Datasheet

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LTC2487

2487fd

FEATURES

APPLICATIONS

DESCRIPTION

16-Bit 2-/4-Channel

ΔΣ

ADC

with PGA, Easy Drive and

C Interface

The LTC

2487 is a 4-channel (2 differential), 16-bit,

No Latency ΔΣ™ 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 LTC2487 includes programmable gain, a high accuracy

temperature sensor and an integrated oscillator. This device

can be conﬁ gured to measure an external signal (from com-

binations of 4 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 LTC2487 can

be conﬁ gured to provide a programmable gain from 1 to

256 in 8 steps.

The LTC2487 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

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

ables optional external ampliﬁ ers to be shared between all

analog inputs and auto calibration continuously removes

their associated offset and drift.

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

2-Wire I

C Interface with 9 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 4mm × 3mm DFN Package

Direct Sensor Digitizer

Direct Temperature Measurement

Instrumentation

Industrial Process Control

Built-In High Performance Temperature Sensor

SDA

SCL

REF

2.7V TO 5.5V

0.1μF

COM

REF

–

16-BIT ΔΣ ADC

WITH EASY-DRIVE

4-CHANNEL

MUX

–

2487 TA01a

2-WIRE

C INTERFACE

1.7k

CA1

CA0

9-PIN SELECTABLE

ADDRESSES

CH0

CH1

CH3

CH2

10μF

OSC

TEMPERATURE

SENSOR

TEMPERATURE (°C)

–55 –30 –5

ABSOLUTE ERROR (°C)

–4

–3

–2

–1

12095704520

2487 TA02

–5

TYPICAL APPLICATION

Summary of content (32 pages)

