LTC1285/LTC1288
3V Micropower Sampling
12-Bit A/D Converters in
SO-8 Packages
DESCRIPTION
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FEATURES
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The LTC®1285/LTC1288 are 3V micropower, 12-bit, successive approximation sampling A/D converters. They
typically draw only 160µA of supply current when converting and automatically power down to a typical supply
current of 1nA whenever they are not performing conversions. They are packaged in 8-pin SO packages and
operate on 3V to 6V supplies. These 12-bit, switchedcapacitor, successive approximation ADCs include
sample-and-holds. The LTC1285 has a single differential
analog input. The LTC1288 offers a software selectable
2-channel MUX.
12-Bit Resolution
8-Pin SO Plastic Package
Low Cost
Low Supply Current: 160µA Typ
Auto Shutdown to 1nA Typ
Guaranteed ±3/4LSB Max DNL
Single Supply 3V to 6V Operation
Differential Inputs (LTC1285)
2-Channel MUX (LTC1288)
On-Chip Sample-and-Hold
100µs Conversion Time
Sampling Rates:
7.5ksps (LTC1285)
6.6ksps (LTC1288)
I/O Compatible with SPI, Microwire, etc.
On-chip serial ports allow efficient data transfer to a wide
range of microprocessors and microcontrollers over three
wires. This, coupled with micropower consumption, makes
remote location possible and facilitates transmitting data
through isolation barriers.
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APPLICATIONS
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These circuits can be used in ratiometric applications or
with an external reference. The high impedance analog
inputs and the ability to operate with reduced spans (to
1.5V full scale) allow direct connection to sensors and
transducers in many applications, eliminating the need for
gain stages.
Pen Screen Digitizing
Battery-Operated Systems
Remote Data Acquisition
Isolated Data Acquisition
Battery Monitoring
Temperature Measurement
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIONS N
12µW, S0-8 Package, 12-Bit ADC
Samples at 200Hz and Runs Off a 3V Supply
MPU
(e.g., 8051)
1
2
VREF
VCC
+IN
CLK
8
P1.4
7
ANALOG INPUT
LTC1285
6
0V TO 3V RANGE 3 –IN
DOUT
4
5
CS/SHDN
GND
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
3V
P1.3
SUPPLY CURRENT (µA)
1µF
Supply Current vs Sample Rate
1000
100
10
P1.2
SERIAL DATA LINK
LTC1285/88 • TA01
1
0.1
1
10
SAMPLE FREQUENCY (kHz)
100
LTC1285/88 • TA02
1
LTC1285/LTC1288
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ABSOLUTE MAXIMUM RATINGS
(Notes 1 and 2)
Supply Voltage (VCC) to GND ................................... 12V
Voltage
Analog and Reference ................ –0.3V to VCC + 0.3V
Digital Inputs......................................... –0.3V to 12V
Digital Output ............................. –0.3V to VCC + 0.3V
Power Dissipation .............................................. 500mW
Operating Temperature Range .................... 0°C to 70°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................ 300°C
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PACKAGE/ORDER INFORMATION
TOP VIEW
VREF 1
8
VCC
+IN 2
7
CLK
–IN 3
6
DOUT
GND 4
5
CS/SHDN
ORDER PART
NUMBER
LTC1285CN8
VREF 1
8
VCC
+IN 2
7
CLK
–IN 3
6
DOUT
GND 4
5
CS/SHDN
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 130°C/W
TJMAX = 150°C, θJA = 175°C/W
CS/SHDN 1
8
VCC (VREF)
CH0 2
7
CLK
CH1 3
6
DOUT
GND 4
5
DIN
ORDER PART
NUMBER
LTC1285CS8
PART MARKING
N8 PACKAGE
8-LEAD PDIP
TOP VIEW
ORDER PART
NUMBER
TOP VIEW
1285C
ORDER PART
NUMBER
TOP VIEW
LTC1288CN8
CS/SHDN 1
8
VCC (VREF)
CH0 2
7
CLK
CH1 3
6
DOUT
GND 4
5
DIN
LTC1288CS8
PART MARKING
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 130°C/W
TJMAX = 150°C, θJA = 175°C/W
1288C
Consult factory for Industrial and Military grade parts.
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RECOM ENDED OPERATING CONDITIONS
SYMBOL
PARAMETER
CONDITIONS
MIN
VCC
Supply Voltage (Note 3)
LTC1285
LTC1288
2.7
2.7
TYP
MAX
6
6
fCLK
Clock Frequency
VCC = 2.7V
(Note 4)
120
tCYC
Total Cycle Time
LTC1285, fCLK = 120kHz
LTC1288, fCLK = 120kHz
thDI
Hold Time, DIN After CLK↑
VCC = 2.7V
tsuCS
Setup Time CS↓ Before First CLK↑ (See Operating Sequence)
LTC1285, VCC = 2.7V
LTC1288, VCC = 2.7V
tsuDI
Setup Time, DIN Stable Before CLK↑
tWHCLK
CLK High Time
tWLCLK
UNITS
V
V
kHz
125.0
141.5
µs
µs
450
ns
2
2
µs
µs
VCC = 2.7V
600
ns
VCC = 2.7V
3.5
µs
CLK Low Time
VCC = 2.7V
3.5
µs
tWHCS
CS High Time Between Data Transfer Cycles
VCC = 2.7V
tWLCS
CS Low Time During Data Transfer
LTC1285, fCLK = 120kHz
LTC1288, fCLK = 120kHz
2
2
µs
123.0
139.5
µs
µs
LTC1285/LTC1288
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CONVERTER AND MULTIPLEXER CHARACTERISTICS
PARAMETER
CONDITIONS
Resolution (No Missing Codes)
Integral Linearity Error
MIN
●
(Note 6)
LTC1285
TYP
MAX
(Note 5)
MIN
12
●
LTC1288
TYP
MAX
12
±3/4
±2
UNITS
Bits
±3/4
±2
LSB
Differential Linearity Error
●
±1/4
±3/4
±1/4
±3/4
LSB
Offset Error
●
±3/4
±3
±3/4
±3
LSB
Gain Error
●
±2
±2
±8
LSB
Analog Input Range
(Note 7 and 8)
REF Input Range (LTC1285)
(Notes 7, 8, and 9)
2.7 ≤ VCC ≤ 6V
Analog Input Leakage Current (Note 10)
●
±8
– 0.05V to VCC + 0.05V
V
V
V
1.5V to VCC + 0.05V
●
±1
±1
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DIGITAL AND DC ELECTRICAL CHARACTERISTICS
(Note 5)
SYMBOL
PARAMETER
VIH
High Level Input Voltage
VCC = 3.6V
●
VIL
Low Level Input Voltage
VCC = 2.7V
●
0.8
V
IIH
High Level Input Current
VIN = VCC
●
2.5
µA
IIL
Low Level Input Current
VIN = 0V
●
– 2.5
µA
VOH
High Level Output Voltage
VCC = 2.7V, IO = 10µA
VCC = 2.7V, IO = 360µA
●
●
VOL
Low Level Output Voltage
VCC = 2.7V, IO = 400µA
●
IOZ
Hi-Z Output Leakage
CS = High
●
ISOURCE
Output Source Current
VOUT = 0V
– 10
mA
ISINK
Output Sink Current
VOUT = VCC
15
mA
RREF
Reference Input Resistance
(LTC1285)
CS = VIH
CS = VIL
2700
54
MΩ
kΩ
IREF
Reference Current (LTC1285)
CS = VCC
tCYC ≥ 640µs, fCLK ≤ 25kHz
tCYC = 134µs, fCLK = 120kHz
●
ICC
CONDITIONS
µA
Supply Current
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DYNAMIC ACCURACY
MIN
TYP
MAX
2
2.4
2.1
UNITS
V
2.64
2.30
V
V
0.4
±3
V
µA
2.5
●
0.001
50
50
70
µA
µA
µA
CS = VCC
●
0.001
±3.0
µA
LTC1285, tCYC ≥ 640µs, fCLK ≤ 25kHz
LTC1285, tCYC = 134µs, fCLK = 120kHz
●
150
160
320
µA
µA
LTC1288, tCYC ≥ 720µs, fCLK ≤ 25kHz
LTC1288, tCYC = 150µs, fCLK = 120kHz
●
200
210
390
µA
µA
TYP
MAX
UNITS
fSMPL = 7.5kHz (LTC1285), fSMPL = 6.6kHz (LTC1288) (Note 5)
SYMBOL
PARAMETER
CONDITIONS
MIN
S/(N +D)
Signal-to-Noise Plus Distortion Ratio
1kHz Input Signal
67
dB
THD
Total Harmonic Distortion (Up to 5th Harmonic)
1kHz Input Signal
– 80
dB
SFDR
Spurious-Free Dynamic Range
1kHz Input Signal
88
dB
Peak Harmonic or Spurious Noise
1kHz Input Signal
– 88
dB
3
LTC1285/LTC1288
AC CHARACTERISTICS
(Note 5)
SYMBOL
PARAMETER
CONDITIONS
MIN
tSMPL
Analog Input Sample Time
See Operating Sequence
TYP
MAX
1.5
●
●
UNITS
CLK Cycles
fSMPL (MAX) Maximum Sampling Frequency
LTC1285
LTC1288
tCONV
Conversion Time
See Operating Sequence
tdDO
Delay Time, CLK↓ to DOUT Data Valid
See Test Circuits
●
600
1500
ns
tdis
Delay Time, CS↑ to DOUT Hi-Z
See Test Circuits
●
220
660
ns
ten
Delay Time, CLK↓ to DOUT Enable
See Test Circuits
●
180
500
ns
thDO
Time Output Data Remains Valid After CLK↓
CLOAD = 100pF
tf
DOUT Fall Time
See Test Circuits
●
60
180
ns
tr
DOUT Rise Time
See Test Circuits
●
80
180
ns
CIN
Input Capacitance
Analog Inputs, On Channel
Analog Inputs, Off Channel
Digital Input
The ● denotes specifications which apply over the full operating
temperature range.
