Datasheet LTC1090MJ, LTC1090CSW, LTC1090CN, LTC1090AMJ, LTC1090ACN Datasheet (Linear Technology)

...

Page 1

FEATURES

■

Software Programmable Features:

Unipolar/Bipolar Conversions 4 Differential/8 Single Ended Inputs MSB or LSB First Data Sequence Variable Data Word Length

■

Built-In Sample and Hold

■

Single Supply 5V, 10V or ±5V Operation

■

Direct 4 Wire Interface to Most MPU Serial Ports and All MPU Parallel Ports

■

30kHz Maximum Throughput Rate

KEY SPECIFICATIO S

■

Resolution: 10 Bits

■

Total Unadjusted Error (LTC1090A): ±1/2LSB Max

■

Conversion Time: 22µs

■

Supply Current: 2.5mA Max, 1.0mA Typ

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

LTCMOS is a trademark of Linear Technology Corp.

LTC1090

Single Chip 10-Bit Data

Acquisition System

DESCRIPTIO

The LTC®1090 is a data acquisition component which contains a serial I/O successive approximation A/D converter. It uses LTCMOSTM switched capacitor technology to perform either 10-bit unipolar, or 9-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.

The serial I/O is designed to be compatible with industry standard full duplex serial interfaces. It allows either MSB or 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, 10, 12 or 16 bits. This allows easy interface to shift registers and a variety of processors.

The LTC1090A is specified with total unadjusted error (including the effects of offset, linearity and gain errors) less than ±0.5LSB.

The LTC1090 is specified with offset and linearity less than ±0.5LSB but with a gain error limit of ±2LSB for applications where gain is adjustable or less critical.

TYPICAL APPLICATIO

LTC1090 MPU

DIFFERENTIAL INPUT

BIPOLAR INPUT

–5V

OUT

–5V

(+)

(–)

UNIPOLAR INPUTS

– UNIPOLAR INPUT

SERIAL DATA

FOR 8051 CODE SEE

APPLICATIONS INFORMATION

SECTION

(e.g., 8051)

P1.1D

P1.2D

P1.3SCLK

P1.4CS

LINK

LTC1090 • TA01

Linearity Plot

1.0

0.5

0.0

ERROR (LSBs)

–0.5

–1.0

0 512 1024

OUTPUT CODE

LTC1090 • TA02

1090fc

Page 2

LTC1090

PACKAGE/ORDER I FOR ATIO

WWWU

ABSOLUTE AXI U RATI GS

(Notes 1 and 2)

Supply Voltage (VCC) to GND or V Negative Supply Voltage (V Voltage:

Analog and Reference

Inputs .................................... (V–) –0.3V to V

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

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

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

Operating Temperature Range

LTC1090AC/LTC1090C ........................–40°C to 85°C

LTC1090AM/LTC1090M (OBSOLETE) ...... –55°C to 125°C

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

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

–

) ..................... – 6V to GND

................................

12V

0.3V

TOP VIEW

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

DGND

SW PACKAGE

20-LEAD PLASTIC SO WIDE

= 150°C, θ

JMAX

= 110°C, θ

JMAX

20-LEAD PDIP

= 70°C/W

= 90°C/W

ACLK

SCLK

OUT

REF

–

AGND

N PACKAGE

ORDER PART

NUMBER

LTC1090ACN LTC1090CN LTC1090CSW

–

20-LEAD CERDIP

= 150°C θ

JMAX

J PACKAGE

= 70° C/W

LTC1090AMJ LTC1090MJ LTC1090ACJ LTC1090CJ

OBSOLETE PACKAGE

Consider the SW or N Package for Alternate Source

Consult LTC Marketing for parts specified with wider operating temperature ranges.

LTC1090 • POI01

UUUUWW

RECO E DED OPERATI G CO DITIO S

LTC1090/LTC1090A

SYMBOL PARAMETER CONDITIONS MIN MAX UNITS

CC –

SCLK

ACLK

CYC

hCS

hDI

suCS

suDI

WHACLK

WLACLK

WHCS

Positive Supply Voltage V– = 0V 4.5 10 V Negative Supply Voltage VCC = 5V –5.5 0 V

Shift Clock Frequency VCC = 5V 0 1.0 MHz A/D Clock Frequency VCC = 5V 25°C 0.01 2.0 MHz

85°C 0.05 2.0 125°C 0.25 2.0

Total Cycle Time See Operating Sequence 10 SCLK + Cycles

48 ACLK Hold Time, CS Low After Last SCLK Hold Time, DIN After SCLK Setup Time CS↓ Before Clocking in First Address Bit (Note 9) VCC = 5V 2 ACLK Cycles

Setup Time, DIN Stable Before SCLK ACLK High Time VCC = 5V 127 ns ACLK Low Time VCC = 5V 200 ns CS High Time During Conversion VCC = 5V 44 ACLK

↑

↓

↑

VCC = 5V 0 ns VCC = 5V 150 ns

1µs

VCC = 5V 400 ns

Cycles

1090fc

Page 3

LTC1090

CO VERTER A D ULTIPLEXER CHARACTERISTICS

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

PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS

Offset Error (Note 4) ● ±0.5 ±0.5 LSB Linearity Error (Notes 4 and 5) ● ±0.5 ±0.5 LSB Gain Error (Note 4) ● ±1.0 ±2.0 LSB Total Unadjusted Error V

Reference Input Resistance 10 10 kΩ Analog and REF Input Range (Note 7) (V–) – 0.05V to VCC 0.05V V On Channel Leakage Current On Channel = 5V ● 11µA

(Note 8) Off Channel = 0V

Off Channel Leakage Current On Channel = 5V ● –1 –1 µA (Note 8) Off Channel = 0V

The ● denotes specifications which

= 25°C. (Note 3)

LTC1090A LTC1090

= 5.000V ● ±1.0 LSB

REF

(Notes 4 and 6)

On Channel = 0V ● –1 –1 µA Off Channel = 5V

On Channel = 0V ● 11µA Off Channel = 5V

AC ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating

temperature range, otherwise specification are TA = 25°C. (Note 3)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

ACC

SMPL

CONV

dDO

dis

hDO

Delay Time From CS↓ to D Analog Input Sample Time See Operating Sequence 5 SCLK Cycles Conversion Time See Operating Sequence 44 ACLK Cycles Delay Time, SCLK↓ to D Delay Time, CS↑ to D Delay Time, 2nd CLK↓ to D Time Output Data Remains Valid After SCLK D

