LTC2435-1IGN Linear Technology, LTC2435-1IGN Datasheet - Page 32

• Manufacturer Part Number: LTC2435-1IGN
• Description: IC CONV A/D 20BIT DIFF 16-SSOP
• Manufacturer: Linear Technology

Specifications of LTC2435-1IGN

• Number Of Bits: 20
• Sampling Rate (per Second): 13.75
• Data Interface: MICROWIRE™, Serial, SPI™
• Number Of Converters: 2
• Power Dissipation (max): 1mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP (0.150", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC2435/LTC2435-1
32
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over con-
ventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2435/LTC2435-1 sig-
nificantly simplifies antialiasing filter requirements.
The sinc
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (f
pendent of the operating mode, f
f
maximum output data rate. In the internal oscillator mode,
for the LTC2435, F
and f
LTC2435-1, f
oscillator mode, f
The normal mode rejection performance is shown in
Figure 34. The regions of low rejection occurring at
integer multiples of f
Magnified details of the normal mode rejection curves are
shown in Figure 35 (rejection near DC) and Figure 36
(rejection at f
frequency. For the LTC2435, the bandwidth is 13.6Hz
(F
12.4Hz for the LTC2435-1 (F
OUTMAX
O
= GND) and 11.4Hz (F
S
= 15360Hz with a 60Hz notch setting. For the
4
where f
digital filter provides greater than 120dB normal
Figure 34a. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch (LTC2435)
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
S
S
0
N
= 13980Hz (F
0 f
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
= 256f
is the notch frequency and f
S
S
S
U
F
= f
O
2f
= 12800Hz with a 50Hz notch setting
= HIGH
S
EOSC
S
3f
N
S
) where f
have a very narrow bandwidth.
4f
U
S
/10.
5f
O
S
O
O
6f
= V
= GND).
S
= LOW). In the external
7f
N
S
CC
S
8f
W
represents the notch
). The Bandwidth is
S
= 256 • f
9f
S
10f
S
11f
2435 F34a
OUTMAX
S
12f
N
S
U
= 1024 •
S
). Inde-
is the
Through F
110dB input differential mode rejection at 50Hz or 60Hz
±2%. While for the LTC2435-1, it has a notch frequency of
about 55Hz with better than 70db rejection over 48Hz to
62.4Hz, which covers both 50Hz ±2% and 60Hz ±2%. In
order to achieve better rejection over the range of 48Hz to
62.4Hz, a running average can be performed. By averaging
two consecutive LTC2435-1 readings, a sinc
combined with the sinc
quency response shown in Figure 37. The averaging
operation still keeps the output rate with the following
algorithm:
The user can expect to achieve in practice this level of
performance using the internal oscillator as it is demon-
strated by Figures 38 to 40. Typical measured values of the
normal mode rejection of the LTC2435-1 operating with an
internal oscillator and a 54.6Hz notch setting are shown in
Figure 38 and 39 superimposed over the theoretical calcu-
lated curve. The same normal mode rejection perfor-
mance is obtained for the LTC2435 with the frequency
scaled to have the notch frequency at 60Hz (F
50Hz (F
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
Result n = average (sample n-1, sample n)
Figure 34b. Input Normal Mode Rejection, Internal
Oscillator and FO = Low or External Oscillator
O
= V
O
–140
–100
–120
connection, the LTC2435 provides better than
–20
–60
–40
–80
CC
0
0
).
INPUT FREQUENCY
4
digital filter, yielding the fre-
f
S
/2
2435 F34b
f
S
O
1
= GND) or
notch is
24351fb