LINEAR TECHNOLOGY LTC1290 Technical data

FEATURES

■

Software Programmable Features – Unipolar/Bipolar Conversion – Four Differential/Eight Single-Ended Inputs – MSB- or LSB-First Data Sequence – Variable Data Word Length – Power Shutdown

■

Built-In Sample-and-Hold

■

Single Supply 5V or ± 5V Operation

■

Direct Four-Wire Interface to Most MPU Serial Ports and All MPU Parallel Ports

■

50kHz Maximum Throughput Rate

■

Available in 20-Lead PDIP and SO Wide Packages

KEY SPECIFICATIO S

■

Resolution: 12 Bits

■

Fast Conversion Time: 13µs Max Over Temp

■

Low Supply Current: 6.0mA

LTC1290

Single Chip 12-Bit Data

Acquisition System

DESCRIPTIO

The LTC®1290 is a data acquisition component which contains a serial I/O successive approximation A/D converter. It uses LTCMOS to perform either 12-bit unipolar or 11-bit plus sign bipolar A/D conversions. The 8-channel input multiplexer can be configured for either single-ended or differential inputs (or combinations thereof). An on-chip sample-and-hold is included for all single-ended input channels. When the LTC1290 is idle it can be powered down with a serial word in applications where low power consumption is desired.

The serial I/O is designed to be compatible with industry standard full duplex serial interfaces. It allows either MSBor LSB-first data and automatically provides 2's complement output coding in the bipolar mode. The output data word can be programmed for a length of 8, 12 or 16 bits. This allows easy interface to shift registers and a variety of processors.

, LTC and LT are registered trademarks of Linear Technology Corporation. LTCMOS is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5287525.

switched capacitor technology

TYPICAL APPLICATIO

SINGLE-ENDED INPUT

±15V OVERVOLTAGE RANGE*

0V TO 5V OR ±5V

DIFFERENTIAL INPUT (+)

±5V COMMON MODE RANGE (–)

* FOR OVERVOLTAGE PROTECTION ON ONLY ONE CHANNEL LIMIT THE INPUT CURRENT TO 15mA. FOR OVERVOLTAGE PROTECTION ON MORE THAN ONE CHANNEL LIMIT THE INPUT CURRENT TO 7mA PER CHANNEL AND 28mA FOR ALL CHANNELS. (SEE SECTION ON OVERVOLTAGE PROTECTION IN THE APPLICATIONS INFORMATION SECTION.) CONVERSION RESULTS ARE NOT VALID WHEN THE SELECTED OR ANY OTHER CHANNEL IS OVERVOLTAGED (V

12-Bit 8-Channel Sampling Data Acquisition System

CH0

•

CH1

•

CH2

CH3

CH4

LTC1290

CH5

•

CH6

•

CH7

•

COM

DGND

ACLK

SCLK

OUT

REF

–

REF

–

AGND

< V– OR VIN > VCC).

22µF TANTALUM

TO AND FROM MICROPROCESSOR

0.1µF

–5V

1N5817

4.7µF

TANTALUM

1N4148

LT®1027

8V TO 40V

1µF

1290 • TA01

1290fe

LTC1290

LUTEXI TIS

WUW

ARB

(Notes 1, 2)

Supply Voltage (VCC) to GND or V–........................ 12V

Negative Supply Voltage (V

–

) .................... – 6V to GND

Voltage

Analog/Reference Inputs .........(V

) – 0.3V to V

+ 0.3V

–

Digital Inputs ........................................ – 0.3V to 12V

Digital Outputs ........................... – 0.3V to V

+ 0.3V

Power Dissipation............................................. 500mW

PACKAGE

RDER I FOR ATIO

TOP VIEW

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

DGND

N PACKAGE

20-LEAD PDIP

= 110°C, θJA = 100°C/W (N)

JMAX

ACLK

SCLK

OUT

REF

–

REF

–

AGND

Operating Temperature Range

LTC1290BC, LTC1290CC, LTC1290DC .... 0°C to 70°C

LTC1290BI, LTC1290CI, LTC1290DI .... –40°C to 85°C

LTC1290BM, LTC1290CM,

LTC1290DM (OBSOLETE) ............ –55°C to 125°C

Storage Temperature Range ................ –65°C to 150°C

Lead Temperature (Soldering, 10 sec.)................ 300°C

TOP VIEW

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

DGND

SW PACKAGE

20-LEAD PLASTIC SO WIDE

= 110°C, θJA = 130°C/W (SW)

JMAX

ACLK

SCLK

OUT

REF

–

REF

–

AGND

ORDER PART NUMBER

LTC1290BIN LTC1290CIN LTC1290DIN LTC1290BCN LTC1290CCN LTC1290DCN

J PACKAGE

20-LEAD CERAMIC DIP

= 150°C, qJA = 80°C/W (J)

JMAX

N PART MARKING

ORDER PART NUMBER SW PART MARKING

LTC1290BCSW LTC1290CCSW LTC1290DCSW LTC1290BISW LTC1290CISW LTC1290DISW

LTC1290BMJ LTC1290CMJ LTC1290DMJ LTC1290BIJ LTC1290CIJ LTC1290DIJ

OBSOLETE PACKAGE

Consider N Package for Alternate Source

Order Options

Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/

*The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for parts specified with wider operating temperature ranges.

Tape and Reel: Add #TR

1290fe

LTC1290

UU W

CO VERTER A D ULTIPLEXER CHARACTERISTICS

which apply over the full operating temperature range, otherwise specifications are at T

LTC1290B LTC1290C LTC1290D

PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX MIN TYP MAX UNITS

Offset Error (Note 4) Linearity Error (INL) (Notes 4, 5) Gain Error (Note 4) Minimum Resolution for Which

No Missing Codes are Guaranteed

Analog and REF Input Range (Note 7) (V–) – 0.05V to VCC + 0.05V (V–) – 0.05V to VCC + 0.05V (V–) – 0.05V to VCC + 0.05V V On Channel Leakage Current On Channel = 5V

(Note 8) Off Channel = 0V

On Channel = 0V Off Channel = 5V

Off Channel Leakage Current On Channel = 5V (Note 8) Off Channel = 0V

On Channel = 0V Off Channel = 5V

●

±1.5 ±1.5 ±1.5 LSB ±0.5 ±0.5 ± 0.75 LSB ±0.5 ±1.0 ±4.0 LSB

12 12 12 Bits

±1 ±1 ±1 µA

±1 ± 1 ±1 µA

= 25°C. (Note 3)

The ● denotes the specifications

1290fe

LTC1290

AC CHARACTERISTICS

otherwise specifications are at T

= 25°C. (Note 3)

The ● denotes the specifications which apply over the full operating temperature range,

LTC1290B/LTC1290C/LTC1290D

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

SCLK

ACLK

ACC

Shift Clock Frequency VCC = 5V (Note 6) 0 2.0 MHz A/D Clock Frequency VCC = 5V (Note 6) (Note 10) 4.0 MHz Delay Time from CS↓ to D

Data Valid (Note 9) 2 ACLK

OUT

Cycles

SMPL

Analog Input Sample Time See Operating Sequence 7 SCLK

Cycles

CONV

Conversion Time See Operating Sequence 52 ACLK

Cycles

CYC

Total Cycle Time See Operating Sequence (Note 6) 12 SCLK + Cycles

56 ACLK

dDO

Delay Time, SCLK↓ to D

Data Valid See Test Circuits LTC1290BC, LTC1290CC

OUT

●

130 220 ns LTC1290DC, LTC1290BI LTC1290CI, LTC1290DI

LTC1290BM, LTC1290CM

●

180 270 ns LTC1290DM (OBSOLETE)

dis

hCS

hDI

hDO

suDI

suCS

Delay Time, CS↑ to D Delay Time, 2nd ACLK↓ to D

Hi-Z See Test Circuits

OUT

Enabled See Test Circuits

OUT

●

70 100 ns

130 200 ns

Hold Time, CS After Last SCLK↓ VCC = 5V (Note 6) 0 ns Hold Time, DIN After SCLK↑ VCC = 5V (Note 6) 50 ns Time Output Data Remains Valid After SCLK↓ 50 ns D

Fall Time See Test Circuits

OUT

Rise Time See Test Circuits

OUT

●

65 130 ns 25 50 ns

Setup Time, DIN Stable Before SCLK↑ VCC = 5V (Note 6) 50 ns Setup Time, CS↓ Before Clocking in (Notes 6, 9) 2 ACLK Cycles

First Address Bit + 100ns

WHCS

CS High Time During Conversion VCC = 5V (Note 6) 52 ACLK

Cycles

Input Capacitance Analog Inputs On Channel 100 pF

Analog Inputs Off Channel 5 pF Digital Inputs 5 pF

1290fe

LTC1290

DIGITAL

apply over the full operating temperature range, otherwise specifications are at T