Fall Time See Test Circuits ● 90 300 ns ns

OUT

Rise Time See Test Circuits ● 60 300 ns ns

OUT

Input Capacitance Analog Inputs On Channel 65 pF

Data Valid (Note 9) 2 ACLK Cycles

OUT

Data Valid See Test Circuits ● 250 450 ns

OUT

Hi-Z See Test Circuits ● 140 300 ns ns

OUT

Enabled See Test Circuits ● 150 400 ns ns

OUT

↓

Digital Inputs 5 pF

Off Channel 5 pF

LTC1090/LTC1090A

50 ns

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Page 4

LTC1090

DIGITAL A D DC ELECTRICAL CHARACTERISTICS

over the full operating temperature range, otherwise specification are T

= 25°C. (Note 3)

The ● denotes specifications which apply

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

LTC1090/LTC1090A

SOURCE

SINK

REF –

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 = 2.0MHz, SCLK = 0.5MHz 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: Linearity error is specified between the actual end points of the A/D transfer curve. Note 6: Total unadjusted error includes offset, gain, linearity, multiplexer and hold step errors.

Note 7: Two on-chip diodes are tied to each reference and analog input

High Level lnput Voltage VCC = 5.25V ● 2.0 V Low Level Input Voltage VCC = 4.75V ● 0.8 V High Level lnput Current VIN = V

● 2.5 µA

Low Level Input Current VIN = 0V ● –2.5 µA High Level Output Voltage VCC = 4.75V, lO = 10µA 4.7 V

= 4.75V, lO = 360µA ● 2.4 4.0 V

Low Level Output Voltage VCC = 4.75V, lO = 1.6mA ● 0.4 V Hi-Z Output Leakage V

Output Source Current V Output Sink Current V

= VCC, CS High ● 3 µA

OUT

= 0V, CS High ● –3 µA

OUT

= 0V –10 mA

OUT

= V

10 mA Positive Supply Current CS High, REF+ Open ● 1.0 2.5 mA Reference Current V

= 5V ● 0.5 1.0 mA

REF

Negative Supply Current CS High, V– = –5V ● 150 µA

–

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

levels (4.5V), as high level reference or analog inputs (5V) can cause

–

wired together (unless otherwise noted).

= 5V, V

+ = 5V, V

REF

– = 0V, V– = 0V for unipolar mode and

REF

below V V 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,

) divided by 1024.

= 5V, 1LSB (bipolar) = 2(5V)/1024 = 9.77mV.

REF

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 edges 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.

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

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Page 5

TEST CIRCUITS

LTC1090

On and Off Channel Leakage Current Voltage Waveforms for D

OFF

ON CHANNELS

SCLK 0.8V

OUT

Voltage Waveforms for D

OUT

dis

POLARITY

ACLK

OFF CHANNELS

LTC1090 • TC01

Voltage Waveforms for ten and t

Delay Time, t

OUT

dDO

Rise and Fall Times, tr, t

OUT

2.0V

2.4V

0.4V

2.4V

0.4V

LTC1090 • TC02

dDO

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

Load Circuit for t

TEST

POINT

OUT

100pF

dis

and t

WAVEFORM 25V

WAVEFORM 1

LTC1090 • TC04

2.4V

0.4V 10%

dis

Load Circuit for t

OUT

1.4V

90%

LTC1090 • TC03

, tr, and t

dDO

TEST POINT

100pF

LTC1090 • TC05

1090fc

Page 6

LTC1090

PI FU CTIO S

# PIN FUNCTION DESCRIPTION

1-8 CH0 to CH7 Analog Inputs The analog inputs must be free of noise with respect to AGND. 9 COM 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. 10 DGND Digital Ground This is the ground for the internal logic. Tie to the ground plane. 11 AGND Analog Ground AGND should be tied directly to the analog ground plane. 12 V 13,14 REF 15 CS Chip Select Input A logic low on this input enables data transfer. 16 D 17 D 18 SCLK Shift Clock This clock synchronizes the serial data transfer. 19 ACLK A/D Conversion Clock This clock controls the A/D conversion process. 20 V

–

OUT IN

–

, REF

Negative Supply Tie V– to most negative potential in the circuit. (Ground in single supply applications.)

Reference Inputs The reference inputs must be kept free of noise with respect to AGND.

Digital Data Output The A/D conversion result is shifted out of this output. Data Input The A/D configuration word is shifted into this input.

Positive Supply This supply must be kept free of noise and ripple by bypassing directly to the analog ground

plane.

BLOCK DIAGRA

INPUT SHIFT

CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7

COM

1 2 3 4 5 6 7 8 9

DGND

ANALOG

INPUT

MUX

AGND

SAMPLE

AND HOLD

SCLK

OUTPUT

SHIFT

COMP

10-BIT

SAR

10-BIT

CAPACITIVE

DAC

–

REF

–

REF

CONTROL

AND

TIMING

OUT

ACLK

LTC1090 • BD01

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Page 7

AMBIENT TEMPERATURE, TA (°C)

–50

REFERENCE CURRENT, I

REF

(mA)

0.4

0.5

0.6

25 75

LTC1090 • TPC03

0.3

0.2

–25 0

50 100 125

0.1

REF

= 5V

TYPICAL PERFOR A CE CHARACTERISTICS

LTC1090

Supply Current vs Supply Voltage

REF +OPEN ACLK = 2MHz

CS = V

TA = 25°C

(mA)

SUPPLY CURRENT, I

678

SUPPLY VOLTAGE, V

Unadjusted Offset Error vs Reference Voltage

VCC = 5V

)

REF

1024

OFFSET ERROR (LSBs = • V

VOS = 1mV

VOS = 0.5mV

0.2 REFERENCE VOLTAGE, V

1.0 5.0

(V)

REF

910

LTC1090 • TPC01

(V)

LTC1090 • TPC04

Supply Current vs Temperature Reference Current vs Temperature

1.4

1.2

(mA)

1.0

0.8

0.6 REF +OPEN

SUPPLY CURRENT, I

ACLK = 2MHz

0.4 CS = 5V

= 5V

0.2 –50

–25 0

AMBIENT TEMPERATURE, TA (°C)

Linearity Error vs Reference Voltage

1.25 VCC = 5V

)