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 GND.
Note 3: These devices are specified at 3V. For 5V specified devices, see
LTC1286 and LTC1298.
Note 4: Increased leakage currents at elevated temperatures cause the
sample-and-hold to droop, therefore it is recommended that fCLK ≥ 75kHz
at 70° and fCLK ≥ 1kHz at 25°C.
Note 5: VCC = 2.7V, VREF = 2.5V and CLK = 120kHz unless otherwise
specified.
Note 6: Linearity error is specified between the actual end points of the
A/D transfer curve.
7.5
6.6
kHz
kHz
12
CLK Cycles
520
ns
20
5
5
pF
pF
pF
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 GND or one diode drop above VCC. This spec allows 50mV forward
bias of either diode for 2.7V ≤ VCC ≤ 6V. 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 2.7V
input voltage range will therefore require a minimum supply voltage of
2.650V over initial tolerance, temperature variations and loading. For 2.7V
< VCC ≤ 6V, reference and analog input range cannot exceed 6.05V. If
reference and analog input range are greater than 6.05V, the output code
will not be guaranteed to be correct.
Note 8: The supply voltage range for the LTC1285 and the LTC1288 is
from 2.7V to 6V.
Note 9: Recommended operating conditions
Note 10: Channel leakage current is measured after the channel selection.
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TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Sample Rate
250
LTC1285
LTC1285
fSMPL = 7.5kHz
150
100
50
1
0.1
1
SAMPLE RATE (kHz)
10
LTC1285/88 • TPC01
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
0
–55 –35 –15
TA = 25°C
VCC = 2.7V
VREF = 2.5V
8
200
LTC1288
100
10
9
LTC1288
fSMPL = 6.6kHz
7
SUPPLY CURRENT (µA)
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
SUPPLY CURRENT (µA)
SUPPLY CURRENT (µA)
1000
4
Shutdown Supply Current vs Clock
Rate with CS High and CS Low
Supply Current vs Temperature
6
5
4
CS = 0
(AFTER CONVERSION)
3
2
1
0.002
5 25 45 65 85 105 125
TEMPERATURE (°C)
LTC1285/88 • TPC02
CS = VCC
0
1
20
60
80
40
FREQUENCY (kHz)
100
120
LTC1285/88 • TPC03
LTC1285/LTC1288
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TYPICAL PERFORMANCE CHARACTERISTICS
Reference Current vs
Sample Rate (LTC1285)
53
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
40
35
30
25
20
15
10
3.0
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
fSMPL = 7.5kHz
52
REFERENCE CURRENT (µA)
REFERENCE CURRENT (µA)
45
51
CHANGE IN OFFSET (LSB = 1/4096 × VREF)
50
50
49
48
47
46
45
44
5
43
–55 –35 –15
0
0
1
2
6
4
3
5
SAMPLE RATE (kHz)
7
8
TA = 25°C
VCC = 2.7V
fCLK = 120kHz
fSMPL = 7.5kHz
2.5
2.0
1.5
1.0
0.5
0
5 25 45 65 85 105 125
TEMPERATURE (°C)
0.5
–10
TA = 25°C
VCC = 2.7V
fCLK = 120kHz
fSMPL = 7.5kHz
– 0.05
– 0.10
0.40
–9
–8
CHANGE IN GAIN (LSB)
CHANGE IN LINEARITY (LSB)
0.45
0
0.35
0.30
0.25
0.20
0.15
–6
–5
–4
–3
0.10
–2
0.05
–1
– 0.20
0
20
40
50
30
TEMPERATURE (°C)
60
70
0
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
REFERENCE VOLTAGE (V)
LTC1285/88 • TPC07
EFFECTIVE NUMBER OF BITS (ENOBs)
0
– 0.5
12
74
11
68
10
62
9
56
8
50
S/(N + D) (dB)
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
LTC1285/88 • TPC09
Effective Bits and S/(N + D)
vs Input Frequency
1
0.5
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
REFERENCE VOLTAGE (V)
LTC1285/88 • TPC08
Differential Nonlinearity vs Code
DIFFERENTIAL NONLINEARITY ERROR (LSB)
10
TA = 25°C
VCC = 2.7V
fCLK = 120kHz
fSMPL = 7.5kHz
–7
– 0.15
0
3.0
Change in Gain
vs Reference Voltage
0.50
0.20
0.05
1.5
2.0
2.5
REFERENCE VOLTAGE (V)
LTC1285/88 • TPC06
Change in Linearity
vs Reference Voltage
Change in Offset vs Temperature
VCC = 2.7V
0.15 VREF = 2.5V
fCLK = 120kHz
0.10 fSMPL = fSMPL(MAX)
1.0
LTC1285/88 • TPC05
LTC1285/88 • TPC04
CHANGE IN OFFSET (LSB)
Change in Offset
vs Reference Voltage
Reference Current vs Temperature
7
6
5
4
3
TA = 25°C
VCC = 2.7V
fCLK = 120kHz
2
1
0
–1
0
512 1024 1536 2048 2560 3072 3584 4096
CODE
LTC1285/88 • TPC11
1
10
INPUT FREQUENCY (kHz)
100
LTC1285/88 • TPC12
5
LTC1285/LTC1288
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TYPICAL PERFORMANCE CHARACTERISTICS
Spurious-Free Dynamic Range
vs Input Frequency
S/(N + D) vs Input Level
90
80
70
60
50
40
30
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fSMPL = fSMPL(MAX)
20
10
0
10
INPUT FREQUENCY (kHz)
1
TA = 25°C
70 VCC = 2.7V
VREF = 2.5V
60 fIN = 1kHz
fSMPL = fSMPL(MAX)
10
20
50
40
30
20
10
70
100
0
– 100
10M
LTC1285/86 • TPC15
– 40
TA = 25°C
VCC = 2.7V (VRIPPLE = 1mV)
VREF = 2.5V
fCLK = 120kHZ
– 10
– 20
FEEDTHROUGH (dB)
– 80
10k
100k
1M
INPUT FREQUENCY (Hz)
1k
Power Supply Feedthrough
vs Ripple Frequency
TA = 25°C
VCC = 2.7V
VREF = 2.5V
f1 = 2.05kHz
f2 = 3.05kHz
fSMPL = 7.5kHz
– 20
– 60
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fSMPL = fSMPL(MAX)
0
0
MAGNITUDE (dB)
MAGNITUDE (dB)
60
Intermodulation Distortion
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fIN = 3.05kHz
fCLK = 120kHz
fSMPL = 7.5kHz
– 40
50
LTC1285/88 • TPC14
4096 Point FFT Plot
– 20
40
90
0
– 45 – 40 – 35 – 30 – 25 – 20 – 15 – 10 – 5
INPUT LEVEL (dB)
100
30
80
LTC1285/88 • G13
0
Attenuation vs Input Frequency
0
80
