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LTC1285/LTC1288

3V Micropower Sampling

12-Bit A/D Converters in

SO-8 Packages

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12-Bit Resolution

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8-Pin SO Plastic Package

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Low Cost

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Low Supply Current: 160µA Typ

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Auto Shutdown to 1nA Typ

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Guaranteed ±3/4LSB Max DNL

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Single Supply 3V to 6V Operation

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Differential Inputs (LTC1285)

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2-Channel MUX (LTC1288)

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On-Chip Sample-and-Hold

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100µs Conversion Time

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Sampling Rates:

7.5ksps (LTC1285)

6.6ksps (LTC1288)

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I/O Compatible with SPI, Microwire, etc.

The LTC

1285/LTC1288 are 3V micropower, 12-bit, suc-

cessive approximation sampling A/D converters. They

typically draw only 160µA of supply current when con-

verting and automatically power down to a typical supply

current of 1nA whenever they are not performing conver-

sions. They are packaged in 8-pin SO packages and

operate on 3V to 6V supplies. These 12-bit, switched-

capacitor, successive approximation ADCs include

sample-and-holds. The LTC1285 has a single differential

analog input. The LTC1288 offers a software selectable

2-channel MUX.

On-chip serial ports allow efficient data transfer to a wide

range of microprocessors and microcontrollers over three

wires. This, coupled with micropower consumption, makes

remote location possible and facilitates transmitting data

through isolation barriers.

These circuits can be used in ratiometric applications or

with an external reference. The high impedance analog

inputs and the ability to operate with reduced spans (to

1.5V full scale) allow direct connection to sensors and

transducers in many applications, eliminating the need for

gain stages.

APPLICATIONS

SAMPLE FREQUENCY (kHz)

0.1

SUPPLY CURRENT (µA)

100

1000

1 10 100

LTC1285/88 • TA02

= 25°C

= 2.7V

REF

= 2.5V

CLK

= 120kHz

Supply Current vs Sample Rate

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Pen Screen Digitizing

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Battery-Operated Systems

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Remote Data Acquisition

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Isolated Data Acquisition

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Battery Monitoring

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Temperature Measurement

12µW, S0-8 Package, 12-Bit ADC

Samples at 200Hz and Runs Off a 3V Supply

3V1µF

ANALOG INPUT

0V TO 3V RANGE

–IN

GND

CLK

OUT

REF

LTC1285

MPU

(e.g., 8051)

P1.4

P1.3

P1.2

+IN

LTC1285/88 • TA01

CS/SHDN

SERIAL DATA LINK

FEATURES

DESCRIPTION

TYPICAL APPLICATIONS N

, LTC and LT are registered trademarks of Linear Technology Corporation.

Summary of content (24 pages)