REF

1.0

1024

0.75

0.5

0.25

LINEARITY ERROR (LSBs = • V

REFERENCE VOLTAGE, V

50 100 125

25 75

REF

LTC1090 • TPC02

(V)

LTC1090 • TPC05

Change in Gain Error vs Reference Voltage

)

1.25 VCC = 5V

REF

1.0

1024

0.75

0.5

0.25

CHANGE IN GAIN ERROR (LSBs = • V

REFERENCE VOLTAGE, V

REF

(V)

LTC1090 • TPC06

Offset Error vs Supply Voltage Linearity Error vs Supply Voltage

1.25

1.0

0.75

0.5

OFFSET ERROR (LSBs)

0.25

= 4V

REF

ACLK = 2MHz

= 1.25mV AT VCC = 5V

678

SUPPLY VOLTAGE, V

(V)

910

LTC1090 • TPC07

1.25 V

= 4V

REF

ACLK = 2MHz

1.0

0.75

0.5

LINEARITY ERROR (LSBs)

0.25

678

SUPPLY VOLTAGE, V

(V)

910

LTC1090 • TPC08

Change in Gain Error vs Supply Voltage

0.5 V

= 4V

REF

ACLK = 2MHz

0.25

– 0.25

– 0.5

CHANGE IN GAIN ERROR (LSBs)

678

SUPPLY VOLTAGE, V

(V)

910

LTC1090 • TPC09

1090fc

Page 8

LTC1090

SUPPLY VOLTAGE, VCC (V)

LTC1090 • TPC15

810

MAXIMUM ACLK FREQUENCY* (MHz)

REF

= 4V

= 25°C

TYPICAL PERFOR A CE CHARACTERISTICS

Change in Offset Error vs Temperature

0.6 V

= 5V

REF

0.5

ACLK = 2MHz

0.4

0.3

0.2

0.1

–50

MAGNITUDE OF OFFSET CHANGE, ∆OFFSET (LSBs)

–25 0

AMBIENT TEMPERATURE, TA (°C)

Maximum Conversion Clock Rate vs Temperature

= 5V

MAXIMUM ACLK FREQUENCY* (MHz)

= 5V

REF

–50

–25 0

AMBIENT TEMPERATURE, TA (°C)

Maximum Conversion Clock Rate vs Source Resistance

INPUT

*MAXIMUM ACLK FREQUENCY REPRESENTS THE ACLK FREQUENCY AT WHICH A 0.1LSB SHIFT IN THE ERROR AT ANY CODE TRANSITION FROM ITS 2MHz VALVE IS FIRST DETECTED.

–

MAXIMUM ACLK FREQUENCY* (MHz)

INPUT

SOURCE

100 1k 10k

50 100 125

25 75

LTC1090 • TPC10

50 100 125

25 75

LTC1090 • TPC13

VCC = 5V V

= 5V

REF

= 25°C

–

(Ω)

SOURCE

LTC1090 • TPC16

Change in Linearity Error vs Temperature

0.6 V

= 5V

REF

0.5

ACLK = 2MHz

0.4

0.3

0.2

0.1

MAGNITUDE OF LINEARITY CHANGE, ∆LINEARITY (LSBs)

–50

–25 0

AMBIENT TEMPERATURE, TA (°C)

50 100 125

25 75

Maximum Conversion Clock Rate vs Reference Voltage

= 5V

= 25°C

MAXIMUM ACLK FREQUENCY* (MHz)

REFERENCE VOLTAGE, V

Maximum Filter Resistor vs Cycle Time

100k

10k

** (Ω)

FILTER

100

MAXIMUM R

FILTER

≥ 1µF

FILTER

CYCLE TIME, t

100 1000 10k

(µs)

CYC

**MAXIMUM R CHANGE IN FULL SCALE ERROR FROM ITS VALUE AT R

REF

Change in Gain Error vs Temperature

0.6 V

= 5V

REF

0.5 ACLK = 2MHz

0.4

0.3

0.2

0.1

LTC1090 • TPC11

MAGNITUDE OF GAIN CHANGE, ∆GAIN (LSBs)

–50

–25 0

AMBIENT TEMPERATURE, TA (°C)

50 100 125

25 75

Maximum Conversion Clock Rate vs Supply Voltage

(V)

LTC1090 • TPC14

Sample-and-Hold Acquisition Time vs Source Resistance

= 5V

REF

= 5V

= 25°C

0 TO 5V INPUT STEP

SOURCE

S & H ACQUISITION TIME TO 0.1% (µs)

0.1

100

LTC1090 • TPC17

REPRESENTS THE FILTER RESISTOR VALVE AT WHICH A 0.1LSB SHIFT

FILTER

1k 10k

(Ω)

SOURCE

= 0 IS FIRST DETECTED.

FILTER

LTC1090 • TPC12

LTC1090 • TPC18

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Page 9

REFERENCE VOLTAGE, V

REF

(V)

0.2

PEAK-TO-PEAK NOISE ERROR (LSBs)

0.5

1.0

2.0

LTC1090 • TPC21

1.5

0.25

0.75

1.75

1.25

LTC1090 NOISE = 200µV PEAK-TO-PEAK

TYPICAL PERFOR A CE CHARACTERISTICS

LTC1090

Digital Input Logic Threshold vs Supply Voltage

TA = 25°C

LOGIC THRESHOLD (V)

5678

SUPPLY VOLTAGE, VCC (V)

910

LTC1090 • TPC19

Input Channel Leakage Current vs Temperature Noise Error vs Reference Voltage

1000

900

800

700

600

500

400

300

200

100

INPUT CHANNEL LEAKAGE CURRENT (nA)

–50

–25

AMBIENT TEMPERATURE, TA (°C)

WUUU

APPLICATIO S I FOR ATIO

The LTC1090 is a data acquisition component which contains the following functional blocks:

1. 10-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

GUARANTEED

ON CHANNEL

OFF CHANNELS

100

LTC1090 • TPC20

125

DIGITAL CONSIDERATIONS

1. Serial Interface

The LTC1090 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).

15810

SCLK

OUT

SGL/

ODD/

DIFF

SIGN

Operating Sequence

(Example: Differential Inputs (CH3 to CH2), Bipolar, MSB First and 10-Bit Word Length)

CYC

DON’T CARE

SEL0 UNI

SEL1

SHIFT CONFIGURATION

WORD IN

MSBF

SMPL

WL1 WL0

DON’T CARE

CONV

B8 B7 B6 B5 B4 B3 B2 B1 B0

(SB)

SHIFT A/D RESULT OUT AND

NEW CONFIGURATION WORD IN

LTC1090 • AI01

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Page 10

LTC1090

WUUU

APPLICATIO S I FOR ATIO

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 DIN input which configures the LTC1090 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.