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

SOURCE

SINK

REF –

High Level Input Voltage VCC = 5.25V Low Level Input Voltage VCC = 4.75V High Level Input Current VIN = V Low Level Input Current VIN = 0V High Level Output Voltage VCC = 4.75V IO = 10µA 4.7 V

Low Level Output Voltage VCC = 4.75V IO = 1.6mA High-Z Output Leakage V

Output Source Current V Output Sink Current V Positive Supply Current CS High

Reference Current V Negative Supply Current CS High

LECTRICAL C CHARA TER ST

= 360µA

= VCC, CS High

OUT

= 0V, CS High

OUT

= 0V –20 mA

OUT

= V

OUT

CS High LTC1290BC, LTC1290CC Power Shutdown LTC1290DC, LTC1290BI ACLK Off LTC1290CI, LTC1290DI

LTC1290BM, LTC1290CM LTC1290DM

(OBSOLETE)

= 5V

REF

= 25°C. (Note 3)

LTC1290B/LTC1290C/LTC1290D

The ● denotes the specifications which

ICS

●

2.0 V

0.8 V

2.5 µA

–2.5 µA

2.4 4.0 V

0.4 V 3 µA

–3 µA

20 mA

612 mA 510 µA

515 µA

10 50 µA

150 µA

Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.

Note 2: All voltage values are with respect to ground with DGND, AGND

–

and REF Note 3: V

–5V for bipolar mode, ACLK = 4.0MHz unless otherwise specified. Note 4: These specs apply for both unipolar and bipolar modes. In bipolar

mode, one LSB is equal to the bipolar input span (2V For example, when V

Note 5: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band.

Note 6: Recommended operating conditions. Note 7: Two on-chip diodes are tied to each reference and analog input

which will conduct for reference or analog input voltages one diode drop below V

wired together (unless otherwise noted).

= 5V, V

–

or one diode drop above VCC. Be careful during testing at low

= 5V, V

REF

= 5V, 1LSB (bipolar) = 2(5V)/4096 = 2.44mV.

REF

–

= 0V, V– = 0V for unipolar mode and

REF

) divided by 4096.

REF

VCC levels (4.5V), as high level reference or analog inputs (5V) can cause this input diode to conduct, especially at elevated temperatures and cause errors for inputs near full scale. This spec allows 50mV forward bias of either diode. This means that as long as the reference or analog input does not exceed the supply voltage by more than 50mV, the output code will be correct. To achieve an absolute 0V to 5V input voltage range will therefore require a minimum supply voltage of 4.950V over initial tolerance, temperature variations and loading.

Note 8: Channel leakage current is measured after the channel selection. Note 9: To minimize errors caused by noise at the chip select input, the

internal circuitry waits for two ACLK falling edge after a chip select falling edge is detected before responding to control input signals. Therefore, no attempt should be made to clock an address in or data out until the minimum chip select setup time has elapsed.

Note 10: Increased leakage currents at elevated temperatures cause the S/H to droop, therefore it's recommended that f

≥ 15kHz at 25°C.

ACLK

≥ 125kHz at 85°C and

ACLK

1290fe

LTC1290

AMBIENT TEMPERATURE, TA (°C)

–50

MAGNITUDE OF OFFSET CHANGE ⏐∆OFFSET⏐ (LSB)

0.5

0.4

0.3

0.2

0.1

–10

50 130

1290 • TPC06

–30 10

110

ACLK = 4MHz V

= 5V

REF

= 5V

REFERENCE VOLTAGE, V

REF

(V)

OFFSET ERROR (LSB = • V

REF

)

1290 • TPC03

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

4096

VOS = 0.25mV

VOS = 0.125mV

VCC = 5V

LPER

Supply Current vs Supply Voltage

ACLK = 4MHz

= 25°C

(mA)

SUPPLY CURRENT, I

ATYPICA

CCHARA TERIST

Supply Current vs Temperature

ACLK = 4MHz

= 5V

(mA)

SUPPLY CURRENT, I

ICS

Unadjusted Offset Voltage vs Reference Voltage

46810

Change in Linearity vs Reference Voltage

1.25

)

REF

1.00

4096

0.75

0.50

0.25

LINEARITY ERROR (LSB = • V

Change in Linearity Error vs Temperature

0.6 ACLK = 4MHz

0.5 V

0.4

0.3

0.2

0.1

–30 10

–50

MAGNITUDE OF LINEARITY CHANGE ⏐∆LINEARITY⏐ (LSB)

SUPPLY VOLTAGE, VCC (V)

VCC = 5V

REFERENCE VOLTAGE, V

= 5V

REF

–10

AMBIENT TEMPERATURE, TA (°C)

50 130

REF

1290 • TPC01

(V)

1290 • TPC04

110

1290 • TPC07

3 –50

–10 70

–30

10 90 110 130

AMBIENT TEMPERATURE, TA (°C)

Change in Gain vs Reference Voltage

)

REF

–0.1

4096

–0.2

–0.3

–0.4

VCC = 5V

CHANGE IN GAIN ERROR (LSB = • V

–0.5

REFERENCE VOLTAGE, V

Change in Gain Error vs Temperature

0.5 ACLK = 4MHz

= 5V

0.4

REF

0.3

0.2

0.1

MAGNITUDE OF GAIN CHANGE ⏐∆GAIN⏐ (LSB)

–30 10

–50

–10

AMBIENT TEMPERATURE, TA (°C)

50 130

LT1290 • TPC02

Change in Offset vs Temperature

REF

(V)

1290 • TPC05

Maximum ACLK Frequency vs Source Resistance

MAXIMUM ACLK FREQUENCY* (MHz)

110

1290 • TPC08

100

* MAXIMUM ACLK FREQUENCY REPRESENTS THE

ACLK FREQUENCY AT WHICH A 0.1LSB SHIFT IN THE ERROR AT ANY CODE TRANSITION FROM ITS 4MHz VALUE IS FIRST DETECTED.

1k 10 k 100k

SOURCE

V R

(Ω)

IN SOURCE

VCC = 5V

= 5V

REF

= 25°C

–

INPUT

1290 • TPC09

1290fe

AMBIENT TEMPERATURE, TA (°C)

–50

SUPPLY CURRENT, I

(µA)

–10

50 130

1290 • TPC12

–30 10

110

ACLK OFF DURING POWER SHUTDOWN

TYPICAL PERFOR A CE CHARACTERISTICS

LTC1290

Maximum Filter Resistor vs Cycle Time

10k

** (Ω)

FILTER

100

FILTER

≥ 1µF

MAXIMUM R

1.0 10 1000 10000

** MAXIMUM R

VALUE AT WHICH A 0.1LSB CHANGE IN FULL-SCALE ERROR FROM ITS VALUE AT R

FILTER

100

CYCLE TIME, t

REPRESENTS THE FILTER RESISTOR

(µs)

CYC

= 0 IS FIRST DETECTED.

FILTER

Supply Current (Power Shutdown) vs ACLK

200

VCC = 5V

180

CMOS LEVELS

160

(µA)

140

120

100

SUPPLY CURRENT, I

1.00 2.00 ACLK FREQUENCY (MHz)

3.00

–

1290 • TPC10

1290 • TPC13

4.00

Sample-and-Hold Acquisition Time vs Source Resistance

100

= 5V

REF

= 5V

= 25°C

0V TO 5V INPUT STEP

SOURCE

–

S & H AQUISITION TIME TO 0.02% (µs)

100

1k 10k

+ (Ω)

SOURCE

Input Channel Leakage Current vs Temperature

1000

900

800

700

600

500

400

300

200

100

INPUT CHANNEL LEAKAGE CURRENT (nA)

–30 10

–10

–50

AMBIENT TEMPERATURE, TA (°C)

GUARANTEED

ON CHANNEL

OFF CHANNEL

70 90

50 130

LTC1290 • TPC11

110

1290 • TPC14

Supply Current (Power Shutdown) vs Temperature

Noise Error vs Reference Voltage

2.25 LTC1290 NOISE 200µV

2.00

1.75

1.50

1.25

1.00

0.75

0.50

PEAK-TO-PEAK NOISE ERROR (LSBs)

0.25

REFERENCE VOLTAGE, V

P-P

REF

(V)

1290 • TPC15

PI FU CTIO S

CH0 to CH7 (Pin 1 to Pin 8): Analog Inputs. The analog inputs must be free of noise with respect to AGND.

COM (Pin 9): Common. The common pin defines the zero reference point for all single-ended inputs. It must be free of noise and is usually tied to the analog ground plane.

DGND (Pin 10): Digital Ground. This is the ground for the internal logic. Tie to the ground plane.

AGND (Pin 11): Analog Ground. AGND should be tied directly to the analog ground plane.