PAGE 1
LTC1285/LTC1288 3V Micropower Sampling 12-Bit A/D Converters in SO-8 Packages DESCRIPTION U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1285/LTC1288 are 3V micropower, 12-bit, successive approximation sampling A/D converters. They typically draw only 160µA of supply current when converting and automatically power down to a typical supply current of 1nA whenever they are not performing conversions. They are packaged in 8-pin SO packages and operate on 3V to 6V supplies.
PAGE 2
LTC1285/LTC1288 W W W ABSOLUTE MAXIMUM RATINGS (Notes 1 and 2) Supply Voltage (VCC) to GND ................................... 12V Voltage Analog and Reference ................ –0.3V to VCC + 0.3V Digital Inputs......................................... –0.3V to 12V Digital Output ............................. –0.3V to VCC + 0.3V Power Dissipation .............................................. 500mW Operating Temperature Range .................... 0°C to 70°C Storage Temperature Range .................
PAGE 3
LTC1285/LTC1288 W U U CONVERTER AND MULTIPLEXER CHARACTERISTICS PARAMETER CONDITIONS Resolution (No Missing Codes) Integral Linearity Error MIN ● (Note 6) LTC1285 TYP MAX (Note 5) MIN 12 ● LTC1288 TYP MAX 12 ±3/4 ±2 UNITS Bits ±3/4 ±2 LSB Differential Linearity Error ● ±1/4 ±3/4 ±1/4 ±3/4 LSB Offset Error ● ±3/4 ±3 ±3/4 ±3 LSB Gain Error ● ±2 ±2 ±8 LSB Analog Input Range (Note 7 and 8) REF Input Range (LTC1285) (Notes 7, 8, and 9) 2.
PAGE 4
LTC1285/LTC1288 AC CHARACTERISTICS (Note 5) SYMBOL PARAMETER CONDITIONS MIN tSMPL Analog Input Sample Time See Operating Sequence TYP MAX 1.
PAGE 5
LTC1285/LTC1288 U W TYPICAL PERFORMANCE CHARACTERISTICS Reference Current vs Sample Rate (LTC1285) 53 TA = 25°C VCC = 2.7V VREF = 2.5V fCLK = 120kHz 40 35 30 25 20 15 10 3.0 VCC = 2.7V VREF = 2.5V fCLK = 120kHz fSMPL = 7.5kHz 52 REFERENCE CURRENT (µA) REFERENCE CURRENT (µA) 45 51 CHANGE IN OFFSET (LSB = 1/4096 × VREF) 50 50 49 48 47 46 45 44 5 43 –55 –35 –15 0 0 1 2 6 4 3 5 SAMPLE RATE (kHz) 7 8 TA = 25°C VCC = 2.7V fCLK = 120kHz fSMPL = 7.5kHz 2.5 2.0 1.5 1.0 0.
PAGE 6
LTC1285/LTC1288 U W TYPICAL PERFORMANCE CHARACTERISTICS Spurious-Free Dynamic Range vs Input Frequency S/(N + D) vs Input Level 90 80 70 60 50 40 30 TA = 25°C VCC = 2.7V VREF = 2.5V fSMPL = fSMPL(MAX) 20 10 0 10 INPUT FREQUENCY (kHz) 1 TA = 25°C 70 VCC = 2.7V VREF = 2.5V 60 fIN = 1kHz fSMPL = fSMPL(MAX) 10 20 50 40 30 20 10 70 100 0 – 100 10M LTC1285/86 • TPC15 – 40 TA = 25°C VCC = 2.7V (VRIPPLE = 1mV) VREF = 2.
PAGE 7
LTC1285/LTC1288 U W TYPICAL PERFORMANCE CHARACTERISTICS Digital Input Logic Threshold vs Supply Voltage VCC = 2.7V VREF = 2.5V CLOCK FREQUENCY (kHz) 100 80 60 40 20 2 0 0 10 20 30 50 40 TEMPERATURE (°C) 60 70 Input Channel Leakage Current vs Temperature 3.0 1000 TA = 25°C VCC = 2.7V VREF = 2.5V 2.5 100 LEAKAGE CURRENT (nA) 120 DIGITAL INPUT LOGIC THRESHOLD VOLTAGE (V) Minimum Clock Frequency for 0.1 LSB Error vs Temperature 2.0 1.5 1.0 1 ON CHANNEL OFF CHANNEL 0.1 0.5 0 2.5 10 3.
PAGE 8
LTC1285/LTC1288 W BLOCK DIAGRAM CS/SHDN BIAS AND SHUTDOWN CIRCUIT IN + (CH0) CSAMPLE CLK (DIN) VCC (VCC/ V REF) SERIAL PORT DOUT – IN – (CH1) + SAR MICROPOWER COMPARATOR CAPACITIVE DAC LTC1285/88 • BD VREF GND PIN NAMES IN PARENTHESES REFER TO THE LTC1288 TEST CIRCUITS Load Circuit for tdDO, tr and tf Voltage Waveforms for DOUT Rise and Fall Times, tr, tf 1.
PAGE 9
LTC1285/LTC1288 TEST CIRCUITS Voltage Waveforms for tdis Voltage Waveforms for ten LTC1285 CS VIH CS DOUT WAVEFORM 1 (SEE NOTE 1) 90% 1 CLK 2 tdis DOUT WAVEFORM 2 (SEE NOTE 2) B11 10% DOUT VOL NOTE 1: WAVEFORM 1 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH THAT THE OUTPUT IS HIGH UNLESS DISABLED BY THE OUTPUT CONTROL. NOTE 2: WAVEFORM 2 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH THAT THE OUTPUT IS LOW UNLESS DISABLED BY THE OUTPUT CONTROL.