ATTENUATION (%)
SIGNAL-TO-NOISE PLUS DISTORTION (dB)
SPURIOUS-FREE DYNAMIC RANGE (dB)
100
– 60
– 80
– 30
– 40
– 50
– 60
– 70
– 80
– 100
– 90
– 120
0.5
1.0
1.5 2.0 2.5 3.0
FREQUENCY (kHz)
3.5
4.0
– 120
0
0.5
1.0
1.5 2.0 2.5 3.0
FREQUENCY (kHz)
LTC1285/88 • TPC16
160
S & H ACQUISITION TIME (ns)
CLOCK FREQUENCY (kHz)
180
140
120
100
80
VIN
+INPUT
60
40
0
0.1
–INPUT
RSOURCE–
300
TA = 25°C
VCC = 2.7V
VREF = 2.5V
280
1000
RSOURCE+
VIN
+INPUT
1
SOURCE RESISTANCE (kΩ)
10
TA = 25°C
VREF = 2.5V
260
240
220
200
180
160
140
–INPUT
LTC1285/88 • G19
6
Maximum Clock Frequency
vs Supply Voltage
10000
TA = 25°C
VCC = 2.7V
VREF = 2.5V
10M
LTC1285/86 • TPC18
Sample-and-Hold Acquisition
Time vs Source Resistance
200
10k
100k
1M
RIPPLE FREQUENCY (Hz)
1k
4.0
LTC1285/88 • TPC17
Maximum Clock Frequency
vs Source Resistance
20
3.5
CLOCK FREQUENCY (kHz)
0
–100
100
1
120
100
10
100
1000
SOURCE RESISTANCE (Ω)
10000
LTC1285/88 • TPC20
2.5
3.0
3.5 4.0 4.5 5.0
SUPPLY VOLTAGE (V)
5.5
6.0
LTC1285/88 • TPC21
LTC1285/LTC1288
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TYPICAL PERFORMANCE CHARACTERISTICS
Digital Input Logic Threshold
vs Supply Voltage
VCC = 2.7V
VREF = 2.5V
CLOCK FREQUENCY (kHz)
100
80
60
40
20
2
0
0
10
20
30
50
40
TEMPERATURE (°C)
60
70
Input Channel Leakage Current
vs Temperature
3.0
1000
TA = 25°C
VCC = 2.7V
VREF = 2.5V
2.5
100
LEAKAGE CURRENT (nA)
120
DIGITAL INPUT LOGIC THRESHOLD VOLTAGE (V)
Minimum Clock Frequency
for 0.1 LSB Error vs Temperature
2.0
1.5
1.0
1
ON CHANNEL
OFF CHANNEL
0.1
0.5
0
2.5
10
3.0
5.0
3.5 4.0 4.5
SUPPLY VOLTAGE (V)
LTC1285/88 • TPC22
5.5
6.0
0.01
–55 –35 –15
LTC1285/88 • TPC23
5 25 45 65 85 105 125
TEMPERATURE (°C)
LTC1285/88 • TPC24
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PIN FUNCTIONS
LTC1285
LTC1288
VREF (Pin 1): Reference Input. The reference input defines
the span of the A/D converter.
CS/SHDN (Pin 1): Chip Select Input. A logic low on this
input enables the LTC1288. A logic high on this input
disables and powers down the LTC1288.
IN + (Pin 2): Positive Analog Input.
IN – (Pin 3): Negative Analog Input.
CH0 (Pin 2): Analog Input.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
CH1 (Pin 3): Analog Input.
CS/SHDN (Pin 5): Chip Select Input. A logic low on this
input enables the LTC1285. A logic high on this input
disables and powers down the LTC1285.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
DIN (Pin 5): Digital Data Input. The multiplexer address is
shifted into this input.
DOUT (Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
DOUT (Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
CLK (Pin 7): Shift Clock. This clock synchronizes the serial
data transfer and determines conversion speed.
CLK (Pin 7): Shift Clock. This clock synchronizes the
serial data transfer and determines conversion speed.
VCC (Pin 8): Power Supply Voltage. This pin provides
power to the A/D converter. It must be kept free of noise
and ripple by bypassing directly to the analog ground
plane.
VCC /VREF (Pin 8): Power Supply and Reference Voltage.
This pin provides power and defines the span of the A/D
converter. It must be kept free of noise and ripple by
bypassing directly to the analog ground plane.
7
LTC1285/LTC1288
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BLOCK DIAGRAM
CS/SHDN
BIAS AND
SHUTDOWN CIRCUIT
IN + (CH0)
CSAMPLE
CLK
(DIN)
VCC (VCC/ V REF)
SERIAL PORT
DOUT
–
IN – (CH1)
+
SAR
MICROPOWER
COMPARATOR
CAPACITIVE DAC
LTC1285/88 • BD
VREF
GND
PIN NAMES IN PARENTHESES REFER TO THE LTC1288
TEST CIRCUITS
Load Circuit for tdDO, tr and tf
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
1.4V
VOH
DOUT
VOL
3k
DOUT
TEST POINT
tr
tf
LTC1285/88 • TC02
100pF
LTC1285/88 • TC01
Voltage Waveforms for DOUT Delay Times, tdDO
Load Circuit for tdis and ten
TEST POINT
CLK
VIL
tdDO
DOUT
3k
VCC tdis WAVEFORM 2, ten
DOUT
VOH
100pF
tdis WAVEFORM 1
VOL
LTC1285/88 • TC04
LTC1285/88 • TC03
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LTC1285/LTC1288
TEST CIRCUITS
Voltage Waveforms for tdis
Voltage Waveforms for ten
LTC1285
CS
VIH
CS
DOUT
WAVEFORM 1
(SEE NOTE 1)
90%
1
CLK
2
tdis
DOUT
WAVEFORM 2
(SEE NOTE 2)
B11
10%
DOUT
VOL
NOTE 1: WAVEFORM 1 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS HIGH UNLESS DISABLED BY THE OUTPUT CONTROL.
NOTE 2: WAVEFORM 2 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS LOW UNLESS DISABLED BY THE OUTPUT CONTROL.
ten
LTC1285/88 • TC06
LTC1285/88 • TC05
Voltage Waveforms for ten
LTC1288
CS
DIN
CLK
START
1
2
3
4
B11
DOUT
VOL
ten
LTC1285/88 • TC07
9
LTC1285/LTC1288
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OVERVIEW
basic design, the LTC1285 and LTC1288 differ in some
respects. The LTC1285 has a differential input and has an
external reference input pin. It can measure signals floating on a DC common-mode voltage and can operate with
reduced spans to 1.5V. Reducing the spans allows it to
achieve 366µV resolution. The LTC1288 has a two-channel input multiplexer and can convert either channel with
respect to ground or the difference between the two. The
reference input is tied to the supply pin.