–

(Pin 12): Negative Supply. Tie V– to most negative

potential in the circuit. (Ground in single supply applications.)

–

, REF+ (Pins 13, 14): Reference Inputs. The refer-

REF

ence inputs must be kept free of noise with respect to AGND.

CS (Pin 15): Chip Select Input. A logic low on this input enables data transfer.

(Pin 16): Digital Data Output. The A/D conversion

OUT

result is shifted out of this output.

1290fe

LTC1290

OUT

100pF

TEST POINT

5V WAVEFORM 2

WAVEFORM 1

LTC1290 • TC02

PI FU CTIO S

(Pin 17): Digital Data Input. The A/D configuration

word is shifted into this input after CS is recognized.

SCLK (Pin 18): Shift Clock. This clock synchronizes the serial data transfer.

BLOCK DIAGRAM

INPUT

SHIFT

ANALOG

INPUT MUX

SAMPLE-

AND-

HOLD

COMP

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

ACLK (Pin 19): A/D Conversion Clock. This clock controls the A/D conversion process.

VCC (Pin 20): Positive Supply. This supply must be kept free of noise and ripple by bypassing directly to the analog ground plane.

SCLK

OUTPUT

SHIFT

12-BIT

SAR

12-BIT

CAPACITIVE

DAC

OUT

ACLK

TEST CIRCUITS

DGND

AGND

On and Off Channel Leakage Current

ON CHANNEL

•

OFF CHANNELS

LTC1290 • TC01

POLARITY

OFF

–

REF

–

REF

CONTROL

AND

TIMING

Load Circuit for t

LTC1290 • BD

and t

dis

1290fe

TEST CIRCUITS

LTC1290

Voltage Waveforms for D

SCLK

OUT

0.8V

Voltage Waveform for D

OUT

Load Circuit for t

Delay Time, t

OUT

dDO

Rise and Fall Times, tr, t

OUT

, tr and t

dDO

2.4V

0.4V

LTC1290 • TC03

2.4V

0.4V

LTC1290 • TC04

1.4V

OUT

100pF

Voltage Waveforms for ten and t

ACLK

OUT

WAVEFORM 1

(SEE NOTE 1)

OUT

WAVEFORM 2

(SEE NOTE 2)

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.

2.4V

0.8V

TEST POINT

1290 • TC05

dis

2.0V

90%

dis

10%

LTC1290 • TC06

1290fe

LTC1290

PPLICATI

I FOR ATIO

The LTC1290 is a data acquisition component which contains the following functional blocks:

1. 12-bit successive approximation capacitive A/D converter

2. Analog multiplexer (MUX)

3. Sample-and-hold (S/H)

4. Synchronous, full duplex serial interface

5. Control and timing logic

DIGITAL CONSIDERATIONS Serial Interface

The LTC1290 communicates with microprocessors and other external circuitry via a synchronous, full duplex, four-wire serial interface (see Operating Sequence). The shift clock (SCLK) synchronizes the data transfer with each bit being transmitted on the falling SCLK edge and captured on the rising SCLK edge in both transmitting and receiving systems. The data is transmitted and received simultaneously (full duplex).

Data transfer is initiated by a falling chip select (CS) signal. After the falling CS is recognized, an 8-bit input word is shifted into the D

input which configures the LTC1290

for the next conversion. Simultaneously, the result of the

previous conversion is output on the D

line. At the end

OUT

of the data exchange the requested conversion begins and CS should be brought high. After t

, the conversion is

CONV

complete and the results will be available on the next data transfer cycle. As shown below, the result of a conversion is delayed by one CS cycle from the input word requesting it.

DIN

OUTDOUT

WORD 1

WORD 0

DATA

TRANSFER

CONV

A/D

CONVERSION

WORD 2

WORD 1

OUT

DATA

TRANSFER

CONVERSION

CONV

A/D

OUT

WORD 3

WORD 2

LTC1290 • AI01

Input Data Word

The LTC1290 8-bit data word is clocked into the D

input

on the first eight rising SCLK edges after chip select is recognized. Further inputs on the D

pin are then ignored

until the next CS cycle. The eight bits of the input word are defined as follows:

SGL/ DIFF

SELECT

ODD/ SIGN

MUX ADDRESS

UNIPOLAR/

BIPOLAR

SELECT

UNI MSBF WL1

MSB-FIRST/

LSB-FIRST

WORD

LENGTH

WL0

LTC1290 • AI02

SCLK

OUT

Operating Sequence

(Example: Differential Inputs (CH3-CH2), Bipolar, MSB-First and 12-Bit Word Length)

123456789101112

SHIFT CONFIGURATION

WORD IN

CYC

SMPL

DON’T CARE

CONV

B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 (SB)

SHIFT A/D RESULT OUT AND

NEW CONFIGURATION WORD IN

LTC1290 • AI03

1290fe

LTC1290

PPLICATI

I FOR ATIO

MUX Address

The first four bits of the input word assign the MUX configuration for the requested conversion. For a given channel selection, the converter will measure the voltage between the two channels indicated by the + and – signs in the selected row of Table 1. Note that in differential

Table 1. Multiplexer Channel Selection

MUX ADDRESS

ODD

SIGN

SELECT

1 0

SGL/

DIFF

00 00+– 00 01 +– 00 10 +– 00 11 + – 01 00–+ 01 01 –+ 01 10 –+ 01 11 – +

DIFFERENTIAL CHANNEL SELECTION

567

mode (SGL/DIFF = 0) measurements are limited to four adjacent input pairs with either polarity. In single-ended mode, all input channels are measured with respect to COM.

MUX ADDRESS

SGL/ DIFF

10 00 + – 10 01 + – 10 10 + – 10 11 + – 11 00 + – 11 01 + – 11 10 + – 11 11 + –

ODD

SIGN

SELECT

1 0

SINGLE-ENDED CHANNEL SELECTION

0 1 2 3 4 5 6 7 COM

CHANNEL

0,1

{

2,3

{

4,5

{

6,7

{

4 Differential

(–) (+)

–

(–)

(+)

– +

(–) (+)

– +

(–) (+)

–

4,5

6,7

8 Single-Ended

CHANNEL

COM (–)

Changing the MUX Assignment “On the Fly”

{

– +

{

–

COM (UNUSED)

1ST CONVERSION 2ND CONVERSION

5,4

{

COM (–)

Combinations of Differential and Single-Ended

CHANNEL

0,1

2,3

{

– –

{

COM (–)

– +

+ +

Figure 1. Examples of Multiplexer Options on the LTC1290

LTC1290 • F01

1290fe

LTC1290

PPLICATI

I FOR ATIO

Unipolar/Bipolar (UNI)

The fifth input bit (UNI) determines whether the conversion will be unipolar or bipolar. When UNI is a logical one, a unipolar conversion will be performed on the selected

Unipolar Output Code (UNI = 1)

OUTPUT CODE

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

•

• 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

INPUT VOLTAGE

– 1LSB

REF

– 2LSB

REF

•

1LSB

INPUT VOLTAGE

= 5V)

REF

4.9988V

4.9976V

•

0.0012V 0V

LTC1290 • AI04a

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

input voltage. When UNI is a logical zero, a bipolar conversion will result. The input span and code assignment for each conversion type are shown in the figures below.

Unipolar Transfer Curve (UNI = 1)

•

1LSB

REF

– 2LSB

REF

– 1LSB

REF

LTC1290 AI04b

OUTPUT CODE

0 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 0

•

• 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

–V

REF

+ 1LSB

–V

REF

INPUT VOLTAGE

– 1LSB

REF

– 2LSB

REF

•

1LSB

0 1 1 1 1 1 1 1 1 1 1 1

0 1 1 1 1 1 1 1 1 1 1 0

0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

Bipolar Output Code (UNI = 0)

INPUT VOLTAGE

= 5V)

REF

4.9976V

4.9851V

0.0024V

•

OUTPUT CODE

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

•

• 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0

Bipolar Transfer Curve (UNI = 0)

•

–2LSB

–1LSB

1LSB

1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 0

•

INPUT VOLTAGE

–1LSB

–2LSB

•

) + 1LSB

–(V

REF

)

– (V

REF

INPUT VOLTAGE

= 5V)

REF

–0.0024V –0.0048V

•

• –4.9976V –5.0000V

LTC1290 AI05a

REF

– 1LSB

– 2LSB

REF

1 0 0 0 0 0 0 0 0 0 0 1

1 0 0 0 0 0 0 0 0 0 0 0

LTC1290 AI05b

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MSB-First/LSB-First Format (MSBF)

The output data of the LTC1290 is programmed for MSBfirst or LSB-first sequence using the MSBF bit. For MSB first output data the input word clocked to the LTC1290 should always contain a logical one in the sixth bit location (MSBF bit). Likewise for LSB-first output data the input word clocked to the LTC1290 should always contain a zero in the MSBF bit location. The MSBF bit affects only the order of the output data word. The order of the input word is unaffected by this bit.