PAGE 10
LTC1285/LTC1288 U W U U APPLICATION INFORMATION OVERVIEW basic design, the LTC1285 and LTC1288 differ in some respects. The LTC1285 has a differential input and has an external reference input pin. It can measure signals floating on a DC common-mode voltage and can operate with reduced spans to 1.5V. Reducing the spans allows it to achieve 366µV resolution. The LTC1288 has a two-channel input multiplexer and can convert either channel with respect to ground or the difference between the two.
PAGE 11
LTC1285/LTC1288 U W U U APPLICATION INFORMATION MSB-First Data (MSBF = 0) tCYC CS tsuCS POWER DOWN CLK ODD/ SIGN START DIN DON’T CARE MSBF NULL HI-Z BIT SGL/ DIFF DOUT B11 B10 B9 (MSB) tSMPL B8 B6 B7 B5 B4 B3 B2 B1 B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11* HI-Z tDATA tCONV MSB-First Data (MSBF = 1) tCYC CS POWER DOWN tsuCS CLK ODD/ SIGN START DIN DON’T CARE SGL/ DIFF DOUT MSBF NULL BIT B11 B10 B9 HI-Z tSMPL (MSB) B8 B7 B6 tCONV B5 B4 B3 B2 HI-Z B1 B0* tDATA
PAGE 12
LTC1285/LTC1288 U W U U APPLICATION INFORMATION Data Transfer The CLK synchronizes the data transfer with each bit being transmitted on the falling CLK edge and captured on the rising CLK edge in both transmitting and receiving systems. The LTC1285 does not require a configuration input word and has no DIN pin. A falling CS initiates data transfer as shown in the LTC1285 operating sequence. After CS falls the second CLK pulse enables DOUT.
PAGE 13
LTC1285/LTC1288 U W U U APPLICATION INFORMATION Transfer Curve The LTC1285/LTC1288 are permanently configured for unipolar only. The input span and code assignment for this conversion type are shown in the following figures. Transfer Curve either an input or an output. The LTC1288 will take control of the data line and drive it low on the 4th falling CLK edge after the start bit is received (see Figure 3).
PAGE 14
LTC1285/LTC1288 U W U U APPLICATION INFORMATION Several things must be taken into account to achieve such a low power consumption. Shutdown The LTC1285/LTC1288 are equipped with automatic shutdown features. They draw power when the CS pin is low and shut down completely when that pin is high. The bias circuit and comparator powers down and the reference input becomes high impedance at the end of each conversion leaving the CLK running to clock out the LSB first data or zeroes (see Figures 1 and 2).
PAGE 15
LTC1285/LTC1288 U W U U APPLICATION INFORMATION of Maximum Clock Rate vs Supply Voltage). If the maximum clock frequency is used, care must be taken to ensure that the device converts correctly. Mixed Supplies It is possible to have a microprocessor running off a 5V supply and communicate with the LTC1285/LTC1288 operating on a 3V supply. The inputs of CS, CLK and DIN of the LTC1285/LTC1288 have no problem to take a voltage swing from 0V to 5V.
PAGE 16
LTC1285/LTC1288 U W U U APPLICATION INFORMATION Single-Ended Inputs “+” Input Settling The sample-and-hold of the LTC1288 allows conversion of rapidly varying signals. The input voltage is sampled during the tSMPL time as shown in Figure 7. The sampling interval begins as the bit preceding the MSBF bit is shifted in and continues until the falling CLK edge after the MSBF bit is received. On this falling edge, the S&H goes into hold mode and the conversion begins.
PAGE 17