PAGE 1
LTC2487 16-Bit 2-/4-Channel ΔΣ 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 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 2-Wire I2C Interface with 9 Addresses Plus One Global Address for Synchronization 600nV RMS Noise Programmable Gain from 1 to 256 Integrated High Accuracy Temperature Sensor GND to VCC Input
PAGE 2
LTC2487 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 LTC2487C ................................................
PAGE 3
LTC2487 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 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) = 1.25V (Note 6) l l 2 1 20 Offset Error 2.
PAGE 4
LTC2487 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 V IN– Absolute/Common Mode IN– Voltage (IN– Corresponds to the Selected Negative Input Channel) GND – 0.3V VCC + 0.
PAGE 5
LTC2487 POWER REQUIREMENTS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. (Note 3) SYMBOL PARAMETER VCC Supply Voltage ICC Supply Current CONDITIONS MIN l l l l Conversion Current (Note 11) Temperature Measurement (Note 11) Sleep Mode (Note 11) TYP 2.7 160 200 1 MAX UNITS 5.
PAGE 6
LTC2487 TYPICAL PERFORMANCE CHARACTERISTICS Integral Nonlinearity (VCC = 5V, VREF = 5V) –45°C 1 2 INL (ppm of VREF) 2 25°C 0 85°C –1 –2 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 1 –45°C, 25°C, 85°C 0 –1 2 –3 –1.25 2.5 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 2487 G01 12 8 85°C 25°C 4 0 –45°C –4 –8 2 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V fO = GND 12 85°C –45°C 0 –4 Noise Histogram (6.8sps) 12 12 NUMBER OF READINGS (%) 2 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.
PAGE 7
LTC2487 TYPICAL PERFORMANCE CHARACTERISTICS RMS Noise vs Input Differential Voltage 0.9 0.7 0.6 0.5 0.8 RMS NOISE (μV) VCC = 5V VREF = 5V VIN = 0V TA = 25°C fO = GND GAIN = 256 0.7 0.6 0.5 0.4 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 INPUT DIFFERENTIAL VOLTAGE (V) 0.4 –1 2.5 0 VREF = 2.5V VIN = 0V VIN(CM) = GND TA = 25°C fO = GND GAIN = 256 0.6 0.5 0.8 3.5 3.9 4.3 VCC (V) 4.7 5.1 0.1 –0.1 –0.2 0 15 30 45 60 TEMPERATURE (°C) 0.1 0 –0.1 –0.
PAGE 8
LTC2487 TYPICAL PERFORMANCE CHARACTERISTICS On-Chip Oscillator Frequency vs Temperature On-Chip Oscillator Frequency vs VCC 310 310 308 308 PSRR vs Frequency at VCC 0 VCC = 4.1V DC ±0.7V VREF = 2.5V IN+ = GND IN– = GND fO = GND TA = 25°C 306 304 VCC = 4.1V VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND 300 –45 –30 –15 306 304 VREF = 2.5V VIN = 0V VIN(CM) = GND fO = GND TA = 25°C 302 300 0 15 30 45 60 TEMPERATURE (°C) 75 90 2.5 3.0 3.5 –120 4.0 VCC (V) 4.5 5.
PAGE 9
LTC2487 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 85°C 0 –45°C, 25°C –1 –2 Noise Histogram (2x Speed Mode) 16 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V fO = GND 1 85°C 0 –45°C, 25°C –1 RMS = 0.85μV 10,000 CONSECUTIVE AVERAGE = 0.
PAGE 10
LTC2487 TYPICAL PERFORMANCE CHARACTERISTICS –40 –60 –20 RREJECTION (dB) –20 REJECTION (dB) 0 VCC = 4.1V DC ±0.7V REF+ = 2.5V REF– = GND IN+ = GND IN– = GND fO = GND TA = 25°C –80 –40 –60 0 VCC = 4.1V DC ±1.4V REF+ = 2.5V REF– = GND IN+ = GND IN– = GND fO = GND TA = 25°C VCC = 4.1V DC ±0.7V REF+ = 2.
PAGE 11
LTC2487 PIN FUNCTIONS GND – 0.3V to VCC + 0.3V. Within these limits, the two selected inputs (IN+ and IN– ) provide a bipolar input range VIN = (IN+ – IN–) from –0.5 • VREF/Gain to 0.5 • VREF /Gain. Outside this input range, the converter produces unique over-range and under-range output codes. CH0 to CH3 (Pin 8-Pin 11): Analog Inputs. May be programmed for single-ended or differential mode. VCC (Pin 12): Positive Supply Voltage. Bypass to GND with a 10μF tantalum capacitor in parallel with a 0.
PAGE 12
LTC2487 APPLICATIONS INFORMATION CONVERTER OPERATION Converter Operation Cycle The LTC2487 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 LTC2487 performs a conversion. Once the conversion is complete, the device enters the sleep state.
PAGE 13
LTC2487 APPLICATIONS INFORMATION 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). This unique architecture does not require on-chip buffers, thereby enabling signals to swing beyond ground and VCC.
PAGE 14
LTC2487 APPLICATIONS INFORMATION I2C INTERFACE The Start and Stop Conditions The LTC2487 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 15
LTC2487 APPLICATIONS INFORMATION dress matches the hard wired LTC2487’s address (one of 9 pin-selectable addresses) the device is selected. When the device is addressed during the conversion state, it will not acknowledge R/W requests and will issue a NAK by leaving the SDA line high. If the conversion is complete, the LTC2487 issues an ACK by pulling the SDA line low. second bit is the most signiﬁcant bit (MSB) of the result.
PAGE 16
LTC2487 APPLICATIONS INFORMATION INPUT DATA FORMAT If the ﬁrst three bits are 000 or 100, the following data is ignored (don’t care) and the previously selected input channel and conﬁguration remain valid for the next conversion. The serial input word to the LTC2487 is 16 bits long and is written into the device input register in two 8-bit words. The ﬁrst word (SGL, ODD, A2, A1, A0) is used to select the input channel.