MSBF OUTPUT FORMAT

0 LSB First 1 MSB First

Word Length (WL1, WL0) and Power Shutdown

The last two bits of the input word (WL1 and WL0) program the output data word length and the power shutdown feature of the LTC1290. Word lengths of 8, 12 or 16 bits can be selected according to the following table. The WL1 and WL0 bits in a given DIN word control the length of the present, not the next, D

word. WL1 and

OUT

WL0 are never “don’t cares” and must be set for the correct D

word length even when a “dummy” DIN word

OUT

is sent. On any transfer cycle, the word length should be made equal to the number of SCLK cycles sent by the MPU. Power down will occur when WL1 = 0 and WL0 = 1 is selected. The previous conversion result will be clocked out as a 10 bit word so a “dummy” conversion is required before powering down the LTC1290. Conversions are

resumed once CS goes low or an SCLK is applied, if CS is already low.

WL1 WL0 OUTPUT WORD LENGTH

0 0 8-Bits 0 1 Power Shutdown 1 0 12-Bits 1 1 16-Bits

Deglitcher

A deglitching circuit has been added to the Chip Select input of the LTC1290 to minimize the effects of errors caused by noise on that input. This circuit ignores changes in state on the CS input that are shorter in duration than one ACLK cycle. After a change of state on the CS input, the LTC1290 waits for two falling edge of the ACLK before recognizing a valid chip select. One indication of CS recognition is the D

line becoming active (leaving the

OUT

Hi-Z state). Note that the deglitching applies to both the rising and falling CS edges.

CS Low During Conversion

In the normal mode of operation, CS is brought high during the conversion time. The serial port ignores any SCLK activity while CS is high. The LTC1290 will also operate with CS low during the conversion. In this mode, SCLK must remain low during the conversion as shown in the following figure. After the conversion is complete, the D

line will become active with the first output bit. Then

OUT

the data transfer can begin as normal.

ACLK

OUT

Low CS Recognized Internally

ACLK

HI-Z

VALID OUTPUT

LTC1290 • AI06

OUT

VALID OUTPUT

High CS Recognized Internally

HI-Z

LTC1290 • AI07

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8-Bit Word Length

SCLK

OUT

MSB-FIRST

OUT

LSB-FIRST

12-Bit Word Length

(SB)

B11

B10 B9 B8 B7 B4

B0 B1 B2 B3 B4 B7B5 B6

SMPL

B6 B5

SMPL

CONV

THE LAST FOUR BITS ARE TRUNCATED

CONV

SCLK

OUT

MSB-FIRST

OUT

LSB-FIRST

16-Bit Word Length

SCLK

OUT

MSB-FIRST

OUT

LSB-FIRST

* IN UNIPOLAR MODE, THESE BITS ARE FILLED WITH ZEROS. IN BIPOLAR MODE, THE SIGN BIT IS EXTENDED INTO THESE LOCATIONS.

(SB)

B11

B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11

(SB)

B11

B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11

10 12

SMPL

(SB)

12 16

FILL

ZEROS

(SB)

CONV

***

LTC1290 F02

Figure 2. Data Output (D

) Timing with Different Word Lengths

OUT

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SCLK

OUT

SCLK

SHIFT

MUX ADDRESS

SHIFT

MUX ADDRESS

U S

I FOR ATIO

SMPL

SAMPLE ANALOG

INPUT

SMPL

SAMPLE ANALOG

INPUT

DON’T CARE

48 TO 52

ACLK CYC

B11B10B9B8B7B6B5B4B3B2B1B0B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

Figure 3. CS High During Conversion

48 TO 52

ACLK CYC

SCLK MUST

REMAIN LOW

SHIFT RESULT OUT

AND NEW ADDRESS IN

SHIFT RESULT OUT

AND NEW ADDRESS IN

LTC1290 F03

OUT

DON’T CARE

Figure 4. CS Low During Conversion (CS Must go High to Low Once to Insure Proper Operation in this Mode)

Microprocessor Interfaces

The LTC1290 can interface directly (without external hardware) to most popular microprocessor (MPU) synchronous serial formats (see Table 2). If an MPU without a serial interface is used, then four of the MPU’s parallel port lines can be programmed to form the serial link to the LTC1290. Included here are two serial interface examples and one example showing a parallel port programmed to form the serial interface

B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

LTC1290 F04

Serial Port Microprocessors

Most synchronous serial formats contain a shift clock (SCLK) and two data lines, one for transmitting and one for receiving. In most cases data bits are transmitted on the falling edge of the clock (SCLK) and captured on the rising edge. However, serial port formats vary among MPU manufactures as to the smallest number of bits that can be sent in one group (e.g., 4-bit, 8-bit or 16-bit transfers). They also vary as to the order in which the bits are transmitted (LSB or MSB first). The following examples show how the LTC1290 accommodates these differences.

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Table 2. Microprocessors with Hardware Serial Interfaces Compatible with the LTC1290**

PART NUMBER TYPE OF INTERFACE

Motorola

MC6805S2, S3 SPI MC68HC11 SPI MC68HC05 SPI

RCA

CDP68HC05 SPI

Hitachi

HD6305 SCI Synchronous HD6301 SCI Synchronous HD63701 SCI Synchronous HD6303 SCI Synchronous HD64180 SCI Synchronous

National Semiconductor

COP400 Family MICROWIRE COP800 Family MICROWIRE/PLUS NS8050U MICROWIRE/PLUS HPC16000 Family MICROWIRE/PLUS

Texas Instruments

TMS7002 Serial Port TMS7042 Serial Port TMS70C02 Serial Port TMS70C42 Serial Port TMS32011* Serial Port TMS32020 Serial Port TMS370C050 SPI

*Requires external hardware ** Contact factory for interface information for processors not on this list

Hardware and Software Interface to COP402 Processor

ANALOG

INPUTS

LTC1290

•

SCLK

OUT

COP402

LTC1290 • AI08

National MICROWIRE (COP402)

The COP402 transfers data MSB first and in 4-bit increments (nibbles). This is easily accommodated by setting the LTC1290 to MSB-first format and 12-bit word length. The data output word is then received by the COP402 in three 4-bit blocks.

COP402 Code

MNEMONIC COMMENTS

CLRA Must be First Instruction LBI 1,0 BR = 1BD = 0 Initialize B Reg. STII 8 First D STII E Second D STII 0 Null Data in $12, B = $13 LEI C Set EN to (1100) BIN

LOOP SC Carry Set

LDD 1,0 Load First D OGI 0 Go (CS) Cleared XAS ACC to Shift Reg. Begin Shift LDD 1,1 Load Next D NOP Timing XAS Next Nibble, Shift Continues XIS 0 First Nibble D LDD 1,2 Put Null Data in ACC XAS Shift Continues, D XIS 0 Next Nibble D RC Clear Carry CLRA Clear ACC XAS Third Nibble D OGI 1 Go (CS) Set XIS 0 Third Nibble D LBI 1,3 Set B Reg. For Next Loop

Nibble in $10

Nibble in $11

Nibble In ACC

Nibble in ACC

to $13

OUT

to ACC

OUT

to $14

to ACC

to $15

Motorola SPI (MC68HC05C4)

The MC68HC05C4 transfers data MSB first and in 8-bit increments. Programming the LTC1290 for MSB-first format and 16-bit word length allows the 12-bit data output to be received by the MPU as two 8-bit bytes with the final four unused bits filled with zeros by the LTC1290.

from LTC1290 Stored in COP402 RAM

OUT

†

MSB

LOCATION $13

LOCATION $14

LOCATION $15

†

B11 IS MSB IN UNIPOLAR OR SIGN BIT IN BIPOLAR

B10 B9 B8B11

B6 B5

B2 B1

LSB

FIRST 4 BITS

SECOND 4 BITS

THIRD 4 BITS

MICROWIRE and MICROWIRE PLUS are trademarks of National Semiconductor Corp

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Hardware and Software Interface to Motorola MC68HC05C4 Processor

LTC1290

ANALOG

INPUTS

LOCATION $61

LOCATION $62

*B11 IS MSB IN UNIPOLAR OR SIGN BIT IN BIPOLAR

•

from LTC1290 Stored in MC68HC05C4 RAM

OUT

MSB*

SCLK

OUT

B10 B9 B8B11 B6 B5 B4B7

LSB

B2 B1 B0B3 0 0 00

MC68HC05C4

SCK

MOSI

MISO

LTC1290 • AI09

MC68HC05C4 Code

MNEMONIC COMMENTS

LDA #$50 Configuration Data for SPCR STA $0A Load Data Into SPCR ($0A) LDA #$FF Config. Data for Port C DDR STA $06 Load Data Into Port C DDR LDA #$0F Load LTC1290 D STA $50 Load LTC1290 D

START BCLR 0,$20 CO Goes Low (CS Goes Low)

LDA $50 Load D STA $0C Load D

NOP 8 NOPs for Timing

Into ACC from $50

Into SPI, Start SCK

Data Into ACC

Data Into $50

BYTE 1

BYTE 2

Parallel Port Microprocessors

When interfacing the LTC1290 to an MPU which has a parallel port, the serial signals are created on the port with software. Three MPU port lines are programmed to create the CS, SCLK and D port line reads the D

signals for the LTC1290. A fourth

line. An example is made of the

OUT

Intel 8051/8052/80C252 family.