DIND

OUT

Word 1

Word 0

Data

Transfer

CONV

A/D

Conversion

OUT

Word 2

Word 1

Data

Transfer

CONV

A/D

Conversion

OUT

Word 3

Word 2

LTC1090 • AI02

2. Input Data Word

The LTC1090 8-bit input data word is clocked into the D

input on the first eight rising SCLK edges after chip select is recognized. Further inputs on the DIN pin are then ignored until the next CS cycle. The eight bits of the input word are defined as follows:

Multiplexer (MLIX) 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 mode (SGL/DIFF = O) measurements are limited to four adjacent input pairs with either polarity. In single ended mode, all input channels are measured with respect to COM. Figure 1 shows some examples of multiplexer assignments.

Table 1. Multiplexer Channel Selection

MUX ADDRESS DIFFERENTIAL CHANNEL SELECTION

SGL/ ODD SELECT DIFF SIGN 1 0 0 1 234567

0000+–

0001 +–

0010 +–

0011 +–

0100–+

0101 –+

0110 –+

0111 –+

Data Input (DIN) Word:

SGL/

ODD/

DIFF

SIGN

MUX Address

SELECT1SELECT

Unipolar/

Bipolar

UNI MSBF WL1

MSB First/

LSB First

Word Length

WL0

LTC1090• AI03

MUX ADDRESS SINGLE ENDED CHANNEL SELECTION

SGL/ ODD/ SELECT DIFF SIGN 1 0 0 1 2 3 4 5 6 7 COM

1000+ –

1001 + –

1010 + –

1011 + –

1100 + –

1101 + –

1110 + –

1111 +–

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Page 11

WUUU

APPLICATIO S I FOR ATIO

4 Differential 8 Single Ended Combinations of Differential and Single Ended

CHANNEL

0,1

2,3

4,5

6,7

+( – ) –( + )

LTC1090 • AI04A

CHANNEL

Changing the MUX Assignment “On the Fly”

LTC1090

COM ( – )

LTC1090 • AI04B

CHANNEL

0,1

2,3

4 5

6 7

+ –

– +

+ + + +

COM ( – )

LTC1090 • AI04C

4,5

6,7

+ –

COM (UNUSED)

1ST CONVERSION

LTC1090 • AI04D

Figure 1. Examples of Multiplexer Options on the LTC1090

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 Transfer Curve (UNI = 1)

1111111111 1111111110

0000000001 0000000000

OV 1LSB

6 7

– +

+ +

COM ( – )

2ND CONVERSION

LTC1090 • AI04E

5,4

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.

REF

LTC1090 • AI05

– 2LSB

REF

– 1LSB

REF

–V

REF

+1LSB

Bipolar Transfer Curve (UNI = 0)

0111111111 0111111110

0000000001 0000000000

–1LSB

–2LSB

1LSB

1111111111 1111111110

1000000001 1000000000

LTC1090 • AI06

1090fc

REF

– 1LSB

REF

– 2LSB

REF

Page 12

LTC1090

WUUU

APPLICATIO S I FOR ATIO

Unipolar Output Code (UNI = 1)

INPUT VOLTAGE

OUTPUT CODE INPUT VOLTAGE (V

1111111111 V 1111111110 V

•••

••• 0000000001 1LSB 0.0049V 0000000000 0V 0V

– 1LSB 4.9951V

REF

– 2LSB 4.9902V

REF

= 5V)

Bipolar Output Code (UNI = 0)

INPUT VOLTAGE

OUTPUT CODE INPUT VOLTAGE (V

0111111111 V 0111111110 V

•••

••• 0000000001 1LSB 0.0098V 0000000000 0V 0V 1111111111 –1LSB –0.0098V 1111111110 –2LSB –0.0195V

•••

••• 1000000001 – (V 1000000000 – (V

– 1LSB 4.9902V

REF

– 2LSB 4.9805V

REF

) + 1LSB –4.9902V

REF

)–5.000V

REF

= 5V)

MSB First/LSB First Format (MSBF)

The output data of the LTC1090 is programmed for MSB first or LSB first sequence using the MSBF bit. For MSB first output data the input word clocked to the LTC1090 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 LTC1090 should always contain a zero in the MSBF bit location. The MSBF bit in a given DIN word will control the order of the next D

word. The MSBF bit

OUT

affects only the order of the output data word. The order of the input word is unaffected by this bit.

MSBF OUTPUT FORMAT

0LSB First 1 MSB First

control the length of the present, not the next, D

OUT

word. WL1 and WL0 are never “don’t cares” and must be set for the correct D

word length even when a “dummy” D

OUT

word is sent. On any transfer cycle, the word length should be made equal to the number of SCLK cycles sent by the MPU.

WL1 WL0 OUTPUT WORD LENGTH

00 8 Bits 01 10 Bits 10 12 Bits 11 16 Bits

Figure 2 shows how the data output (D

) timing can be

OUT

controlled with word length selection and MSB/LSB first format selection.

3. Deglitcher

A deglitching circuit has been added to the Chip Select input of the LTC1090 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 1 ACLK cycle. After a change of state on the CS input, the LTC1090 waits for two falling edges of the ACLK before recognizing a valid chip select. One indication of CS low 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.

