LTC2424 Linear Technology, LTC2424 Datasheet - Page 22

• Manufacturer Part Number: LTC2424
• Description: 4-/8-Channel 20-Bit uPower No Latency ADCs
• Manufacturer: Linear Technology

APPLICATIONS
LTC2424/LTC2428
full-scale shift, see Figure 19. To simplify the analysis of
input dynamic current, two separate cases are assumed:
large capacitance at V
tance at V
If the total capacitance at V
(< 0.01 F), relatively large external source resistances (up
to 20k for 20pF parasitic capacitance) can be tolerated
without any offset/full-scale error.
22
TUE
SOURCE
SIGNAL
INTPUT
0
IN
Figure 20. An RC Network at CH0 to CH7
(C
Figure 19. Offset/Full-Scale Shift
IN
SELECTED
CHANNEL
f
f
EXTERNAL OSCILLATOR: 2.56kHz
OUT
OUT
< 0.01 F).
R
CHX
SOURCE
U
= 50Hz, INTERNAL OSCILLATOR: f = 128kHz
= 60Hz, INTERNAL OSCILLATOR: f = 153.6kHz
IN
I
I
IN(MUX)
IN(MUX)
(C
INFORMATION
U
IN
C
V
IN
REF
V
MUXV
> 0.01 F) and small capaci-
IN
(PIN 8)
IN
/2
(see Figure 20) is small
Figure 18. LTC2424/LTC2428 Equivalent Analog Input Circuit
CC
R
75
C
SW
PAR
20pF
W
f 307.2kHz
CH0 TO
CH7
LTC2424/
LTC2428
MUXOUT
24248 F20
U
24248 F19
V
REF
ADCIN
ZS
I
SET
DC
FS
SET
I
I
IN(LEAK)
IN(LEAK)
I
I
REF
REF
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 LTC2424/LTC2428 signifi-
cantly simplify antialiasing filter requirements.
The digital filter provides very high rejection except at
integer multiples of the modulator sampling frequency
(f
256 • F
or 60Hz). The bandwidth of signals not rejected by the
digital filter is narrow ( 0.2%) compared to the bandwidth
of the frequencies rejected.
S
ADCV
ADCV
), see Figure 21. The modulator sampling frequency is
(PIN 2)
(PIN 2)
CC
CC
O
R
R
R
, where F
5k
5k
5k
SW
SW
SW
–140
–100
–120
–20
–60
–40
–80
0
0
Figure 21. Sync
O
24248 F18
I
DC
is the notch frequency (typically 50Hz
= 0.25(V
AVERAGE INPUT CURRENT:
INPUT FREQUENCY
IN
f
S
4
– 0.5 • V
/2
Filter Rejection
C
1pF (TYP)
EQ
REF
) • f • C
24248 F21
EQ
f
S