Intel 8051

To interface to the 8051, the LTC1290 is programmed for MSB-first format and 12-bit word length. The 8051 generates CS, SCLK and D

on three port lines and reads D

OUT

on the fourth.

Hardware and Software Interface to Intel 8051 Processor

ANALOG

INPUTS

LTC1290

OUT

•

from LTC1290 Stored in 8051 RAM

OUT

MSB*

SCLK

ACLK

B10 B9 B8B11 B6 B5 54B7

8051

P1.1

P1.2

P1.3

P1.4

ALE

LTC1290 • AI10

LDA $0B Check SPI Status Reg LDA $0C Load LTC1290 MSBs Into ACC STA $61 Store MSBs in $61 STA $0C Start Next SPI Cycle

NOP 6 NOPs for Timing

BSET 0,$02 CO Goes High (CS Goes High) LDA $0B Check SPI Status Register LDA $0C Load LTC1290 LSBs Into ACC STA $62 Store LSBs in $62

LSB

*B11 IS MSB IN UNIPOLAR OR SIGN BIT IN BIPOLAR

B2 B1 B0B3 0 0 00

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LTC1290

PPLICATI

8051 Code

MNEMONIC COMMENTS

MOV P1,#02H Bit 1 Port 1 Set as Input CLR P1.3 SCLK Goes Low SETB P1.4 CS Goes High

CONT MOV A,#0EH D

CLR P1.4 CS Goes Low MOV R4,#08H Load Counter NOP Delay for Deglitcher

LOOP MOV C,P1.1 Read Data Bit Into Carry

RLC A Rotate Data Bit Into ACC MOV P1.2,C Output D SETB P1.3 SCLK Goes High CLR P1.3 SCLK Goes Low DJNZ R4,LOOP Next Bit MOV R2,A Store MSBs in R2 MOV C,P1.1 Read Data Bit Into Carry CLR A Clear ACC RLC A Rotate Data Bit Into ACC SETB P1.3 SCLK Goes High CLR P1.3 SCLK Goes Low MOV C,P1.1 Read Data Bit Into Carry RLC A Rotate Data Bit Into ACC SETB P1.3 SCLK Goes High CLR P1.3 SCLK Goes Low MOV C,P1.1 Read Data Bit Into Carry RLC A Rotate Data Bit Into ACC SETB P1.3 SCLK Goes High CLR P1.3 SCLK Goes Low MOV C, P1.1 Read Data Bit Into Carry RRC A Rotate Right Into ACC RRC A Rotate Right Into ACC RRC A Rotate Right Into ACC RRC A Rotate Right Into ACC MOV R3,A Store LSBs in R3 SETB P1.3 SCLK Goes High CLR P1.3 SCLK Goes Low SETB P1.4 CS Goes High

I FOR ATIO

Word for LTC1290

Bit to LTC1290

Sharing the Serial Interface

The LTC1290 can share the same 3-wire serial interface with other peripheral components or other LTC1290s (see Figure 5). In this case, the CS signals decide which LTC1290 is being addressed by the MPU.

ANALOG CONSIDERATIONS

1. Grounding

The LTC1290 should be used with an analog ground plane and single point grounding techniques.

AGND (Pin 11) should be tied directly to this ground plane.

DGND (Pin 10) can also be tied directly to this ground plane because minimal digital noise is generated within the chip itself.

(Pin 20) should be bypassed to the ground plane with

a 22µF tantalum with leads as short as possible. V– (Pin

12) should be bypassed with a 0.1µF ceramic disk. For single supply applications, V– can be tied to the ground plane.

It is also recommended that REF– (Pin 13) and COM (Pin

9) be tied directly to the ground plane. All analog inputs should be referenced directly to the single point ground. Digital inputs and outputs should be shielded from and/or routed away from the reference and analog circuitry.

MOV R5,#0BH Load Counter

DELAY DJNZ R5,DELAY Go to Delay if Not Done

OUTPUT PORT

SERIAL DATA

MPU

Figure 5. Several LTC1290s Sharing One 3-Wire Serial Interface

LTC1290

8 CHANNELS

LTC1290

8 CHANNELS 8 CHANNELS

LTC1290

3-WIRE SERIAL INTERFACE TO OTHER PERIPHERALS OR LTC1290s

LTC1290 F05

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PPLICATI

ANALOG

GROUND

PLANE

Figure 6. Example Ground Plane for the LTC1290

I FOR ATIO

22µF

TANTALUM

–

0.1µF CERAMIC DISK

LTC1290 F06

VERTICAL: 0.5mV/DIV

HORIZONTAL: 10µs/DIV

Figure 7. Poor VCC Bypassing. Noise and Ripple Can Cause A/D Errors

Figure 6 shows an example of an ideal ground plane design for a two-sided board. Of course, this much ground plane will not always be possible, but users should strive to get as close to this ideal as possible.

2. Bypassing

For good performance, VCC must be free of noise and ripple. Any changes in the V

voltage with respect to

analog ground during a conversion cycle can induce errors or noise in the output code. V be kept below 0.5mV by bypassing the V

noise and ripple can

pin directly to

the analog ground plane with a 22µF tantalum capacitor and leads as short as possible. The lead from the device to the VCC supply should also be kept to a minimum and the

supply should have a low output impedance such as

that obtained from a voltage regulator (e.g., LT1761). Figures 7 and 8 show the effects of good and poor V

bypassing.

3. Analog Inputs

Because of the capacitive redistribution A/D conversion techniques used, the analog inputs of the LTC1290 have capacitive switching input current spikes. These current

VERTICAL: 0.5mV/DIV

HORIZONTAL: 10µs/DIV

Figure 8. Good VCC Bypassing Keeps Noise and Ripple on VCC Below 1mV

spikes settle quickly and do not cause a problem. However, if large source resistances are used or if slow settling op amps drive the inputs, care must be taken to insure that the transients caused by the current spikes settle completely before the conversion begins.

Source Resistance

The analog inputs of the LTC1290 look like a 100pF capacitor (C shown in Figure 9.

) in series with a 500Ω resistor (RON) as

CIN gets switched between the 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.

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PPLICATI

VIN +

VIN –

SOURCE

I FOR ATIO

“+”

INPUT

“–”

INPUT

–

4TH SCLK

LAST SCLK

LTC1290

= 500Ω

100pF

LTC1290 F09

Figure 9. Analog Input Equivalent Circuit

“+” Input Settling

This input capacitor is switched onto the “+” input during the sample phase (t

, see Figure 10). The sample

SMPL

phase starts at the 4th SCLK cycle and lasts until the falling edge of the last SCLK (the 8th, 12th or 16th SCLK cycle depending on the selected word length). The voltage on the “+” input must settle completely within this sample time. Minimizing R

SOURCE

and C1 will improve the input settling time. If large “+” input source resistance must be used, the sample time can be increased by using a slower SCLK frequency or selecting a longer word length. With

the minimum possible sample time of 2µs, R

SOURCE

< 1k

and C1 < 20pF will provide adequate settling.

“–” Input Settling

At the end of the sample phase the input capacitor switches to the “–” input and the conversion starts (see Figure 10). During the conversion, the “+” input voltage is effectively “held” by the sample-and-hold and will not affect the conversion result. However, it is critical that the “–” input voltage be free of noise and settle completely during the first four ACLK cycles of the conversion time. Minimizing R

SOURCE

–

and C2 will improve settling time. If large “–” input source resistance must be used, the time allowed for settling can be extended by using a slower ACLK frequency. At the maximum ACLK rate of 4MHz, R

SOURCE

–

< 250Ω and C2 < 20pF will provide adequate settling.