The LTC1285 and LTC1288 are 3V micropower, 12-bit,
successive approximation sampling A/D converters. The
LTC1285 typically draws 160µA of supply current when
sampling at 7.5kHz while the LTC1288 nominally consumes 210µA of supply current when sampling at 6.6 kHz.
The extra 50µA of supply current on the LTC1288 comes
from the reference input which is intentionally tied to the
supply. Supply current drops linearly as the sample rate is
reduced (see Supply Current vs Sample Rate). The ADCs
automatically power down when not performing conversions, drawing only leakage current. They are packaged in
8-pin SO and DIP packages. The LTC1285 and LTC1288
operate on a single supply from 2.7V to 6V.
SERIAL INTERFACE
The 2-channel LTC1288 communicates with microprocessors and other external circuitry via a synchronous,
half duplex, 4-wire serial interface. The single channel
LTC1285 uses a 3-wire interface (see Operating Sequence
in Figures 1 and 2).
Both the LTC1285 and the LTC1288 contain a 12-bit,
switched-capacitor ADC, a sample-and-hold, and a serial
port (see Block Diagram). Although they share the same
tCYC
CS
POWER
DOWN
tsuCS
CLK
DOUT
HI-Z
NULL
BIT B11 B10 B9
B8
(MSB)
tSMPL
B7
B6
B5
B4
B3
B2
B1
tCONV
NULL
BIT B11 B10
HI-Z
B0*
B9
B8
tDATA
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT LSB-FIRST DATA THEN FOLLOWED WITH ZEROS INDEFINITELY.
tCYC
CS
tsuCS
POWER DOWN
CLK
DOUT
HI-Z
NULL
BIT
B11 B10
B9
B8
B7
B6
B5
B4
B3
B2
B1
B0
B1
B2
B3
B4
B5
B6
B7 B8
B9 B10 B11*
HI-Z
(MSB)
tSMPL
tDATA
tCONV
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT ZEROS INDEFINITELY.
tDATA: DURING THIS TIME, THE BIAS CIRCUIT AND THE COMPARATOR POWER DOWN AND THE REFERENCE INPUT
BECOMES A HIGH IMPEDANCE NODE, LEAVING THE CLK RUNNING TO CLOCK OUT LSB-FIRST DATA OR ZEROES.
Figure 1. LTC1285 Operating Sequence
10
LTC1285/88 • F01
LTC1285/LTC1288
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MSB-First Data (MSBF = 0)
tCYC
CS
tsuCS
POWER DOWN
CLK
ODD/
SIGN
START
DIN
DON’T CARE
MSBF
NULL
HI-Z BIT
SGL/
DIFF
DOUT
B11 B10 B9
(MSB)
tSMPL
B8
B6
B7
B5
B4
B3
B2
B1
B0
B1
B2
B3
B4
B5
B6
B7
B8
B9 B10 B11*
HI-Z
tDATA
tCONV
MSB-First Data (MSBF = 1)
tCYC
CS
POWER
DOWN
tsuCS
CLK
ODD/
SIGN
START
DIN
DON’T CARE
SGL/
DIFF
DOUT
MSBF
NULL
BIT B11 B10 B9
HI-Z
tSMPL
(MSB)
B8
B7
B6
tCONV
B5
B4
B3
B2
HI-Z
B1 B0*
tDATA
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT ZEROS INDEFINITELY.
tDATA: DURING THIS TIME, THE BIAS CIRCUIT AND THE COMPARATOR POWER DOWN AND THE REFERENCE INPUT
BECOMES A HIGH IMPEDANCE NODE, LEAVING THE CLK RUNNING TO CLOCK OUT LSB-FIRST DATA OR ZEROES.
LTC1285/88 • F02
Figure 2. LTC1288 Operating Sequence Example: Differential Inputs (CH+, CH–)
11
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Data Transfer
The CLK synchronizes the data transfer with each bit
being transmitted on the falling CLK edge and captured
on the rising CLK edge in both transmitting and receiving
systems.
The LTC1285 does not require a configuration input word
and has no DIN pin. A falling CS initiates data transfer as
shown in the LTC1285 operating sequence. After CS falls
the second CLK pulse enables DOUT. After one null bit the
A/D conversion result is output on the DOUT line. Bringing
CS high resets the LTC1285 for the next data exchange.
The LTC1288 first receives input data and then transmits
back the A/D conversion result (half duplex). Because of
the half duplex operation, DIN and DOUT may be tied
together allowing transmission over just 3 wires: CS, CLK
and DATA (DIN/DOUT).
Data transfer is initiated by a falling chip select (CS) signal.
After CS falls the LTC1288 looks for a start bit. After the
start bit is received, the 3-bit input word is shifted into the
DIN input which configures the LTC1288 and starts the
conversion. After one null bit, the result of the conversion
is output on the DOUT line. At the end of the data exchange
CS should be brought high. This resets the LTC1288 in
preparation for the next data exchange.
the rising edge of the clock. The input data words are
defined as follows:
START
SGL/
DIFF
ODD/
SIGN
MSBF
MUX
MSB FIRST/
ADDRESS LSB FIRST
LTC1285/88 • AI02
Start Bit
The first “logical one” clocked into the DIN input after CS
goes low is the start bit. The start bit initiates the data
transfer. The LTC1288 will ignore all leading zeros which
precede this logical one. After the start bit is received, the
remaining bits of the input word will be clocked in. Further
inputs on the DIN pin are then ignored until the next CS
cycle.
Multiplexer (MUX) Address
The bits of the input word following the START bit 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 the following tables.
In single-ended mode, all input channels are measured
with respect to GND.
LTC1288 Channel Selection
CS
DIN 1
DIN 2
DOUT 1
SINGLE-ENDED
MUX MODE
DOUT 2
DIFFERENTIAL
MUX MODE
SHIFT MUX
ADDRESS IN
1 NULL BIT
CHANNEL #
0
1
+
+
+
–
–
+
GND
–
–
LTC1285/88 • AI03
SHIFT A/D CONVERSION
RESULT OUT
LTC1285/88 • AI01
Input Data Word
The LTC1285 requires no DIN word. It is permanently
configured to have a single differential input. The conversion result appears on the DOUT line. The data format is
MSB first followed by the LSB sequence. This provides
easy interface to MSB or LSB first serial ports. For MSB
first data the CS signal can be taken high after B0 (see
Figure 1). The LTC1288 clocks data into the DIN input on
12
MUX ADDRESS
SGL/DIFF ODD/SIGN
1
0
1
1
0
0
0
1
MSB First/LSB First (MSBF)
The output data of the LTC1288 is programmed for
MSB first or LSB first sequence using the MSBF bit.
When the MSBF bit is a logical one, data will appear on
the DOUT line in MSB first format. Logical zeros will be
filled in indefinitely following the last data bit. When the
MSBF bit is a logical zero, LSB first data will follow the
normal MSB first data on the DOUT line (see Operating
Sequence).
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Transfer Curve
The LTC1285/LTC1288 are permanently configured for
unipolar only. The input span and code assignment for
this conversion type are shown in the following figures.
Transfer Curve
either an input or an output. The LTC1288 will take control
of the data line and drive it low on the 4th falling CLK edge
after the start bit is received (see Figure 3). Therefore the
processor port line must be switched to an input before
this happens to avoid a conflict.
In the Typical Applications section, there is an example of
interfacing the LTC1288 with DIN and DOUT tied together to
the Intel 8051 MPU.
