LTC2446 LINER [Linear Technology], LTC2446 Datasheet - Page 22

• Manufacturer Part Number: LTC2446
• Description: 24-Bit High Speed 8-Channel ?? ADCs with Selectable Multiple Reference Inputs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2446/LTC2447
offset and full-scale performance remain unchanged as
does the first multiple of f
The sample rate f
driving the F
rate is f
clock applied to F
sample rate leads to notch frequencies f
maintaining simple antialiasing requirements. A 100kHz
clock applied to F
harmonics up to 20kHz, see Figure 13. This is useful in
applications requiring digitalization of the DC component
of a noisy input signal and eliminates the need of placing
a 0.6Hz filter in front of the ADC.
An external oscillator operating from 100kHz to 20MHz
can be implemented using the LTC1799 (resistor set
SOT-23 oscillator), see Figure 14. By floating pin 4 (DIV)
of the LTC1799, the output oscillator frequency is:
The normal mode rejection characteristic shown in
Figure 13 is achieved by applying the output of the LTC1799
(with R
with SDI tied HIGH (OSR = 32768).
Multiple Ratiometric and Absolute Measurements
The LTC2446/LTC2447 combine a high precision, high
speed delta-sigma converter with a versatile front-end
22
f
OSC
SET
S
=
Figure 13. LTC2446/LTC2447 Normal
Mode Rejection (External Oscillator at 90kHz)
–100
–120
–140
= f
–20
–40
–60
–80
10
= 100k) to the F
0
O
EOSC
MHz
0
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
pin with an external oscillator. The sample
O
/5, where f
S
. Combining a large OSR with a reduced
•
O
2
and NULL f
U
⎛
⎜
⎝
results in a NULL at 0.6Hz plus all
10
10
•
4
R
U
O
S
k
SET
.
pin on the LTC2446/LTC2447
EOSC
N
⎞
⎟
⎠
6
, may also be adjusted by
is the frequency of the
W
8
24467 F13
N
10
near DC while
U
multiplexer. The unique no latency architecture allows
seamless changes in both input channel and reference
while the absolute accuracy ensures excellent matching
between both analog input channels and reference chan-
nels. Any set of inputs (differential or single-ended) can
perform a conversion with one of two references. For
Bridges, RTDs and other ratiometric devices, each set of
channels can perform a conversion with respect to a
unique reference voltage. For Thermocouples, voltage
sense, current sense and other absolute sensors, each set
of channels can perform a conversion with respect to a
single global reference voltage (see Figure 15). This allows
users to measure both multiple absolute and multiple ratio
metric sensors with the same device in such applications
as flow, gas chromatography, multiple RTDs or bridges,
or universal data acquisition.
Average Input Current
The LTC2446 switches the input and reference to a 2pF
capacitor at a frequency of 1.8MHz. A simplified equivalent
circuit is shown in Figure 16. The sample capacitor for the
LTC2447 is 4pF, and its average input current is externally
buffered from the input source.
The average input and reference currents can be ex-
pressed in terms of the equivalent input resistance of the
sample capacitor, where: Req = 1/(f
USER SELECTABLE
REFERENCES
0.1V TO V
ANALOG
INPUTS
1µF
4.5V TO 5.5V
CC
Figure 14. Simple External Clock Source
28
29
30
11
10
24
23
12
22
8
9
7
V
REFG
REFG
REF01
REF01
REF67
REF67
CH0
CH1
CH2
CH7
COM
CC
. .
.
. . .
LTC2446
+
–
+
–
+
–
BUSY
SDO
GND
SCK
SDI
CS
F
O
37
2
1,4,5,6,31,32,33
34
38
35
36
NC
24467 F14
SW
4-WIRE
SPI INTERFACE
OUT
DIV
LTC1799
• Ceq).
GND
SET
V
+
0.1µF
24467fa
R
SET