Input Op Amps

When driving the analog inputs with an op amp it is important that the op amp settle within the allowed time (see Figure 10). Again, the “+” and “–” input sampling times can be extended as described above to accommodate slower op amps. Most op amps including the LT1797, LT1800 and LT1812 single supply op amps can be made to settle well even with the minimum settling windows of 2µs (“+” input) and 1µs (“–” input) which occur at the

“+” INPUT

“–” INPUT

SCLK

ACLK

SAMPLE HOLD

MUX ADDRESS

SHIFTED IN

1234

Figure 10. “+” and “–” Input Settling Windows

“+” INPUT

MUST SETTLE

DURING THIS TIME

SMPL

• • •

LAST SCLK (8TH, 12TH OR 16TH DEPENDING ON WORD LENGTH)

234

1ST BIT TEST

“–” INPUT MUST SETTLE

DURING THIS TIME

• • •

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maximum clock rates (ACLK = 4MHz and SCLK = 2MHz). Figures 11 and 12 show examples of adequate and poor op amp settling.

VERTICAL: 5mV/DIV

HORIZONTAL: 500ns/DIV

Figure 11. Adequate Settling of Op Amps Driving Analog Input

Input Leakage Current

FILTER

Figure 13. RC Input Filtering

FILTER

"+"

LTC1290

"–"

LTC1290 F13

Input leakage currents can also create errors if the source resistance gets too large. For instance, the maximum input leakage specification of 1µA (at 125°C) flowing through a source resistance of 1kΩ will cause a voltage drop of 1mV or 0.8LSB. This error will be much reduced at lower temperatures because leakage drops rapidly (see the typical curve of Input Channel Leakage Current vs Temperature).

VERTICAL: 5mV/DIV

HORIZONTAL: 20µs/DIV

Figure 12. Poor Op Amp Settling Can Cause A/D Errors

RC Input Filtering

It is possible to filter the inputs with an RC network as shown in Figure 13. For large values of C

(e.g., 1µF), the capacitive

input switching currents are averaged into a net DC current. Therefore, a filter should be chosen with a small resistor and large capacitor to prevent DC drops across the resistor. The magnitude of the DC current is approximately I (100pF)(V

) and is roughly proportional to VIN. When

IN/tCYC

running at the minimum cycle time of 20µs, the input current equals 25µA at V

= 5V. In this case, a filter resistor

of 5Ω will cause 0.1LSB of full-scale error. If a larger filter resistor must be used, errors can be eliminated by increasing the cycle time as shown in the typical curve of Maximum Filter Resistor vs Cycle Time.

Noise Coupling Into Inputs

High source resistance input signals (>500Ω) are more sensitive to coupling from external sources. It is preferable to use channels near the center of the package (i.e., CH2 to CH7) for signals which have the highest output resistance because they are essentially shielded by the pins on the package ends (DGND and CH0). Grounding any unused inputs (especially the end pin, CH0) will also reduce outside coupling into high source resistances.

4. Sample-and-Hold

Single-Ended Inputs

The LTC1290 provides a built-in sample-and-hold (S&H) function for all signals acquired in the single-ended mode (COM pin grounded). This sample-and-hold allows the LTC1290 to convert rapidly varying signals (see the typical curve of S&H Acquisition Time vs Source Resistance). The input voltage is sampled during the t

time as shown in

SMPL

Figure 10. The sampling interval begins after the fourth MUX address bit is shifted in and continues during the remainder of the data transfer. On the falling edge of the final SCLK, the S&H goes into hold mode and the conversion begins. The voltage will be held on either the 8th, 12th or 16th falling edge of the SCLK depending on the word length selected.

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Differential Inputs

I FOR ATIO

With differential inputs or when the COM pin is not tied to ground, the A/D no longer converts just a single voltage but rather the difference between two voltages. In these cases, the voltage on the selected “+” input is still sampled and held and therefore may be rapidly time varying just as in singleended mode. However, the voltage on the selected “–” input must remain constant and be free of noise and ripple throughout the conversion time. Otherwise, the differencing operation may not be performed accurately. The conversion time is 52 ACLK cycles. Therefore, a change in the “–” input voltage during this interval can cause conversion errors. For a sinusoidal voltage on the “–” input this error would be:

ERROR (MAX)

= (V

)(2π)[ f(“–”)](52/f

PEAK

ACLK

)

Where f(“–”) is the frequency of the “–” input voltage,

is its peak amplitude and f

PEAK

the ACLK. In most cases V

ERROR

is the frequency of

ACLK

will not be significant. For a 60Hz signal on the “–” input to generate a 0.25LSB error (300µV) with the converter running at ACLK = 4MHz, its peak value would have to be 61mV.

When driving the reference inputs, two things should be kept in mind:

1. Transients on the reference inputs caused by the capacitive switching currents must settle completely during each bit test (each 4 ACLK cycles). Figures 15 and 16 show examples of both adequate and poor settling. Using a slower ACLK will allow more time for the reference to settle. However, even at the maximum ACLK rate of 4MHz most references and op amps can be made to settle within the 1µs bit time. For example the LT1236 will settle adequately.

2. It is recommended that REF the analog ground plane. If REF

–

input be tied directly to

–

is biased at a voltage other than ground, the voltage must not change during a conversion cycle. This voltage must also be free of noise and ripple with respect to analog ground.

5. Reference Inputs

The voltage between the reference inputs of the LTC1290 defines the voltage span of the A/D converter. The reference inputs will have transient capacitive switching currents due to the switched capacitor conversion technique (see Figure 14). During each bit test of the conversion (every 4 ACLK cycles), a capacitive current spike will be generated on the reference pins by the A/D. These current spikes settle quickly and do not cause a problem. However, if slow settling circuitry is used to drive the reference inputs, care must be taken to insure that transients caused by these current spikes settle completely during each bit test of the conversion.

REF+

OUT

REF

REF–

Figure 14. Reference Input Equivalent Circuit

EVERY 4 ACLK CYCLES

LTC1290

8pF TO 40pF

LTC 1290 F14

VERTICAL: 0.5mV/DIV

HORIZONTAL: 1µs/DIV

Figure 15. Adequate Reference Settling

VERTICAL: 0.5mV/DIV

HORIZONTAL: 1µs/DIV

Figure 16. Poor Reference Settling Can Cause A/D Errors

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PPLICATI

6. Reduced Reference Operation

The effective resolution of the LTC1290 can be increased by reducing the input span of the converter. The LTC1290 exhibits good linearity and gain over a wide range of reference voltages (see the typical curves of Linearity and Gain Error vs Reference Voltage). However, care must be taken when operating at low values of V reduced LSB step size and the resulting higher accuracy requirement placed on the converter. The following factors must be considered when operating at low V

1. Offset

2. Noise

Offset with Reduced V

The offset of the LTC1290 has a larger effect on the output code when the A/D is operated with reduced reference voltage. The offset (which is typically a fixed voltage) becomes a larger fraction of an LSB as the size of the LSB is reduced. The typical curve of Unadjusted Offset Error vs Reference Voltage shows how offset in LSBs is related to reference voltage for a typical value of VOS. For example, a V

of 0.1mV which is 0.1LSB with a 5V reference

becomes 0.4LSB with a 1.25V reference. If this offset is unacceptable, it can be corrected digitally by the receiving system or by offsetting the “–” input to the LTC1290.

I FOR ATIO

REF

because of the

REF

values:

reduce the range of input voltages over which a stable output code can be achieved by 0.64LSB. In this case averaging readings may be necessary.

This noise data was taken in a very clean setup. Any setup induced noise (noise or ripple on VCC, V the internal noise. The lower the reference voltage to be used, the more critical it becomes to have a clean, noise-free setup.

7. LTC1290 AC Characteristics

Two commonly used figures of merit for specifying the dynamic performance of the A/D’s in digital signal processing applications are the Signal-to-Noise Ratio (SNR) and the “effective number of bits (ENOB).” SNR is defined as the ratio of the RMS magnitude of the fundamental to the RMS magnitude of all the nonfundamental signals up to the Nyquist frequency (half the sampling frequency). The theoretical maximum SNR for a sine wave input is given by:

SNR = (6.02N + 1.76dB)

where N is the number of bits. Thus the SNR is a function of the resolution of the A/D. For an ideal 12-bit A/D the SNR is equal to 74dB. A Fast Fourier Transform(FFT) plot of the output spectrum of the LTC1290 is shown in Figures 17a and 17b. The input (fIN) frequencies are 1kHz and 25kHz with the sampling frequency (f obtained from the plot are 73.25dB and 72.54dB.

, VIN or V–) will add to

REF

) at 50.6kHz. The SNR

Noise with Reduced V

The total input referred noise of the LTC1290 can be reduced to approximately 200µV peak-to-peak using a ground plane, good bypassing, good layout techniques and minimizing noise on the reference inputs. This noise is insignificant with a 5V reference but will become a larger fraction of an LSB as the size of the LSB is reduced. The typical curve of Noise Error vs Reference Voltage shows the LSB contribution of this 200µV of noise.