LTC1285/LTC1288 U W U U APPLICATION INFORMATION selected “+” and “–” inputs once during each conversion cycle. Large external source resistors and capacitances will slow the settling of the inputs. It is important that the overall RC time constants be short enough to allow the analog inputs to completely settle within the allowed time. RSOURCE + “+” INPUT LTC1285 LTC1288 VIN + C1 RSOURCE – RON = 500Ω “–” INPUT Input leakage currents can also create errors if the source resistance gets too large.
PAGE 18
LTC1285/LTC1288 U W U U APPLICATION INFORMATION Voltage). However, care must be taken when operating at low values of VREF because of the reduced LSB step size and the resulting higher accuracy requirement placed on the converter. The following factors must be considered when operating at low VREF values: 1. Offset 2. Noise 3. Conversion speed (CLK frequency) noise becomes equal to 3.3LSBs and a stable code may be difficult to achieve. In this case averaging multiple readings may be necessary.
PAGE 19
LTC1285/LTC1288 U W U U APPLICATION INFORMATION Signal-to-Noise Ratio 12 74 11 68 10 62 9 56 8 50 S/(N + D) (dB) EFFECTIVE NUMBER OF BITS (ENOBs) The Signal-to-Noise plus Distortion Ratio (S/N + D) is the ratio between the RMS amplitude of the fundamental input frequency to the RMS amplitude of all other frequency components at the ADC’s output. The output is band limited to frequencies above DC and below one half the sampling frequency. Figure 12 shows a typical spectral content with a 7.
PAGE 20
LTC1285/LTC1288 U TYPICAL APPLICATIONS N MICROPROCESSOR INTERFACES The LTC1285/LTC1288 can interface directly without external hardware to most popular microprocessor (MPU) synchronous serial formats (see Table 1). If an MPU without a dedicated serial port is used, then 3 or 4 of the MPU's parallel port lines can be programmed to form the serial link to the LTC1285/LTC1288. Included here is one serial interface example and one example showing a parallel port programmed to form the serial interface.
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LTC1285/LTC1288 U TYPICAL APPLICATIONS N Timing Diagram for Interface to the MC68HC11 CS CLK DIN SGL/ DIFF START ODD/ SIGN MSBF DON'T CARE DOUT MPU TRANSMIT WORD 0 0 0 0 0 0 SGL/ DIFF 1 0 ODD/ SIGN MSBF ? ? ? ? ? B10 B9 B8 X X X X X B7 X B6 B5 B4 B3 B2 B1 B0 X X X X X X X B2 B1 B0 BYTE 2 BYTE 1 MPU RECEIVED WORD B11 ? ? ? ? ? ? 0 B11 BYTE 3 (DUMMY) B10 B9 B8 B7 B6 BYTE 2 BYTE 1 B5 B4 B3 BYTE 3 LTC1285/88 • TA03 Hardware and Softwar
PAGE 22
LTC1285/LTC1288 U TYPICAL APPLICATIONS N Interfacing to the Parallel Port of the INTEL 8051 Family LABEL The Intel 8051 has been chosen to demonstrate the interface between the LTC1288 and parallel port microprocessors. Normally the CS, CLK and DIN signals would be generated on 3 port lines and the DOUT signal read on a 4th port line. This works very well. However, we will demonstrate here an interface with the DIN and DOUT of the LTC1288 tied together as described in the SERIAL INTERFACE section.
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LTC1285/LTC1288 U TYPICAL APPLICATIONS N A “Quick Look” Circuit for the LTC1285 Micropower Battery Voltage Monitor Users can get a quick look at the function and timing of the LT1285 by using the following simple circuit (Figure 13). VREF is tied to VCC. VIN is applied to the +IN input and the –IN input is tied to the ground. CS is driven at 1/16 the clock rate by the 74C161 and DOUT outputs the data.
PAGE 24
LTC1285/LTC1288 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead Plastic DIP 0.300 – 0.325 (7.620 – 8.255) 0.009 – 0.015 (0.229 – 0.381) ( +0.025 0.325 –0.015 8.255 +0.635 –0.381 ) 0.045 – 0.065 (1.143 – 1.651) 0.400* (10.160) MAX 0.130 ± 0.005 (3.302 ± 0.127) 0.065 (1.651) TYP 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) 0.125 (3.175) MIN 0.005 (0.127) MIN 0.015 (0.380) MIN N8 0695 0.018 ± 0.003 (0.457 ± 0.076) 0.100 ± 0.010 (2.