ACLK

OUT

ACLK

HIGH Z

LOW CS RECOGNIZED

VALID OUTPUT

INTERNALLY

Word Length (WL1, WL0)

The last two bits of the input word (WL1 and WL0) program the output data word length of the LTC1090. Word lengths of 8, 10, 12 or 16 bits can be selected according to the following table. The WL1 and WL0 bits in a given DIN word

OUT

HIGH CS RECOGNIZED

INTERNALLY

HIGH Z

LTC1090 • AI07

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APPLICATIO S I FOR ATIO

8-Bit Word Length

SMPL

LTC1090

CONV

MSB FIRST

LSB FIRST

10-Bit Word Length

MSB FIRST

LSB FIRST

12-Bit Word Length

SCLK

OUT

SCLK

OUT

(SB)

B9 B8 B7 B6 B5 B4 B3

B0 B1 B2 B3 B4 B5 B6B2B7

SMPL

(SB)

B9 B8 B7 B6 B5 B4 B3

B0 B1 B2 B3 B4 B5 B6

B2 B1 B0

B7 B8 B9

SMPL

THE LAST TWO BITS ARE TRUNCATED

LTC1090 • AI08A

(SB)

LTC1090 • AI08B

CONV

SCLK

MSB FIRST

LSB FIRST

16-Bit Word Length

SCLK

OUT

MSB FIRST

OUT

LSB FIRST

OUT

(SB)

B9 B8 B7 B6 B5 B4 B3

B0 B1 B2 B3 B4 B5 B6

(SB)

B9 B8 B7 B6 B5 B4 B3

B0 B1 B2 B3 B4 B5 B6

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

Figure 2. Data Output (D

(SB)

FILL

ZEROES

FILL

ZEROES

B2 B1 B0

B7 B8 B9

SMPL

B2 B1 B0

(SB)

B7 B8 B9

Timing with Different Word Lengths

OUT)

** * ***

1210

LTC1090 • AI08C

LTC1090 • AI08D

CONV

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APPLICATIO S I FOR ATIO

4. CS Low During Conversion

In the normal mode of operation, CS is brought high during the conversion time (see Figure 3). The serial port ignores any SCLK activity while CS is high. The LTC1090 will also operate with CS low during the conversion. In this mode, SCLK must remain low during the conversion as shown in Figure 4. After the conversion is complete, the

SMPL SAMPLE ANALOG

INPUT

40 TO 44 ACLK CYCLES

DON’T CARE

Figure 3. CS High During Conversion

SCLK

OUT

SHIFT

MUX

ADDRESS

DIFF

ODD/

SIGN

SEL

UNI MSBF WL1 WL0

SGL/

line will become active with the first output bit. Then

OUT

the data transfer can begin as normal.

5. Microprocessor Interfaces

The LTC1090 can interface directly (without external hardware) to most popular microprocessor (MPU) synchronous

SHIFT RESULT OUT

AND NEW ADDRESS IN

ODD/

SIGN

B8 B7 B6 B5 B4 B3 B2 B1 B0B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

SEL

UNI MSBF WL1 WL0

LTC1090 • AI09

SGL/ DIFF

SCLK

OUT

SHIFT

MUX

ADDRESS

ODD/

SIGN

SEL

UNI MSBF WL1 WL0

SGL/

DIFF

SMPL SAMPLE ANALOG

INPUT

40 TO 44 ACLK CYCLES

SCLK MUST REMAIN LOW

DON’T CARE

SHIFT RESULT OUT

AND NEW ADDRESS IN

ODD/ SIGN

B8 B7 B6 B5 B4 B3 B2 B1 B0B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

SEL

UNI MSBF WL1 WL0

LTC1090 • AI10

SGL/

DIFF

Figure 4. CS Low During Conversion

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LTC1090

ANALOG

INPUTS

OUT

from LTC1090 stored in COP420 RAM

SCLK

COP420

Location A

Location A + 1

first 4 bits

second 4 bits

third 4 bits

LSB

MSB*

B8 B7 B6

B5 B4 B3 B2

Location A + 2

B1 B0 B0 B0

LTC1090 • AI11

*B9 is MSB in unipolar or sign bit in bipolar

APPLICATIO S I FOR ATIO

LTC1090

serial formats (see Table 2). If an MPU without a serial interface is used, then 4 of the MPU’s parallel port lines can be programmed to form the serial link to the LTC1090. Included here are three serial interface examples and one example showing a parallel port programmed to form the serial interface.

Table 2. Microprocessors with Hardware Serial Interfaces Compatible with the LTC1090**

PART NUMBER TYPE OF INTERFACE Motorola

MC6805S2, S3 SPI MC68HC11 SPI MC68HC05 SPI

RCA

CDP68HC05 SPI

Hitachi

HD6305 SCI Synchronous HD63705 SCI Synchronous HD6301 SCI Synchronous HD63701 SCI Synchronous HD6303 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

*Requires external hardware **Contact LTC Marketing for interface information for processors not on

this list †MICROWIRE and MlCROWIRE/PLUS are trademarks of National Semiconductor Corp.

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 manufacturers 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 LTC1090 accommodates these differences.

National MICROWIRE (COP420)

The COP420 transfers data MSB first and in 4-bit increments (nibbles). This is easily accommodated by setting the LTC1090 to MSB first format and 12-bit word length. The data output word is then received by the COP420 in three 4-bit blocks with the final two unused bits filled with zeroes by the LTC1090.

Hardware and Software Interface to National Semiconductor COP420 Processor

†

MNEMONIC DESCRIPTION

LEI Enable SlO SC Set Carry flag OGI G0 is set to (CS goes low) LDD Load first 4 bits of D XAS Swap ACC with SIO reg. Starts SK Clk LDD Load 2nd 4 bits of DIN to ACC NOP Timing XAS Swap first 4 bits from A/D with ACC. SK continues. XIS Put first 4 bits in RAM (location A) NOP Timing XAS Swap 2nd 4 bits from A/D with ACC. SK continues. XIS Put 2nd 4 bits in RAM (location A + 1) RC Clear Carry NOP Timing XAS Swap 3rd 4 bits from A/D with ACC. SK off XIS Put 3rd 4 bits in RAM (location A + 2) OGI G0 is set to 1 (CS goes high) LEI Disable SlO

to ACC

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LTC1090

ANALOG

INPUTS

OUT

from LTC1090 stored in HD63705 RAM

SCLK

HD63705

Location A

Location A + 1

Bipolar

Sign

byte 1

byte 2

LSB

B6 B5 B4 B3 B2 B1 B0

B9 B9 B9 B9 B9 B9 B9 B8

LTC1090 • AI13

Location A

Location A + 1

Unipolar

byte 1

byte 2

LSB

MSB

B6 B5 B4 B3 B2 B1 B0

000000B9 B8

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APPLICATIO S I FOR ATIO

Motorola SPI (MC68HC05C4)

The MC68HC05C4 transfers data MSB first and in 8-bit increments. Programming the LTC1090 for MSB first format and 16-bit word length allows the 10-bit data output to be received by the MPU as two 8-bit bytes with the final 6 unused bits filled with zeroes by the LTC1090.

