DN463 - Upgrade Your Microcontroller ADC to True 12-Bit Performance

Upgrade Your Microcontroller ADC to True 12-Bit Performance
Design Note DN463
by Guy Hoover
Introduction
Many 8-bit and 16-bit microcontrollers feature 10-bit
internal ADCs. A few include 12-bit ADCs, but these often
have poor or nonexistent AC specifications, and certainly
lack the performance to meet the needs of an increasing
number of applications. The LTC ®2366 and its slower
speed versions offer a high performance alternative, as
shown in the AC specifications in Table 1. Compare these
guaranteed specifications with the ADC built into your
current microcontroller.
This family’s DC specifications are equally impressive. INL
and DNL are guaranteed to be less than ±1LSB. Operating
from a single 2.5V, 3V or 3.3V supply, the current draw
on these parts is a maximum of 4mA during a conversion. This can be reduced to less than 1μA by placing
the part into SLEEP mode during periods of inactivity,
which greatly reduces the average supply current at lower
sample rates.
C3
0.1μF
These ADCs are available in tiny 6-lead and 8-lead TSOT23 packages. The 8-lead devices have adjustable VREF
and OVDD pins. The adjustable VREF pin allows the input
span to be reduced to 1.4V. This, combined with the high
ADC input impedance, can eliminate the need for gain or
buffer stages in many applications. The OVDD pin, which
controls the digital output level, can be adjusted from 1V
to 3.6V, simplifying communication with different logic
families. For applications that do not require an adjustable
reference or adjustable output levels, the 6-lead device
with VREF = OVDD = VDD should suffice.
The SPI interface requires only three wires to communicate
with the microcontroller, keeping the overall solution size
small in low power, high speed applications.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
0
3.3V
8192 Pt FFT
fS = 2.84MHz
VIN = –1dB AT 20kHz
SNR = 72.1dB
THD = –70.1dB
R2
1k
R4
499Ω
AGND
AIN
R1
1k
C4
10μF
AGND
+
AGND
LTC2366
R5
24.9Ω
–
AGND
AV =–4
CS
AIN
LT6202
C2
1μF
1
2
5
VDD VREF OVDD
R3
2.0k
SCK
SDO
GND
C1
0.0047μF
3
AGND
dn463 F01
AGND
Figure 1. Single Supply AC-Coupled Amplifier Level Shifts Input for
Maximum Dynamic Range
8
7
6
FOURIER TRANSFORM
–20
C5
0.1μF
–40
–60
–80
–100
–120
–140
0
0.2
0.4 0.6 0.8 1.0
FREQUENCY (MHz)
1.2
1.4
dn463 F02
Figure 2. FFT Shows Low Noise and
Distortion of Figure 1 Circuit
04/09/463
Application Circuits
Figure 1 shows a single supply AC-coupled amplifier
driving the LTC2366. This circuit is useful in applications where the sensor output level is too low to achieve
full SNR performance from the ADC. The output of the
LT®6202 swings rail-to-rail. This feature maximizes the
circuit’s dynamic range when the op amp output is level
shifted to the center of the ADC’s swing. The FFT of
Figure 2 demonstrates the low noise and distortion of
this circuit.
the LTC6915 is a good choice for high gain applications.
This circuit is useful for very low level signals or for applications with a wide range of input levels.
Conclusion
The 12-bit ADCs in the LTC236x family guarantee AC
specifications that most built-in microcontroller ADCs
cannot meet, thus improving performance when used
in place of on-chip ADCs. The LTC236x family is easily
interfaced to most microcontrollers via its SPI interface.
A wide range of sample rates, an external reference pin
and a separate OVDD pin provide additional flexibility.
In Figure 3, a single supply DC amplifier with a programmable gain of 0 to 4096 drives the LTC2360. With a maximum offset of 10μV and a DC to 10Hz noise of 2.5μVP–P ,
Table 1. LTC236x ADC Family AC Specifications
FULL LINEAR
BANDWIDTH
PART NUMBER
SAMPLE RATE
SINAD
SNR
THD
LTC2366
3Msps
68dB (Min)
69dB (Min)
–72dB (Max)
2.5MHz (Typ)
LTC2365
1Msps
68dB (Min)
70dB (Min)
–72dB (Max)
2.0MHz (Typ)
LTC2362
500ksps
72dB (Typ)
73dB (Typ)
–85dB (Typ)
1.0MHz (Typ)
LTC2361
250ksps
72dB (Typ)
73dB (Typ)
–85dB (Typ)
1.0MHz (Typ)
LTC2360
100ksps
72dB (Typ)
73dB (Typ)
–85dB (Typ)
1.0MHz (Typ)
D3 D2 D1 D0 GAIN
0 0 0 0
0
0 0 0 1
1
0 0 1 0
2
0 0 1 1
4
0 1 0 0
8
0 1 0 1
16
0 1 1 0
32
0 1 1 1
64
1 0 0 0 128
1 0 0 1 256
1 0 1 0 512
1 0 1 1 1024
1 1 0 0 2048
1 1 0 1 4096
1 1 1 0 4096
1 1 1 1 4096
3.3V
C10
0.1μF
AIN
P/SL
+
D0
D1
D2
D3
+
LTC6915 V –
V
–
R10 AGND
24.9Ω
REF
C9
10μF
AGND
4
C6
300pF
DGND
1
2
VDD VREF
AIN
LTC2360
5
OVDD
CSL
SCK
SDO
AGND
7
6
GND
3
AGND
8
dn463 F03
AGND
AGND
Figure 3. Single Supply DC Amplifier Provides Programmable Gain from 0 to 4096
Data Sheet Download
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For applications help,
call (408) 432-1900, Ext. 2602
Linear Technology Corporation
dn463 LT/TP 0409 155K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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