111111111111
111111111110
•
•
•
ACHIEVING MICROPOWER PERFORMANCE
000000000001
VIN
000000000000
VREF
VREF–1LSB
VREF
4096
VREF–2LSB
1LSB
0V
1LSB =
LTC1285/88 • AI04
With typical operating currents of 160µA and automatic
shutdown between conversions, the LTC1285/LTC1288
achieves extremely low power consumption over a wide
range of sample rates (see Figure 4). The auto-shutdown
allows the supply curve to drop with reduced sample rate.
Output Code
OUTPUT CODE
INPUT VOLTAGE
11111111111111
11111111111110
•
•
•
00000000000001
00000000000000
VREF – 1LSB
VREF – 2LSB
•
•
•
1LSB
0V
4.99878V
4.99756V
•
•
•
0.00122V
0V
SUPPLY CURRENT (µA)
1000
INPUT VOLTAGE
(VREF = 5.000V)
LTC1285/88 • AI05
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fCLK = 120kHz
100
10
Operation with DIN and DOUT Tied Together
The LTC1288 can be operated with DIN and DOUT tied
together. This eliminates one of the lines required to
communicate to the microprocessor (MPU). Data is transmitted in both directions on a single wire. The processor
pin connected to this data line should be configurable as
1
0.1
1
10
SAMPLE FREQUENCY (kHz)
100
LTC1285/88 • F04
Figure 4. Automatic Power Shutdown Between Conversions
Allows Power Consumption to Drop with Sample Rate
MSBF BIT LATCHED
BY LTC1288
CS
1
2
3
4
START
SGL/DIFF
ODD/SIGN
MSBF
CLK
DATA
(DIN/DOUT)
MPU CONTROLS DATA LINE AND SENDS
MUX ADDRESS TO LTC1288
PROCESSOR MUST RELEASE DATA LINE AFTER
4TH RISING CLK AND BEFORE THE 4TH FALLING CLK
B11
B10
• • •
LTC1288 CONTROLS DATA LINE AND SENDS
A/D RESULT BACK TO MPU
LTC1288 TAKES CONTROL OF DATA LINE
ON 4TH FALLING CLK
LTC1285/88 F03
Figure 3. LTC1288 Operation with DIN and DOUT Tied Together
13
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Several things must be taken into account to achieve such
a low power consumption.
Shutdown
The LTC1285/LTC1288 are equipped with automatic shutdown features. They draw power when the CS pin is low
and shut down completely when that pin is high. The bias
circuit and comparator powers down and the reference
input becomes high impedance at the end of each conversion leaving the CLK running to clock out the LSB first data
or zeroes (see Figures 1 and 2). If the CS is not running railto-rail, the input logic buffer will draw current. This current
may be large compared to the typical supply current. To
obtain the lowest supply current, bring the CS pin to
ground when it is low and to supply voltage when it is high.
When the CS pin is high (= supply voltage), the converter
is in shutdown mode and draws only leakage current. The
status of the DIN and CLK input have no effect on supply
current during this time. There is no need to stop DIN and
CLK with CS = high; they can continue to run without
drawing current.
Minimize CS Low Time
In systems that have significant time between conversions, lowest power drain will occur with the minimum CS
low time. Bringing CS low, transferring data as quickly as
possible, and then bringing it back high will result in the
9
TA = 25°C
VCC = 2.7V
VREF = 2.5V
8
SUPPLY CURRENT (µA)
7
6
5
4
CS = 0
(AFTER CONVERSION)
3
2
1
0.002
CS = VCC
0
1
20
60
80
40
FREQUENCY (kHz)
100
120
LTC1285/88 • TPC03
Figure 5. Shutdown Current with CS High is 1nA Typically,
Regardless of the Clock. Shutdown Current with CS = Ground
Varies From 1µA at 1kHz to 9µA at 120kHz
14
lowest current drain. This minimizes the amount of time
the device draws power. After a conversion the ADC
automatically shuts down even if CS is held low (see
Figures 1 and 2). If the clock is left running to clock out
LSB-data or zero, the logic will draw a small current.
Figure 5 shows that the typical supply current with CS =
ground varies from 1µA at 1kHz to 9µA at 120kHz. When
CS = VCC, the logic is gated off and no supply current is
drawn regardless of the clock frequency.
DOUT Loading
Capacitive loading on the digital output can increase
power consumption. A 100pF capacitor on the DOUT pin
can add more than 16.2µA to the supply current at a
120kHz clock frequency. An extra 16.2µA or so of current
goes into charging and discharging the load capacitor. The
same goes for digital lines driven at a high frequency by
any logic. The C × V × f currents must be evaluated and the
troublesome ones minimized.
OPERATING ON OTHER THAN 3V SUPPLIES
Both the LTC1285 and the LTC1288 operate from a 2.7V
to 6V supply. To operate the LTC1285/LTC1288 on other
than 3V supplies a few things must be kept in mind.
Input Logic Levels
The input logic levels of CS, CLK and DIN are made to
meet TTL on a 3V supply. When the supply voltage varies,
the input logic levels also change. For the LTC1285/
LTC1288 to sample and convert correctly, the digital
inputs have to be in the proper logical low and high levels
relative to the operating supply voltage (see typical curve
of Digital Input Logic Threshold vs Supply Voltage). If
achieving micropower consumption is desirable, the
digital inputs must go rail-to-rail between supply voltage
and ground (see ACHIEVING MICROPOWER PERFORMANCE section).
Clock Frequency
The maximum recommended clock frequency is 120kHz
for the LTC1285/LTC1288 running off a 3V supply. With
the supply voltage changing, the maximum clock frequency for the devices also changes (see the typical curve
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of Maximum Clock Rate vs Supply Voltage). If the maximum clock frequency is used, care must be taken to
ensure that the device converts correctly.
Mixed Supplies
It is possible to have a microprocessor running off a 5V
supply and communicate with the LTC1285/LTC1288
operating on a 3V supply. The inputs of CS, CLK and DIN
of the LTC1285/LTC1288 have no problem to take a
voltage swing from 0V to 5V. With the LTC1285 operating
on a 3V supply, the output of DOUT may only go between
0V and 3V. The 3V output level is higher enough to trip a
TTL input of the MPU. Figure 6 shows a 3V powered
LTC1285 interfacing a 5V system.
3V 4.7µF
MPU
(e.g. 8051)
3V
DIFFERENTIAL INPUTS
COMMON-MODE RANGE
0V TO 3V
VREF
VCC
P1.4
+IN
CLK
P1.3
–IN
DOUT
P1.2
GND
5V
CS
LTC1285
LTC1285/88 • F06
Figure 6. Interfacing a 3V Powered LTC1285 to a 5V System
BOARD LAYOUT CONSIDERATIONS
Grounding and Bypassing
The LTC1285/LTC1288 are easy to use if some care is
taken. They should be used with an analog ground plane
and single point grounding techniques. The GND pin
should be tied directly to the ground plane.
The VCC pin should be bypassed to the ground plane with
a 10µF tantalum capacitor with leads as short as possible.
If the power supply is clean, the LTC1285/LTC1288 can
also operate with smaller 1µF or less surface mount or
ceramic bypass capacitors. 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.
SAMPLE-AND-HOLD
Both the LTC1285 and the LTC1288 provide a built-in
sample-and-hold (S&H) function to acquire signals. The
S&H of the LTC1285 acquires input signals from “+” input
relative to “–” input during the tSMPL time (see Figure 1).
However, the S&H of the LTC1288 can sample input
signals in the single-ended mode or in the differential
inputs during the tSMPL time (see Figure 7).
SAMPLE
HOLD
"+" INPUT MUST
SETTLE DURING
THIS TIME
CS
tSMPL
tCONV
CLK
DIN
START
SGL/DIFF
MSBF
DOUT
DON’T CARE
B11
1ST BIT TEST "–" INPUT MUST
SETTLE DURING THIS TIME
"+" INPUT
"–" INPUT
LTC1285/88 • F07
Figure 7. LTC1288 “+” and “–” Input Settling Windows
15
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Single-Ended Inputs
“+” Input Settling
The sample-and-hold of the LTC1288 allows conversion
of rapidly varying signals. The input voltage is sampled
during the tSMPL time as shown in Figure 7. The sampling
interval begins as the bit preceding the MSBF bit is shifted
in and continues until the falling CLK edge after the MSBF
bit is received. On this falling edge, the S&H goes into hold
mode and the conversion begins.