For operation with a 5V reference, the 200µV noise is only

0.16LSB peak-to-peak. In this case, the LTC1290 noise will contribute virtually no uncertainty to the output code. However, for reduced references, the noise may become a significant fraction of an LSB and cause undesirable jitter in the output code. For example, with a 1.25V reference, this same 200µV noise is 0.64LSB peak-to-peak. This will

REF

Rewriting the SNR expression it is possible to obtain the equivalent resolution based on the SNR measurement.

N = (SNR – 1.76dB)/6.02

This is the so-called effective number of bits (ENOB). For the example shown in Figures 17a and 17b, N = 11.9 bits and 11.8 bits, respectively. Figure 18 shows a plot of ENOB as a function of input frequency. The curve shows the A/D’s ENOB remain in the range of 11.9 to 11.8 for input frequencies up to f

Figure 19 shows an FFT plot of the output spectrum for two tones applied to the input of the A/D. Nonlinearities in the A/D will cause distortion products at the sum and difference frequencies of the fundamentals and products of the fundamentals. This is classically referred to as intermodulation distortion (IMD).

/2.

1290fe

LTC1290

PPLICATI

MAGNITUDE (dB)

–100

–120

–140

EFFECTIVE NUMBER OF BITS

Figure 18. LTC1290 ENOB vs Input Frequency Figure 19. LTC1290 FFT Plot

I FOR ATIO

–20

–40

–60

–80

0 8 12 16 20424

FREQUENCY (kHz)

fIN = 1kHz

SAMPLE

SNR = 73.25dB

= 50.6kHz

1290 • F17a

Figure 17a. LTC1290 FFT Plot

SAMPLE

= 50.6kHz

1290 F18

100

11.6

11.2

10.8

10.4

9.6

9.2

8.8

FREQUENCY (kHz)

fIN = 25kHz

–20

–40

–60

–80

MAGNITUDE (dB)

–100

–120

–140

= 50.6kHz

SAMPLE

SNR = 72.54dB

0 8 12 16 20424

FREQUENCY (kHz)

1290 • F17b

Figure 17b. LTC1290 FFT Plot

–20

–40

–60

–80

MAGNITUDE (dB)

–100

–120

0 8 12 16 20424

FREQUENCY (kHz)

= 5.1kHz

IN1

= 5.6kHz

IN2

SAMPLE

= 50.6kHz

1290 • F19

8. Overvoltage Protection

Applying signals to the analog MUX that exceed the positive or negative supply of the device will degrade the accuracy of the A/D and possibly damage the device. For example this condition would occur if a signal is applied to the analog MUX before power is applied to the LTC1290. Another example is the input source is operating from different supplies of larger value than the LTC1290. These conditions should be prevented either with proper supply sequencing or by use of external circuitry to clamp or current limit the input source. As shown in Figure 20, a 1k resistor is enough to stand off ±15V (15mA for one only channel). If more than one channel exceeds the supplies

then the following guidelines can be used. Limit the current to 7mA per channel and 28mA for all channels. This means four channels can handle 7mA of input current each. Reducing the ACLK and SCLK frequencies from the maximum of 4MHz and 2MHz, respectively, (see Typical Performance Characteristics curves Maximum ACLK Frequency vs Source Resistance and Sample-and-Hold Acquisition Time vs Source Resistance) allows the use of larger current limiting resistors. Use 1N4148 diode clamps from the MUX inputs to VCC and V– if the value of the series resistor will not allow the maximum clock speeds to be used or if an unknown source is used to drive the LTC1290 MUX inputs.

1290fe

LTC1290

PPLICATI

I FOR ATIO

CH0

DGND

LTC1290

AGND

–

22µF

0.1µF

1290 F20

–5V

Figure 20. Overvoltage Protection for MUX

How the various power supplies to the LTC1290 are applied can also lead to overvoltage conditions. For single supply operation (i.e., unipolar mode), if V not tied together, then V

. If this sequence cannot be met, connecting a diode

REF from REF

to VCC is recommended (see Figure 21).

should be turned on first, then

and REF+ are

For dual supplies (bipolar mode) placing two Schottky diodes from V

and V– to ground (Figure 23) will prevent

power supply reversal from occurring when an input source is applied to the analog MUX before power is applied to the device. Power supply reversal occurs, for

–

example, if the input is pulled below V

then VCC will pull a diode drop below ground which could cause the device not to power up properly. Likewise, if the input is pulled above VCC then V– will be pulled a diode drop above ground. If no inputs are present on the MUX, the Schottky diodes are not required if V

–

is applied first, then VCC.

Because a unique input protection structure is used on the digital input pins, the signal levels on these pins can exceed the device V

without damaging the device.

LTC1290

REF

1N4148

22µF

1290 F21

REF

DGND

LTC1290

AGND

–

Figure 21 Figure 22. Power Supply Reversal

1N5817

22µF

–5V

0.1µF

1290 F22

1290fe

LTC1290

PPLICATITYPICAL

A “Quick Look” Circuit for the LTC1290

Users can get a quick look at the function and timing of the LTC1290 by using the following simple circuit. REF+ and

are tied to VCC selecting a 5V input span, CH7 as a

single-ended input, unipolar mode, MSB-first format and 16-bit word length. ACLK and SCLK are tied together and driven by an external clock. CS is driven at 1/128 the clock rate by the CD4520 and D

outputs the data. All other

OUT

pins are tied to a ground plane. The output data from the

pin can be viewed on an oscilloscope which is set up

OUT

to trigger on the falling edge of CS.

Scope Trace of LTC1290 “Quick Look” Circuit Showing A/D Output of 010101010101 (555

A “Quick Look” Circuit for the LTC1290

22µF

LTC1290

CHO

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

DGND

ACLK

SCLK

OUT

REF

AGND

HEX

–

OSCILLOSCOPE

)

f/128

{

CLK

RESET

CLOCK IN 2MHz MAX

CD4520

RESET

CLK

0.1µF

1290 TA02

ACLK/

SCLK

OUT

DEGLITCHER

TIME

MSB

(B11)

VERTICAL: 5V/DIV HORIZONTAL: 1µs/DIV

LSB

(B0)

FILLS

ZEROS

1290fe

LTC1290

PPLICATITYPICAL

SNEAK-A-BIT

The LTC1290’s unique ability to software select the polarity of the differential inputs and the output word length is used to achieve one more bit of resolution. Using the circuit below with two conversions and some software, a 2’s complement 12-bit + sign word is returned to memory inside the MPU. The MC68HC05C4 was chosen as an example, however, any processor could be used.

SNEAK-A-BIT Circuit

22µF

LT1021-5

2MHz

CLOCK

MC68HC05C4

SCLK

–

MOSI

MISO

1290 TA04

0.1µF –5V

OTHER CHANNELS

OR SNEAK-A-BIT

INPUTS

–5V TO 5V

CHO

CH1

CH2

CH3

CH4

LTC1290

CH5

CH6

CH7

COM

DGND

ACLK

SCLK

OUT

REF

AGND

Two 12-bit unipolar conversions are performed: the first over a 0V to 5V span and the second over a 0V to –5V span (by reversing the polarity of the inputs). The sign of the input is determined by which of the two spans contained it. Then the resulting number (ranging from –4095 to +4095 decimal) is converted to 2’s complement notation and stored in RAM.

SNEAK-A-BIT

(+) CH6

(–) CH7

1ST CONVERSION

(–) CH6

(+) CH7

2ND CONVERSION

0V 0V

–5V

1ST CONVERSION 4096 STEPS

SOFTWARE

2ND CONVERSION 4096 STEPS

–5V

8191 STEPS

1290 TA05

SNEAK-A-BIT is a trademark of Linear Technology Corp.

1290fe

LTC1290

PPLICATITYPICAL

SNEAK-A-BIT Code

from LTC1290 in MC68HC05C4 RAM

OUT

Sign

LOCATION $77 B12 B11 B10 B9 B8 B7 B6 B5

LSB

LOCATION $87 B4 B3 B2 B1 B0 Filled with 0s

Words for LTC1290

MUX Addr.