PAGE 17
LTC2487 APPLICATIONS INFORMATION The ﬁrst input bit (SGL) following the 101 sequence determines if the input selection is differential (SGL = 0) or single-ended (SGL = 1). For SGL = 0, two adjacent channels can be selected to form a differential input. For SGL = 1, one of 4 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).
PAGE 18
LTC2487 APPLICATIONS INFORMATION The conﬁguration remains valid until a new input word with EN = 1 (the ﬁrst three bits are 101 for the ﬁrst word) and EN2 = 1 (for the second write byte) is shifted into the device. temperature measurements, the 1x mode is always used independent of the value of SPD. GAIN (GS2, GS1, GS0) The input referred gain of the LTC2487 is adjustable from 1 to 256 (see Tables 5a and 5b).
PAGE 19
LTC2487 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 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 formula: SLOPE = 1020 VCC = 5V VREF = 5V 960 SLOPE = 2.
PAGE 20
LTC2487 APPLICATIONS INFORMATION 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 concluded and a valid address selects the device, the LTC2487 generates a NAK signal indicating the conversion cycle is in progress. Table 6.
PAGE 21
LTC2487 APPLICATIONS INFORMATION can be written into the device and a Stop command will initiate the next conversion (see Figure 9). Synchronizing Multiple LTC2487s with a Global Address Call In applications where several LTC2487s (or other I2C delta-sigma ADCs from Linear Technology Corporation) are used on the same I2C bus, all converters can be synchronized through the use of a global address call. Prior to issuing the global address call, all converters must have completed a conversion cycle.
PAGE 22
LTC2487 APPLICATIONS INFORMATION Driving the Input and Reference The input and reference pins of the LTC2487 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 11.
PAGE 23
LTC2487 APPLICATIONS INFORMATION For many applications, the sensor output impedance combined with external input bypass capacitors produces RC time constants much greater than the 580ns required for 1ppm accuracy. For example, a 10kΩ bridge driving a 0.1μF capacitor has a time constant an order of magnitude greater than the required maximum. common mode input current does not degrade the accuracy if the source impedances tied to IN+ and IN– are matched.
PAGE 24
LTC2487 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 12 and 13)). 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 25
LTC2487 APPLICATIONS INFORMATION One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital ﬁltering. Combined with a large oversample ratio, the LTC2487 signiﬁcantly simpliﬁes antialiasing ﬁlter requirements. Additionally, the input current cancellation feature allows external low pass ﬁltering without degrading the DC performance of the device.
PAGE 26
LTC2487 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) 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) 8fN 2487 F20 2487 F19 Figure 19. Input Normal Mode Rejection at DC Figure 20.
PAGE 27
LTC2487 APPLICATIONS INFORMATION Traditional high order delta-sigma modulators suffer from potential instabilities at large input signal levels. The proprietary architecture used for the LTC2487 third order modulator resolves this problem and guarantees stability with input signals 150% of full scale. In many industrial 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 12.5 25 37.5 50 62.5 75 87.5 100 112.
PAGE 28
LTC2487 APPLICATIONS INFORMATION 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 26 and 27. 1x type frequency rejection can be achieved using the 2x mode by performing a running average of the previous two conversion results (see Figure 28). Output Data Rate When using its internal oscillator, the LTC2487 produces up to 15 samples per second (sps) with a notch frequency of 60Hz.
PAGE 29
LTC2487 APPLICATIONS INFORMATION 50 OFFSET ERROR (ppm OF VREF) NORMAL MODE REJECTION (dB) –80 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) –70 30 TA = 85°C 20 10 2500 TA = 85°C 2000 1500 0 500 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 DATA RATE (READINGS/SEC) 2487 F28 20 –2000 –2500 14 12 VIN(CM) = VREF(CM) VCC
PAGE 30
LTC2487 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 LTC2487 solves this problem by balancing the input currents, thus simplifying or eliminating the need for signal conditioning circuits.
PAGE 31
LTC2487 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 11/09 Update Tables 1 and 2 15 D 7/10 Revised Typical Application drawing 1 Revised parameter for VIHA in I2C Inputs and Digital Outputs 4 Added text to I2C Interface section 14 2487fd Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 32
LTC2487 TYPICAL APPLICATION 5V 5V R1 51.1k C4 0.1μF 12 10μF IIN+ = 0 R3 10k TO 100k 14 8 5V 9 10 5V 102k 11 + 0.1μF 10k TO 100k 7 1k LT1494 REF– 5 SDA 4 2-WIRE I2C INTERFACE CH0 CH1 CA0 CH2 CA1 2 3 9-PIN SELECTABLE ADDRESSES CH3 COM GND 6 2487 F37 0.1μF – 1.7k REF+ SCL IIN– = 0 R4 51.1k = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 fO LTC2487 13 0.1μF C3 0.1μF VCC 1k 0.1μF Figure 37.