = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2486 13 0.1μF C3 0.1μF VCC 1k 0.1μF Figure 38.