Hardware and Software Interface to Motorola MC68HC05C4 Processor

LTC1090

ANALOG

INPUTS

from LTC1090 stored in MC68HCO5C4 RAM

OUT

SCLK

OUT

MSB*

Location A

B8 B7 B6 B5 B4 B3 B2

LSB

Location A + 1

B1 B0 0 0 0 0 0 0

MC68HCO5C4

SCK

MOSI

MISO

byte 1

byte 2

Hitachi Synchronous SCI (HD63705)

The HD63705 transfers serial data in 8-bit increments, LSB first. To accommodate this, the LTC1090 is programmed for 16-bit word length and LSB first format. The 10-bit output data is received by the processor as two 8-bit bytes, LSB first. The LTC1090 fills the final 6 unused bits (after the MSB) with zeroes in unipolar mode and with the sign bit in bipolar mode.

Hardware and Software Interface to Hitachi HD63705 Processor

*B9 is MSB in unipolar or sign bit in bipolar

MNEMONIC DESCRIPTION

BCLR n C0 is cleared (CS goes Low) LDA Load D STA Load D

↑

NOP 8 NOPs for timing

↓

LDA Load contents of SPI status reg. into ACC LDA Load LTC1090 D STA Load LTC1090 D STA Start next SPl cycle

↑

NOP 6 NOPs for timing

↓

BSET n C0 is set (CS goes high) LDA Load contents of SPI status reg. into ACC LDA Load LTC1090 D STA Load LTC1090 D

IN IN

for LTC1090 into ACC from ACC to SPI data reg. Start SCK

from SPI data reg. into ACC (byte 1)

OUT

into RAM (location A)

OUT

from SPI data reg. into ACC (byte 2)

OUT

into RAM (location A + 1)

OUT

LTC1090 • AI12

MNEMONIC DESCRIPTION

LDA Load DIN word for LTC1090 into ACC from RAM BCLR n C0 cleared (CS goes low) STA Load DIN word for LTC1090 into SCI data reg. from ACC

and start clocking data (LSB first)

↑

NOP 6 NOPs for timing

↓

LDA Load contents of SCI data reg. into ACC (byte 1)

Start next SCI cycle STA Load LTC1090 D NOP Timing BSET n C0 set (CS goes high) LDA Load contents of SCI data reg. into ACC (byte 2) STA Load LTC1090 D

word into RAM (Location A)

OUT

word into RAM (Location A + 1)

OUT

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LTC1090

Parallel Port Microprocessors

When interfacing the LTC1090 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 DIN signals for the LTC1090. A fourth port line reads the D

line. An example is made of the

OUT

Intel 8051/8052/80C252 family.

Intel 8051

To interface to the 8051, the LTC1090 is programmed for MSB first format and 10-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

LTC1090

OUT

ANALOG

INPUTS

from LTC1090 stored in 8051 RAM

OUT

SCLK

ACLK

MSB*

B9R2

B8 B7 B6 B5 B4 B3 B2

LSB

B1 B0 000000

8051

P1.1

P1.2

P1.3

P1.4

ALE

8051 Code

MNEMONIC DESCRIPTION

MOV PI,#02H Initialize port 1 (bit 1 is made

an input) CLR P1.3 SCLK goes low SETB P1.4 CS goes high

CONTINUE: MOV A,#0DH D

CLR P1.4 CS goes low MOV R4,#08 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 CIear 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 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 MOV R5,#07H Load counter

DELAY: DJNZ R5, DELAY Delay for LTC1090 to perform

AJMP CONTINUE Repeat program

word for the LTC1090 is

placed in ACC.

bit to LTC1090

conversion

*B9 is MSB in unipolar or sign bit in bipolar

210

OUTPUT PORT

SERIAL DATA

MPU

LTC1090

8 CHANNELS

LTC1090

8 CHANNELS

LTC1090

8 CHANNELS

LTC1090 • AI14

Figure 5. Several LTC1090’s Sharing One 3-Wire Serial Interface

3-WIRE SERIAL INTERFACE TO OTHER PERIPHERALS OR LTC1090s

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6. Sharing the Serial Interface

The LTC1090 can share the same 3-wire serial interface with other peripheral components or other LTC1090s (see Figure 5). In this case, the CS signals decide which LTC1090 is being addressed by the MPU.

ANALOG CONSIDERATIONS

1. Grounding

The LTC1090 should be used with an analog ground plane and single point grounding techniques.

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

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

Pin 20 (VCC) should be bypassed to the ground plane with a 4.7µF tantalum with leads as short as possible. Pin 12 (V–) should be bypassed with a 0.1µF ceramic disk. For single supply applications, V– can be tied to the ground plane.

4.7µF TANTALUM

ANALOG

GROUND

PLANE

Figure 6. Example Ground Plane for the LTC1090

10 11

V–

0.1µF CERAMIC DISK

LTC1090 • AI15

It is also recommended that pin 13 (REF–) and pin 9 (COM) 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.

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 VCC voltage with respect to analog ground during a conversion cycle can induce errors or noise in the output code. VCC noise and ripple can be kept below 1mV by bypassing the VCC pin directly to the analog ground plane with a 4.7µF tantalum with leads as short as possible. Figures 7 and 8 show the effects of good and poor VCC bypassing.

VERTICAL: 0.5mV/DIV

HORIZONTAL: 10µs/DIV

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

VERTICAL: 0.5mV/DIV

HORIZONTAL: 10µs/DIV

Figure 8. Good V on V

Below 1mV

Bypassing Keeps Noise and Ripple

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LTC1090

3. Analog Inputs

Because of the capacitive redistribution A/D conversion techniques used, the analog inputs of the LTC1090 have capacitive switching input current spikes. These current 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 LTC1090 look like a 60pF capacitor (CIN) in series with a 500Ω resistor (RON) as shown in Figure 9. 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.

”

“

SOURCE

–

Figure 9. Analog Input Equivalent Circuit

INPUT

4TH SCLK

= 500Ω

“ – ”

–

INPUT

LAST SCLK

LTC1090

= 60pF

LTC1090 • AI16

“+” 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, 10th, 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 4µs, R

SOURCE

< 2k

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 2MHz, R

SOURCE

–

< 1kΩ and C2 < 20pF will provide adequate settling.

