Download DN294 - 250ksps, 16-Bit Micropower ADC Offers an Excellent Combination of SIze, Power and Speed

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250ksps, 16-Bit Micropower ADC Offers an Excellent
Combination of Size, Power and Speed – Design Note 294
Guy Hoover
MSOP Package for Portable Applications
Available in the MSOP package, the LTC1864 is well suited
for portable applications where space is limited. As seen
in Figure 1, the MSOP package provides considerable
space savings over other serial parts available in 20-lead
SW or 28-lead SSOP packages.
20-LEAD SW
28-LEAD SSOP
MSOP
DN294 F01
Figure 1. This 16-Bit ADC—Available in the Tiny MSOP
Package—is an Obvious Space Saver Over Other Serial
ADCs
Low ADC Supply Current for
Battery-Operated Applications
With a typical supply current of only 850µA at the
maximum sample rate of 250ksps, the LTC1864 already
has one of the lowest power consumptions of any 16-bit
ADC available. After a conversion, the LTC1864 goes into
a low power SLEEP mode (ICC = 1nA typ, ICC = 3µA max)
further reducing supply current. The LTC1864 can therefore run at true micropower levels in applications that do
not require the maximum LTC1864 sampling rate. At
1ksps, the supply current is typically only 2µA as shown
in Figure 2. The part’s low power consumption combined with the ability to go into sleep mode after a
09/02/294
1000
100
SUPPLY CURRENT (µA)
Introduction
The demand for ever smaller portable devices is increasing. In addition, there is a corresponding demand for
longer battery life in these devices that quite often have
smaller batteries than their predecessors. Optimized for
battery-operated, portable, isolated and remote data
acquisition systems, the LTC®1864, a 16-bit, 250ksps
micropower ADC is remarkably fast and offers µpower
operation and small size.
10
1
0.1
0.01
0.01
0.1
10
100
1
SAMPLING FREQUENCY (kHz)
1000
DN294 F02
Figure 2. Micropower Performance
conversion, make the LTC1864 ideal for battery-operated applications.
Simple Serial I/O Eases Isolated
and Remote Applications
The simple 3-wire serial I/O used by the LTC1864 is
compatible with industry standard SPI/MICROWIRETM
interfaces. The LTC1864 has an internal conversion clock
so that the shift clock (SCK) rate does not effect the
conversion. This allows the shift clock rate to run from DC
to 20MHz without concern over sample-and-hold droop
at low clock frequencies or clocking the ADC too fast at
high clock frequencies. The data transfer requires only 16
clock cycles as shown in the timing diagram of Figure 3.
Running the shift clock (SCK) at the maximum rate of
20MHz, an entire conversion can be transferred in only
800ns. Using only three wires in the serial interface makes
the LTC1864 easy to use for isolated or remote applications as shown in Figure 4.
Real Convert Start Input Allows
More Precise Control
The rising edge of CONV immediately causes the LTC1864
sample-and-hold to acquire the input voltage and starts a
conversion. This compares favorably to some ADCs that
require several shift clocks after the convert signal before
, LTC and LT are registered trademarks of Linear Technology Corporation.
MICROWIRE is a trademark of National Semiconductor Corporation.
www.BDTIC.com/Linear
CONV
t SMPL
SLEEP MODE
tCONV
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16
SCK
B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0*
SDO
Hi-Z
Hi-Z
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER SCK CLOCKS ARE
APPLIED WITH CONV LOW, THE ADC WILL OUTPUT ZEROS INDEFINITELY
DN294 F03
Figure 3. Data Transfer Only Requires 16 Clock Cycles
5V
IN
10k
OUT
5V
10k
9V
LT1021-5
150Ω
POWER
DOWN
4N28
10k
1Ω
Q2
Q1
GND
5V
10µF
VREF
VCC
5.1k
ANALOG
INPUT
0V TO 5V
5.1k
LTC1864
150Ω
SCK
Q3
4N28
SCK
IN+
10k
51k
SDO
5V
CONV
IN –
10k
51k
GND
150Ω
CONV
Q4
4N28
Q1-Q4: 2N3906
Q5: 2N3904
51k
300Ω
4N28
5V
10k
Q5
SDO
51k
5.1k
DN294 F04
Figure 4. Micropower 500V Optoisolated 16-Bit Data Acquisition System
acquiring the input voltage. For fast moving signals, the
LTC1864 can more precisely acquire the desired input
voltage.
True Differential Input Rejects
Common Mode Noise
The LTC1864 has a true differential input with sampleand-holds on both IN+ and IN– inputs. The LTC1864
samples both the IN+ and IN– inputs simultaneously; so
common mode noise on the inputs is rejected. The IN+
range of the LTC1864 is ground to VCC. Most competing
parts allow the minus input to go a few hundred millivolts
Data Sheet Download
http://www.linear.com/go/dnLTC1864
Linear Technology Corporation
above ground. The IN– range of the LTC1864 is ground to
VCC/2 which makes the LTC1864 a good choice for remote
applications where large common mode voltages or common mode noise can be present.
Conclusion
The LTC1864 16-bit 250ksps micropower ADC is a good
choice for battery-operated, portable, isolated and remote
ADC applications. The features that make this true include
its small size, micropower operation, simple serial I/O,
real convert start input and true differential inputs.
For literature on our A/D Converters,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2360
dn294f LT/TP 0902 316.5K • PRINTED IN THE USA
www.BDTIC.com/Linear
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2002