The input capacitor of the LTC1285 is switched onto “+”
input during the tSMPL time (see Figure 1) and samples
the input signal within that time. However, the input
capacitor of the LTC1288 is switched onto “+” input
during the sample phase (tSMPL, see Figure 7). The
sample phase is 1 1/2 CLK cycles before conversion
starts. The voltage on the “+” input must settle completely within tSMPLE for the LTC1285 and the LTC1288
respectively. Minimizing RSOURCE+ and C1 will improve
the input settling time. If a large “+” input source resistance must be used, the sample time can be increased by
using a slower CLK frequency.
Differential Inputs
With differential inputs, the ADC no longer converts just a
single voltage but rather the difference between two voltages. In this case, 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 12 CLK 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:
VERROR (MAX) = VPEAK × 2 × π × f(“–”) × 12/fCLK
Where f(“–”) is the frequency of the “–” input voltage,
VPEAK is its peak amplitude and fCLK is the frequency of the
CLK. In most cases VERROR will not be significant. For a
60Hz signal on the “–” input to generate a 1/4LSB error
(152µV) with the converter running at CLK = 120kHz, its
peak value would have to be 4.03mV.
ANALOG INPUTS
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1285/
LTC1288 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.
16
“–” Input Settling
At the end of the tSMPL, the input capacitor switches to the
“–” input and conversion starts (see Figures 1 and 7).
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 settles completely during the first CLK cycle of the
conversion time and be free of noise. Minimizing RSOURCE–
and C2 will improve settling time. If a large “–” input
source resistance must be used, the time allowed for
settling can be extended by using a slower CLK frequency.
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 7). Again, the“+” and “–” input sampling times
can be extended as described above to accommodate
slower op amps. Most op amps, including the LT1006 and
LT1413 single supply op amps, can be made to settle well
even with the minimum settling windows of 12.5µs (“+”
input) which occur at the maximum clock rate of 120kHz.
Source Resistance
The analog inputs of the LTC1285/LTC1288 look like a
20pF capacitor (CIN) in series with a 500Ω resistor (RON)
as shown in Figure 8. CIN gets switched between the
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selected “+” and “–” inputs once during each conversion
cycle. Large external source resistors and capacitances
will slow the settling of the inputs. It is important that the
overall RC time constants be short enough to allow the
analog inputs to completely settle within the allowed time.
RSOURCE +
“+”
INPUT
LTC1285
LTC1288
VIN +
C1
RSOURCE –
RON = 500Ω
“–”
INPUT
Input leakage currents can also create errors if the source
resistance gets too large. For instance, the maximum
input leakage specification of 1µA (at 125°C) flowing
through a source resistance of 240Ω will cause a voltage
drop of 240µV or 0.4LSB. This error will be much
reduced at lower temperatures because leakage drops
rapidly (see typical curve of Input Channel Leakage
Current vs Temperature).
CIN = 20pF
REFERENCE INPUTS
VIN –
C2
LTC1285/88 • F08
Figure 8. Analog Input Equivalent Circuit
RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 9. 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 IDC = 20pF × VIN /tCYC and is roughly
proportional to VIN. When running at the minimum cycle
time of 133.3µs, the input current equals 0.375µA at VIN
= 2.5V. In this case, a filter resistor of 160Ω 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.
RFILTER
Input Leakage Current
IDC
“+”
VIN
CFILTER
LTC1285
“–”
LTC1285/88 • F09
Figure 9. RC Input Filtering
The reference input of the LTC1285 is effectively a 50kΩ
resistor from the time CS goes low to the end of the
conversion. The reference input becomes a high impedence
node at any other time (see Figure 10). Since the voltage
on the reference input defines the voltage span of the A/D
converter, the reference input should be driven by a
reference with low ROUT (ex. LT1004, LT1019 and LT1021)
or a voltage source with low ROUT.
REF+
1
LTC1285
ROUT
VREF
GND
4
LTC1285/88 • F10
Figure 10. Reference Input Equivalent Circuit
Reduced Reference Operation
The minimum reference voltage of the LTC1288 is limited
to 2.7V because the VCC supply and reference are internally tied together. However, the LTC1285 can operate
with reference voltages below 1.5V.
The effective resolution of the LTC1285 can be increased
by reducing the input span of the converter. The LTC1285
exhibits good linearity and gain over a wide range of
reference voltages (see typical curves of Change in Linearity vs Reference Voltage and Change in Gain vs Reference
17
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Voltage). However, care must be taken when operating at
low values of VREF because of the reduced LSB step size
and the resulting higher accuracy requirement placed on
the converter. The following factors must be considered
when operating at low VREF values:
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
noise becomes equal to 3.3LSBs and a stable code may
be difficult to achieve. In this case averaging multiple
readings may be necessary.
This noise data was taken in a very clean setup. Any setup
induced noise (noise or ripple on VCC, VREF or VIN) will 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.
Offset with Reduced VREF
Conversion Speed with Reduced VREF
The offset of the LTC1285 has a larger effect on the output
code. When the ADC 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 Change in Offset vs
Reference Voltage shows how offset in LSBs is related to
reference voltage for a typical value of VOS. For example,
a VOS of 122µV which is 0.2LSB with a 2.5V reference
becomes 1LSB with a 1V reference and 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 of the LTC1285.
With reduced reference voltages, the LSB step size is
reduced and the LTC1285 internal comparator overdrive is reduced. Therefore, it may be necessary to
reduce the maximum CLK frequency when low values
of VREF are used.
The total input referred noise of the LTC1285 can be
reduced to approximately 400µV 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 2.5V reference but will become a
larger fraction of an LSB as the size of the LSB is reduced.
For operation with a 2.5V reference, the 400µV noise is
only 0.66LSB peak-to-peak. In this case, the LTC1285
noise will contribute a little bit of 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 400µV noise is 1.32LSB
peak-to-peak. This will reduce the range of input voltages over which a stable output code can be achieved by
1LSB. If the reference is further reduced to 1V, the 400µV
18
The LTC1285/LTC1288 have exceptional sampling capability. Fast Fourier Transform (FFT) test techniques are
used to characterize the ADC’s frequency response, distortion and noise at the rated throughput. By applying a
low distortion sine wave and analyzing the digital output
using an FFT algorithm, the ADC’s spectral content can be
examined for frequencies outside the fundamental. Figure
11 shows a typical LTC1285 plot.
0
TA = 25°C
VCC = 2.7V
VREF = 2.5V
fIN = 3.05kHz
fCLK = 120kHz
fSMPL = 7.5kHz
– 20
MAGNITUDE (dB)
Noise with Reduced VREF
DYNAMIC PERFORMANCE
– 40
– 60
– 80
– 100
– 120
0
0.5
1.0
1.5 2.0 2.5 3.0
FREQUENCY (kHz)
3.5
4.0
LTC1285/88 • TPC16
Figure 11. LTC1285 Non-Averaged, 4096 Point FFT Plot
LTC1285/LTC1288
U
W
U U
APPLICATION INFORMATION
Signal-to-Noise Ratio
12
74
11
68
10
62
9
56
8
50
S/(N + D) (dB)
EFFECTIVE NUMBER OF BITS (ENOBs)
The Signal-to-Noise plus Distortion Ratio (S/N + D) is the
ratio between the RMS amplitude of the fundamental
input frequency to the RMS amplitude of all other frequency components at the ADC’s output. The output is
band limited to frequencies above DC and below one half
the sampling frequency. Figure 12 shows a typical spectral content with a 7.5kHz sampling rate.