SCLK

ACLK

“ + ” INPUT

“ – ” INPUT

SAMPLE

MUX ADDRESS

SHIFTED IN

123 4

“ + ” INPUT MUST

SETTLE DURING THIS TIME

SMPL

HOLD

LAST SCLK (8TH, 10TH, 12TH OR 16TH DEPENDING ON WORK LENGTH)

234

1ST BIT

TEST

“ – ” INPUT MUST SETTLE

DURING THIS TIME

LTC1090 • AI17

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

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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 LT1006 and LT1013 single supply op amps can be made to settle well even with the minimum settling windows of 4µs (“+” input) and 2µs (“–” input) which occur at the maximum clock rates (ACLK = 2MHz and SCLK = 1MHz). Figures 11 and 12 show examples of adequate and poor op amp settling.

VERTICAL: 5mV/DIV

HORIZONTAL: 1µs/DIV

Figure 11. Adequate Settling of Op Amp Driving Analog Input

eliminated by increasing the cycle time as shown in the typical curve of Maximum Filter Resistor vs Cycle Time.

Input Leakage Current

FILTER

Figure 13. RC Input Filtering

“ + ”

–

“

LTC1090

”

LTC1090 • AI18

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 0.2LSB. This error will be much reduced at lower temperatures because leakage drops rapidly (see 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 CF (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 lDC = 60pF x VIN/t

and is roughly propor-

CYC

tional to VIN. When running at the minimum cycle time of 33µs, the input current equals 9µA at VIN = 5V. In this case, a filter resistor of 50Ω will cause 0.1LSB of full-scale error. If a larger filter resistor must be used, errors can be

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 LTC1090 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 LTC1090 to convert rapidly varying signals (see typical curve of S&H Acquisition Time vs Source Resistance). The input voltage is sampled during the t

time as shown

SMPL

in Figure 10. The sampling interval begins after the fourth

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LTC1090 • AI19

OUT

REF

–

REF

EVERY 4 ACLK CYCLES

10k TYP

5pF – 30pF

LTC1090

APPLICATIO S I FOR ATIO

LTC1090

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, 10th, 12th or 16th falling edge of the SCLK depending on the word length selected.

Differential Inputs

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 single ended 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 44 ACLK cycles. Therefore, a change in the “–” input voltage during this interval can “–” input this error would be:

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

1. The source resistance (R

) driving the reference

OUT

inputs should be low (less than 1Ω) to prevent DC drops caused by the 1mA maximum reference current (I

REF

2. 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 2MHz most references and op amps can be made to settle within the 2µs bit time.

3. It is recommended that the REF– input be tied directly to the analog ground plane. If REF– 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.

ERROR (MAX)

= V

x 2 x π x f(“–”) x 44/f

PEAK

ACLK

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

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 1/4LSB error (1.25mV) with the converter running at ACLK = 2MHz, its peak value would have to be 150mV.

5. Reference Inputs

Figure 14. Reference Input Equivalent Circuit

The voltage between the reference inputs of the LTC1090 defines the voltage span of the A/D converter. The reference inputs look primarily like a 10kΩ resistor but 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

VERTICAL: 0.5mV/DIV

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

Figure 15. Adequate Reference Settling

HORIZONTAL: 1µs/DIV

must be taken to insure that transients caused by these current spikes settle completely during each bit test of the conversion.

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VERTICAL: 0.5mV/DIV

HORIZONTAL: 1µs/DIV

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

6. Reduced Reference Operation

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,

of 0.5mV which is 0.1LSB with a 5V reference

a V

becomes 0.5LSB with a 1V reference and 2.5LSBs with a

0.2V reference. If this offset is unacceptable, it can be corrected digitally by the receiving system or by offsetting the “–” input to the LTC1090.

The effective resolution of the LTC1090 can be increased by reducing the input span of the converter. The LTC1090 exhibits good linearity and gain over a wide range of reference voltages (see typical curves of Linearity and Gain Error vs Reference Voltage). However, care must be taken when operating at low values of V

because of the

REF

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

REF

values:

1. Conversion speed (ACLK frequency)

2. Offset

3. Noise

Conversion Speed with Reduced V

REF

With reduced reference voltages, the LSB step size is reduced and the LTC1090 internal comparator overdrive is reduced. With less overdrive, more time is required to perform a conversion. Therefore, the maximum ACLK frequency should be reduced when low values of V

REF

are used. This is shown in the typical curve of Maximum Conversion Clock Rate vs Reference Voltage.

Offset with Reduced V

REF

The offset of the LTC1090 has a larger effect on the output code when the A/D is operated with reduced reference

Noise with Reduced V

REF

The total input referred noise of the LTC1090 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

0.04LSB peak-to-peak. In this case, the LTC1090 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 1V reference, this same 200µV noise is 0.2LSB peak-to-peak. This will reduce the range of input voltages over which a stable output code can be achieved by 0.2LSB. If the reference is further reduced to 200mV, the 200µV noise becomes equal to one LSB and a stable code may be difficult to achieve. 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

, VIN or V–) will

REF

add to the internal noise. The lower the reference voltage to be used, the more critical it becomes to have a clean, noise-free setup.

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Page 23

TYPICAL APPLICATIO

A “Quick Look” Circuit for the LTC1090

SNEAK-A-BIT

LTC1090

Users can get a quick look at the function and timing of the LTC1090 by using the following simple circuit. REF+ and DIN 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 1/64 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 D

pin can be viewed on an oscilloscope which is set up

OUT

to trigger on the falling edge of CS.

A “Quick Look” Circuit for the LTC1090

4.7µF

f/64

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

DGND

LTC1090

ACLK

SCLK

OUT

REF

AGND

–

CLK

RESET

LTC1090

RESET

CLK

0.1

The LTC1090’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 10-bit + sign word is returned to memory inside the MPU. The MC68HC05C4 was chosen as an example; however, any processor could be used.

Two 10-bit unipolar conversions are performed: the first over a 0 to 5V span and the second over a 0 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 –1023 to 1023 decimal) is converted to 2’s complement notation and stored in RAM.