(ODD/SIGN)

0011 1111

0111 1111

0011 1111

UNI

MSBF

Word

Length

1290 TA06

SNEAK-A-BIT Code for the LTC1290 Using the MC68HC05C4

MNEMONIC DESCRIPTION

LDA #$50 Configuration Data for SPCR STA $0A Load Configuration Data into $0A LDA #$FF Configuration Data for Port C DDR STA $06 Load Configuration Data into Port C DDR BSET 0,$02 Make Sure CS is High JSR READ –/+ Dummy Read Configures LTC1290

JSR READ –/+ Read CH6 with Respect to CH7 JSR READ –/+ Read CH7 with Respect to CH6 JSR CHK Sign Determines which Reading has Valid Data,

for next read

Converts to 2’s Complement and Stores in RAM

SNEAK-A-BIT Code for the LTC1290 Using the MC68HC05C4

MNEMONIC DESCRIPTION

READ –/+: LDA #$3F Load DIN Word for LTC1290 into ACC

JSR TRANSFER Read LTC1290 Routine LDA $60 Load MSBs from LTC1290 into ACC STA $71 Store MSBs in $71 LDA $61 Load LSBs from LTC1290 into ACC STA $72 Store LSBs in $72 RTS Return

READ +/–: LDA #$7F Load D

JSR TRANSFER Read LTC1290 Routine LDA $60 Load MSBs from LTC1290 into ACC STA $73 Store MSBs in $73 LDA $61 Load LSBs from LTC1290 into ACC STA $74 Store LSBs in $74 RTS Return

TRANSFER: BCLR 0,$02 CS Goes Low

STA $0C Load D

LOOP 1: TST $0B Test Status of SPIF

BPL LOOP 1 Loop to Previous Instruction if Not Done LDA $0C Load Contents of SPI Data Reg. into ACC STA $0C Start Next Cycle STA $60 Store MSBs in $60

LOOP 2: TST $0B Test Status of SPIF

BPL LOOP 2 Loop to Previous Instruction if Not Done BSET 0,$02 CS Goes High LDA $0C Load Contents of SPI Data Reg. into ACC STA $61 Store LSBs in $61 RTS Return

CHK SIGN: LDA $73 Load MSBs of ± Read into ACC

ORA $74 Or ACC (MSBs) with LSBs of ± Read BEQ MINUS If Result is 0 Go to Minus CLC Clear Carry ROR $73 Rotate Right $73 Through Carry ROR $74 Rotate Right $74 Through Carry LDA $73 Load MSBs of ± Read into ACC STA $77 Store MSBs in RAM Location $77 LDA $74 Load LSBs of ± Read into ACC STA $87 Store LSBs in RAM Location $87 BRA END Go to End of Routine

MINUS: CLC Clear Carry

ROR $71 Shift MSBs of ± Read Right ROR $72 Shift LSBs of ± Read Right COM $71 1’s Complement of MSBs COM $72 1’s Complement of LSBs LDA $72 Load LSBs into ACC ADD #$01 Add 1 to LSBs STA $72 Store ACC in $72 CLRA Clear ACC ADC $71 Add with Carry to MSBs. Result in ACC STA $71 Store ACC in $71 STA $77 Store MSBs in RAM Location $77 LDA $72 Load LSBs in ACC STA $87 Store LSBs in RAM Location $87

END: RTS Return

Word for LTC1290 into ACC

into SPI, Start Transfer

1290fe

LTC1290

PPLICATITYPICAL

Power Shutdown

For battery-powered applications it is desirable to keep power dissipation at a minimum. The LTC1290 can be powered down when not in use reducing the supply current from a nominal value of 5mA to typically 5µA (with ACLK turned off). See the curve for Supply Current (Power Shutdown) vs ACLK if ACLK cannot be turned off when the LTC1290 is powered down. In this case the supply current is proportional to the ACLK frequency and is independent of temperature until it reaches the magnitude of the supply current attained with ACLK turned off.

As an example of how to use this feature let’s add this to the previous application, SNEAK-A-BIT. After the CHK SIGN subroutine call insert the following:

•

JSR CHK SIGN Determines which reading has valid

data, converts to 2’s complement and stores in RAM

JSR SHUTDOWN LTC1290 power shutdown routine

The actual subroutine is:

SHUTDOWN: LDA #$3D Load D

word for

LTC1290 into ACC JSR TRANSFER Read LTC1290 routine RTS Return

To place the device in power shutdown the word length bits are set to WL1 = 0 and WL0 = 1. The LTC1290 is powered up on the next request for a conversion and it’s ready to digitize an input signal immediately.

Power Shutdown Timing Considerations

After power shutdown has been requested, the LTC1290 is powered up on the next request for a conversion. This request can be initiated either by bringing CS low or by starting the next cycle of SCLKs if CS is kept low (see Figures 3 and 4). When the SCLK frequency is much slower than the ACLK frequency a situation can arise where the LTC1290 could power down and then prematurely power back up. Power shutdown begins at the negative going edge of the 10th SCLK once it has been requested. A dummy conversion is executed and the LTC1290 waits for the next request for conversion. If the SCLKs have not finished once the LTC1290 has finished its dummy conversion, it will recognize the next remaining SCLKs as a request to start a conversion and power up the LTC1290 (see Figure 23). To prevent this, bring either CS high at the 10th SCLK (Figure 24) or clock out only 10 SCLKs (Figure 25) when power shutdown is requested.

SCLK

110

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS POWER UP 1290 TAF23

Figure 23. Power Shutdown Timing Problem

110

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

Figure 24. Power Shutdown Timing

110

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

Figure 25. Power Shutdown Timing

POWER UP

1290 TAF24

POWER UP

1290 TAF25

1290fe

LTC1290

PACKAGE DESCRIPTIO

0.300 BSC

(0.762 BSC)

0.008 – 0.018

(0.203 – 0.457)

0° – 15°

NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE OR TIN PLATE LEADS

CORNER LEADS

OPTION

(4 PLCS)

0.015 – 0.060

(0.381 – 1.524)

0.023 – 0.045

(0.584 – 1.143)

HALF LEAD

OPTION

0.045 – 0.068

(1.143 – 1.727)

FULL LEAD

OPTION

0.125

(3.175)

MIN

J Package

20-Lead CERDIP (Narrow 0.300, Hermetic)

(LTC DWG # 05-08-1110)

0.200

(5.080)

MAX

0.220 – 0.310

(5.588 – 7.874)

0.045 – 0.065

(1.143 – 1.651)

0.014 – 0.026

(0.356 – 0.660)

OBSOLETE PACKAGE

0.100

(2.54)

BSC

0.025

(0.635)

RAD TYP

20 16 1517 14 13 1219 18

0.005

(0.127)

MIN

1.060

(26.924)

MAX

109

J20 1298

N Package

20-Lead PDIP (Narrow .300 Inch)

(Reference LTC DWG # 05-08-1510)

19 1112

.255 ± .015*

(6.477 ± 0.381)

.300 – .325

(7.620 – 8.255)

.008 – .015

(0.203 – 0.381)

+.035

.325

–.015

+0.889

8.255

()

–0.381

NOTE:

1. DIMENSIONS ARE

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)

INCHES

MILLIMETERS

.020

(0.508)

MIN

.125 – .145

(3.175 – 3.683)

.120

(3.048)

MIN

.005

(0.127)

MIN

.100

(2.54)

BSC

1.060*

(26.924)

MAX

.045 – .065

(1.143 – 1.651)

1517

131416

910

.065

(1.651)

TYP

.018 ± .003

(0.457 ± 0.076)

N20 0405

1290fe

PACKAGE DESCRIPTIO

LTC1290

SW Package

20-Lead Plastic Small Outline (Wide .300 Inch)

(Reference LTC DWG # 05-08-1620)

.030 ±.005

TYP

.420 MIN

123 N/2

RECOMMENDED SOLDER PAD LAYOUT

.291 – .299

(7.391 – 7.595)

NOTE 4

.010 – .029

.005

(0.127)

RAD MIN

.009 – .013

(0.229 – 0.330)

NOTE:

1. DIMENSIONS IN

2. DRAWING NOT TO SCALE

3. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS

4. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)

(0.254 – 0.737)

NOTE 3

(0.406 – 1.270)

INCHES

(MILLIMETERS)

.050 BSC

× 45°

.016 – .050

.045 ±.005

.325 ±.005

0° – 8° TYP

NOTE 3

.093 – .104

(2.362 – 2.642)

(1.270)

.050

BSC

.014 – .019

(0.356 – 0.482)

(12.598 – 13.005)

19 18

2345

TYP

.496 – .512

NOTE 4

14 13

1112

.394 – .419

(10.007 – 10.643)

N/2

910

.037 – .045

(0.940 – 1.143)

.004 – .012

(0.102 – 0.305)

S20 (WIDE) 0502

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 circuits as described herein will not infringe on existing patent rights.

1290fe

LTC1290

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LTC1286/LTC1298 12-Bit, Micropower Serial ADC in SO-8 1- or 2-Channel, Autoshutdown LTC1293/LTC1294/LTC1296 12-Bit, Multiplexed Serial ADC 6-, 8- or 8-Channel with Shutdown Output LTC1594/LTC1598 12-Bit, Micropower Serial ADC 4- or 8-Channel, 3V Versions Available

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear.com

1290fe

LT/LT 0805 REV E • PRINTED IN USA

LINEAR TECHNOLOGY LTC1290 Technical data

Specifications and Main Features

Frequently Asked Questions

User Manual