7
6
5
4
3
TA = 25°C
VCC = 2.7V
fCLK = 120kHz
2
1
V22 + V32 + V42 + ... + VN2
THD = 20log
V1
where V1 is the RMS amplitude of the fundamental frequency and V2 through VN are the amplitudes of the
second through the Nth harmonics. The typical THD specification in the Dynamic Accuracy table includes the 2nd
through 5th harmonics. With a 1kHz input signal, the
LTC1285/LTC1288 have typical THD of 80dB with
VCC = 2.7V.
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition
to THD. IMD is the change in one sinusoidal input
caused by the presence of another sinusoidal input at a
different frequency.
0
1
10
INPUT FREQUENCY (kHz)
100
LTC1285/88 • TPC12
Figure 12. Effective Bits and S/(N + D) vs Input Frequency
Effective Number of Bits
The Effective Number of Bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to S/(N+D)
by the equation:
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer function can create distortion products at sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb) and
(fa – fb) while 3rd order IMD terms include (2fa + fb),
(2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine
waves are equal in magnitudes, the value (in dB) of the 2nd
order IMD products can be expressed by the following
formula:
ENOB = [S/(N + D) – 1.76]/6.02
(
)
amplitude fa ± fb
IMD fa ± fb = 20log
amplitude at fa
(
)
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 7.5kHz with a 2.7V supply, the LTC1285
maintains above 10.7 ENOBs at 10kHz input frequency.
Above 10kHz the ENOBs gradually decline, as shown in
Figure 12, due to increasing second harmonic distortion.
The noise floor remains low.
For input frequencies of 2.05kHz and 3.05kHz, the IMD of
the LTC1285/LTC1288 is 72dB with a 2.7V supply.
Total Harmonic Distortion
Peak Harmonic or Spurious Noise
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamental
itself. The out-of-band harmonics alias into the frequency
band between DC and half of the sampling frequency. THD
is defined as:
The peak harmonic or spurious noise is the largest spectral component excluding the input signal and DC. This
value is expressed in dBs relative to the RMS value of a fullscale input signal.
19
LTC1285/LTC1288
U
TYPICAL APPLICATIONS N
MICROPROCESSOR INTERFACES
The LTC1285/LTC1288 can interface directly without external hardware to most popular microprocessor (MPU)
synchronous serial formats (see Table 1). If an MPU
without a dedicated serial port is used, then 3 or 4 of the
MPU's parallel port lines can be programmed to form the
serial link to the LTC1285/LTC1288. Included here is one
serial interface example and one example showing a
parallel port programmed to form the serial interface.
Motorola SPI (MC68HC11)
The MC68HC11 has been chosen as an example of an MPU
with a dedicated serial port. This MPU transfers data MSB
-first and in 8-bit increments. The DIN word sent to the data
register starts with the SPI process. With three 8-bit
transfers, the A/D result is read into the MPU. The second
8-bit transfer clocks B11 through B8 of the A/D conversion
result into the processor. The third 8-bit transfer clocks
the remaining bits, B7 through B0, into the MPU. The data
is right justified into two memory locations. ANDing the
second byte with OFHEX clears the four most significant
bits. This operation was not included in the code. It can be
inserted in the data gathering loop or outside the loop
when the data is processed.
MC68HC11 Code
In this example the DIN word configures the input MUX for
a single-ended input to be applied to CHO. The conversion
result is output MSB-first.
20
Table 1. Microprocessor with Hardware Serial Interfaces
Compatible with the LTC1286/LTC1298
PART NUMBER
TYPE OF INTERFACE
Motorola
MC6805S2,S3
MC68HC11
MC68HC05
SPI
SPI
SPI
RCA
CDP68HC05
SPI
Hitachi
HD6305
HD63705
HD6301
HD63701
HD6303
HD64180
SCI Synchronous
SCI Synchronous
SCI Synchronous
SCI Synchronous
SCI Synchronous
CSI/O
National Semiconductor
COP400 Family
COP800 Family
NS8050U
HPC16000 Family
MICROWIRE †
MICROWIRE/PLUS†
MICROWIRE/PLUS†
MICROWIRE/PLUS†
Texas Instruments
TMS7002
TMS7042
TMS70C02
TMS70C42
TMS32011*
TMS32020
Serial Port
Serial Port
Serial Port
Serial Port
Serial Port
Serial Port
Intel
8051
Bit Manipulation on Parallel Port
* Requires external hardware
†
MICROWIRE and MICROWIRE/PLUS are trademarks of
National Semiconductor Corp.
LTC1285/LTC1288
U
TYPICAL APPLICATIONS N
Timing Diagram for Interface to the MC68HC11
CS
CLK
DIN
SGL/
DIFF
START
ODD/
SIGN MSBF
DON'T CARE
DOUT
MPU
TRANSMIT
WORD
0
0
0
0
0
0
SGL/
DIFF
1
0
ODD/
SIGN MSBF
?
?
?
?
?
B10
B9
B8
X
X
X
X
X
B7
X
B6
B5
B4
B3
B2
B1
B0
X
X
X
X
X
X
X
B2
B1
B0
BYTE 2
BYTE 1
MPU
RECEIVED
WORD
B11
?
?
?
?
?
?
0
B11
BYTE 3 (DUMMY)
B10
B9
B8
B7
B6
BYTE 2
BYTE 1
B5
B4
B3
BYTE 3
LTC1285/88 • TA03
Hardware and Software Interface to the MC68HC11
DOUT FROM LTC1298 STORED IN MC68HC11 RAM
MSB
#62
0
0
0
0
B11
B10
B9
B8
ANALOG
INPUTS
LSB
#63
B7
B6
B5
B4
CH0
BYTE 1
B3
B2
B1
B0
CS
D0
CLK
SCK
LTC1288
DOUT
BYTE 2
CH1
DIN
MC68HC11
MISO
MOSI
LTC1285/88 • TA04
LABEL MNEMONIC
LDAA
STAA
LDAA
STAA
LDAA
STAA
LDAA
STAA
LDAA
LOOP
OPERAND
#$50
$1028
#$1B
$1009
#$01
$50
#$A0
$51
#$00
STAA
LDX
$52
#$1000
BCLR
LDAA
STAA
LDAA
$08,X,#$01
$50
$102A
$1029
COMMENTS
CONFIGURATION DATA FOR SPCR
LOAD DATA INTO SPCR ($1028)
CONFIG. DATA FOR PORT D DDR
LOAD DATA INTO PORT D DDR
LOAD DIN WORD INTO ACC A
LOAD DIN DATA INTO $50
LOAD DIN WORD INTO ACC A
LOAD DIN DATA INTO $51
LOAD DUMMY DIN WORD INTO
ACC A
LOAD DUMMY DIN DATA INTO $52
LOAD INDEX REGISTER X WITH
$1000
D0 GOES LOW (CS GOES LOW)
LOAD DIN INTO ACC A FROM $50
LOAD DIN INTO SPI, START SCK
CHECK SPI STATUS REG
LABEL MNEMONIC
WAIT1 BPL
LDAA
STAA
WAIT2 LDAA
BPL
LDAA
STAA
LDAA
STAA
WAIT3 LDAA
BPL
BSET
LDAA
STAA
JMP
OPERAND
WAIT1
$51
$102A
$1029
WAIT2
$102A
$62
$52
$102A
$1029
WAIT3
$08,X#$01
$102A
$63
LOOP
COMMENTS
CHECK IF TRANSFER IS DONE
LOAD DIN INTO ACC A FROM $51
LOAD DIN INTO SPI, START SCK
CHECK SPI STATUS REG
CHECK IF TRANSFER IS DONE
LOAD LTC1288 MSBs INTO ACC A
STORE MSBs IN $62
LOAD DUMMY INTO ACC A
FROM $52
LOAD DUMMY DIN INTO SPI,
START SCK
CHECK SPI STATUS REG
CHECK IF TRANSFER IS DONE
DO GOES HIGH (CS GOES HIGH)
LOAD LTC1288 LSBs IN ACC
STORE LSBs IN $63
START NEXT CONVERSION
21
LTC1285/LTC1288
U
TYPICAL APPLICATIONS N
Interfacing to the Parallel Port of the INTEL 8051
Family
LABEL
The Intel 8051 has been chosen to demonstrate the
interface between the LTC1288 and parallel port microprocessors. Normally the CS, CLK and DIN signals would
be generated on 3 port lines and the DOUT signal read on
a 4th port line. This works very well. However, we will
demonstrate here an interface with the DIN and DOUT of the
LTC1288 tied together as described in the SERIAL INTERFACE section. This saves one wire.