Scope Trace of LTC1090 “Quick Look” Circuit

Showing A/D Output of 0101010101 (155

OUT

HEX

)

CLOCK IN

TO OSCILLOSCOPE

1MHz MAX

OTHER CHANNELS

OR SNEAK-A-BIT

INPUTS

–5V TO 5V

LT1 021-5

CH0

CH1

CH2

CH3

CH4

LTC1090

CH5

CH6

CH7

COM

DGND

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

LTC1090 • TA03

SNEAK-A-BIT Circuit

10µF

ACLK

SCLK

OUT

REF

–

REF

–

AGND

0.1µF –5V

DEGLITCHER

2MHz

CLOCK

TIME

MSB (B9)

MC68HC05C4

SCK

MOSI

MISO

LTC1090 • TA04

LSB

(B0)

FILLS ZERO

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Page 24

LTC1090

TYPICAL APPLICATIO

SNEAK-A-BIT

2047 STEPS

–5V

( + ) CH6

( – ) CH7

1ST CONVERSION

( – ) CH6

( + ) CH7

2ND CONVERSION

1ST CONVERSION 1024 STEPS

SOFTWARE

0V 0V 0V

2ND CONVERSION 1024 STEPS

–5V

SNEAK-A-BIT Code

from LTC1090 in MC68HC05C4 RAM

OUT

Sign

Location $77

Location $87

words for LTC1090

B10 B9 B8

B2 B1 B0

MUX Addr. Word

(ODD/SIGN)

001 11111

011 11111

001 11111

B7 B6 B5 B4 B3

LSB

filled with 0s

MSBF

UNI

Length

LTC1090 • TA05

Sneak-A-Bit Code for the LTC1090 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 LTC1090 for next

read 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,

converts to 2’s complement and stores in

RAM

Sneak-A-Bit Code for the LTC1090 Using the MC68HC05C4

MNEMONIC DESCRIPTION

READ–/+: LDA #$3F Load DIN word for LTC1090 into ACC

JSR TRANSFER Read LTC1090 routine LDA $60 Load MSBs from LTC1090 into ACC STA $71 Store MSBs in $71 LDA $61 Load LSBs from LTC1090 into ACC STA $72 Store LSBs in $72 RTS Return

READ+ /–: LDA #$7F Load D

JSR TRANSFER Read LTC1090 routine LDA $60 Load MSBs from LTC1090 into ACC STA $73 Store MSBs in $73 LDA $61 Load LSBs from LTC1090 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 SPlF

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 SPlF

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 goto 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 Goto 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 LTC1090 into ACC

into SPI. Start transfer

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Page 25

PACKAGE DESCRIPTIO

CORNER LEADS OPTION

(4 PLCS)

J Package

20-Lead CERDIP (Narrow .300 Inch, Hermetic)

(Reference LTC DWG # 05-08-1110)

1.060

(26.924)

MAX

20 16 1517 14 13 1219 18

LTC1090

0.023 – 0.045

(0.584 – 1.143)

HALF LEAD

0.045 – 0.068

(1.143 – 1.727)

FULL LEAD

OPTION

0.008 – 0.018

(0.203 – 0.457)

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

OPTION

0.300 BSC

(0.762 BSC)

0.220 – 0.310

(5.588 – 7.874)

0° – 15°

0.025

(0.635)

RAD TYP

0.015 – 0.060

(0.381 – 1.524)

0.125

(3.175)

MIN

0.005

(0.127)

MIN

0.045 – 0.065

(1.143 – 1.651)

0.014 – 0.026

(0.356 – 0.660)

OBSOLETE PACKAGE

109

0.100 (2.54)

BSC

0.200

(5.080)

MAX

J20 1298

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Page 26

LTC1090

PACKAGE DESCRIPTIO

N Package

20-Lead PDIP (Narrow .300 Inch)

(Reference LTC DWG # 05-08-1510)

19 1112

0.255 ± 0.015* (6.477 ± 0.381)

1.040* (26.416)

MAX

1517

131416

1234

0.300 – 0.325

(7.620 – 8.255)

0.009 – 0.015

(0.229 – 0.381)

+0.035

0.325 –0.015

+0.889

8.255

()

–0.381

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)

0.020

(0.508)

MIN

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

0.005

(0.127)

MIN

0.045 – 0.065

(1.143 – 1.651)

0.100 (2.54)

BSC

910

0.065

(1.651)

TYP

0.018 ± 0.003

(0.457 ± 0.076)

N20 1098

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Page 27

PACKAGE DESCRIPTIO

SW Package

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

(Reference LTC DWG # 05-08-1620)

0.496 – 0.512*

(12.598 – 13.005)

19 18

14 13

LTC1090

1112

NOTE 1

0.291 – 0.299** (7.391 – 7.595)

0.010 – 0.029

(0.254 – 0.737)

0.009 – 0.013

(0.229 – 0.330)

NOTE:

1. 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

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

NOTE 1

× 45°

0° – 8° TYP

0.016 – 0.050

(0.406 – 1.270)

0.093 – 0.104

(2.362 – 2.642)

0.050

(1.270)

BSC

0.014 – 0.019

(0.356 – 0.482)

2345

TYP

0.394 – 0.419

(10.007 – 10.643)

910

0.037 – 0.045

(0.940 – 1.143)

0.004 – 0.012

(0.102 – 0.305)

S20 (WIDE) 1098

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1090fc

Page 28

LTC1090

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LTC1290 8-Channel Configurable, 5V, 12-Bit ADC Pin-Compatible with LTC1090 LTC1391 Serial-Controlled 8-to-1 Analog Multiplexer Low RON, Low Power, 16-Pin SO and SSOP Package LTC1594L/LTC1598L 4-/8-Channel, 3V Micropower 12-Bit ADC Low Power, Small Size LTC1850/LTC1851 10-Bit/12-Bit, 8-Channel, 1.25Msps ADCs 5V, Programmable MUX and Sequencer LTC1852/LTC1853 10-Bit/12-Bit, 8-Channel, 400ksps ADCs 3V or 5V, Programmable MUX and Sequencer LTC2404/LTC2408 24-Bit, 4-/8-Channel, No Latency ∆ΣTM ADC 4ppm INL, 10ppm Total Unadjusted Error, 200µA

LTC2424/LTC2428 20-Bit, 4-/8-Channel, No Latency ∆Σ ADC 1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2404/LTC2408

No Latency ∆Σ is a trademark of Linear Technology Corporation.

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear.com

1090fc

LW/TP 0902 1K REV C • PRINTED IN USA

 LINE AR TE CHNO LOGY CO R P O R ATIO N 1990

Datasheet LTC1090MJ, LTC1090CSW, LTC1090CN, LTC1090AMJ, LTC1090ACN Datasheet (Linear Technology)

Specifications and Main Features

Frequently Asked Questions

User Manual