The 8051 first sends the start bit and MUX address to the
LTC1288 over the data line connected to P1.2. Then P1.2
is reconfigured as an input (by writing to it a one) and the
8051 reads back the 12-bit A/D result over the same data
line.
ANALOG
INPUTS
CS
CLK
LTC1288
DOUT
DIN
P1.4
P1.3
P1.2
LOOP 2
LOOP 3
8051
MUX ADDRESS
A/D RESULT
LOOP 1
LOOP 4
LTC1285/88 • TA01
MNEMONIC
OPERAND
COMMENTS
MOV
SETB
CLR
MOV
RLC
CLR
MOV
SETB
DJNZ
MOV
CLR
MOV
MOV
RLC
SETB
CLR
DJNZ
MOV
CLR
MOV
MOV
RLC
SETB
CLR
DJNZ
MOV
RRC
DJNZ
MOV
SETB
A, #FFH
P1.4
P1.4
R4, #04
A
P1.3
P1.2, C
P1.3
R4, LOOP 1
P1, #04
P1.3
R4, #09
C, P1.2
A
P1.3
P1.3
R4, LOOP 2
R2, A
A
R4, #04
C, P1.2
A
P1.3
P1.3
R4, LOOP 3
R4, #04
A
R4, LOOP 4
R3, A
P1.4
DIN word for LTC1288
Make sure CS is high
CS goes low
Load counter
Rotate DIN bit into Carry
SCLK goes low
Output DIN bit to LTC1288
SCLK goes high
Next bit
Bit 2 becomes an input
SCLK goes low
Load counter
Read data bit into Carry
Rotate data bit into Acc.
SCLK goes high
SCLK goes low
Next bit
Store MSBs in R2
Clear Acc.
Load counter
Read data bit into Carry
Rotate data bit into Acc.
SCLK goes high
SCLK goes low
Next bit
Load counter
Rotate right into Acc.
Next Rotate
Store LSBs in R3
CS goes high
DOUT FROM 1288 STORED IN 8501 RAM
MSB
R2 B11 B10 B9 B8 B7 B6 B5 B4
LSB
R3 B3 B2 B1 B0 0 0 0 0
MSBF BIT LATCHED
INTO LTC1288
CS
CLK
DATA
(DIN/DOUT)
START
SGL/
DIFF ODD/ MSBF
SIGN
8051 P1.2 OUTPUTS DATA
TO LTC1288
8051 P1.2 RECONFIGURED
AS IN INPUT AFTER THE 4TH RISING CLK
AND BEFORE THE 4TH FALLING CLK
22
B11
B10
B9
B8
B7
B6
B5
B4
B3
B2
B1
B0
LTC1288 SENDS A/D RESULT
BACK TO 8051 P1.2
LTC1288 TAKES CONTROL OF DATA
LINE ON 4TH FALLING CLK
LTC1285/88 • TA07
LTC1285/LTC1288
U
TYPICAL APPLICATIONS N
A “Quick Look” Circuit for the LTC1285
Micropower Battery Voltage Monitor
Users can get a quick look at the function and timing of the
LT1285 by using the following simple circuit (Figure 13).
VREF is tied to VCC. VIN is applied to the +IN input and the
–IN input is tied to the ground. CS is driven at 1/16 the
clock rate by the 74C161 and DOUT outputs the data. The
output data from the DOUT pin can be viewed on an
oscilloscope that is set up to trigger on the falling edge of
CS (Figure 14). Note the LSB data is partially clocked out
before CS goes high.
A common problem in battery systems is battery voltage
monitoring. This circuit monitors the 10 cell stack of NiCad
or NiMH batteries found in laptop computers. It draws only
40µA from the 2.7V supply at fSMPL = 0.1kHz and 30µA to
62µA from the battery. The 12-bits of resolution of the
LTC1285 are positioned over the desired range of 8V to
16V. This is easily accomplished by using the ADC’s
differential inputs. Tying the –input to the reference gives
an ADC input span of VREF to 2VREF (1.2V to 2.4V). The
resistor divider then scales the input voltage for 8V to 16V.
2.7V
4.7µF
BATTERY MONITOR
INPUT 8V TO 16V
VIN
VREF
VCC
+IN
CLK
CLR
VCC
CLK
RC
A
QA
B
QB
74HC161
C
QC
D
QD
P
T
GND
LOAD
LTC1285
–IN
DOUT
CS
GND
0.1µF
220k
39k
VCC
+IN
CS
LTC1285
1µF
CLK
DOUT
–IN
39k
LTC1285/88 • F13
2.7V
VREF
GND
LT1004-1.2
3Ω
CLOCK IN 120kHz
LTC1285/88 • F15
TO OSCILLOSCOPE
Figure 13. “Quick Look” Circuit for the LTC1285
NULL
BIT
Figure 15. Micropower Battery Voltage Monitor
LSB
MSB
(B0)
(B11)
VERTICAL: 2V/DIV
HORIZONTAL: 20µs/DIV
Figure 14. Scope Trace the LTC1285 “Quick Look” Circuit
Showing A/D Output 101010101010 (AAAHEX)
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.
23
LTC1285/LTC1288
U
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package
8-Lead Plastic DIP
0.300 – 0.325
(7.620 – 8.255)
0.009 – 0.015
(0.229 – 0.381)
(
+0.025
0.325 –0.015
8.255
+0.635
–0.381
)
0.045 – 0.065
(1.143 – 1.651)
0.400*
(10.160)
MAX
0.130 ± 0.005
(3.302 ± 0.127)
0.065
(1.651)
TYP
8
7
6
5
1
2
3
4
0.255 ± 0.015*
(6.477 ± 0.381)
0.125
(3.175)
MIN
0.005
(0.127)
MIN
0.015
(0.380)
MIN
N8 0695
0.018 ± 0.003
(0.457 ± 0.076)
0.100 ± 0.010
(2.540 ± 0.254)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
S8 Package
8-Lead Plastic SOIC
0.189 – 0.197*
(4.801 – 5.004)
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
8
7
6
5
0°– 8° TYP
0.016 – 0.050
0.406 – 1.270
0.014 – 0.019
(0.355 – 0.483)
0.050
(1.270)
BSC
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
*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
1
2
3
4
SO8 0695
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1096/LTC1098
8-Pin SOIC, Micropower 8-Bit ADC
Low Power, Small Size, Low Cost
LTC1196/LTC1198
8-Pin SOIC, 1Msps 8-bit ADC
Low Power, Small Size, Low Cost
LTC1282
3V High Speed Parallel 12-Bit ADC
Complete, VREF, CLK, Sample-and-Hold, 140ksps
LTC1289
Multiplexed 3V, 1A 12-Bit ADC
8-Channel, 12-Bit Serial I/O
LTC1522
16-Pin SOIC, 3V Micropower 12-Bit ADC
4-Channel, 12-Bit Serial I/O
24
Linear Technology Corporation
LT/GP 0894 10K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
LINEAR TECHNOLOGY CORPORATION 1994
工商网监
湘ICP备2023018690号
