LTC2421 LINER [Linear Technology], LTC2421 Datasheet - Page 25

• Manufacturer Part Number: LTC2421
• Description: 1-/2-Channel 20-Bit UPower No Latency ADCs in MSOP-10
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
0.015ppm/
Figure 26.
In addition to the dynamic reference current, the V
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA ( 10nA max),
results in a fixed full-scale shift of 10 V for a 10k source
resistance.
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 LTC2421/LTC2422 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 band-
width of the frequencies rejected.
As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2421/LTC2422. If passive RC components are
placed in front of the LTC2421/LTC2422, the input dy-
namic current should be considered (see Input Current
section). In cases where large effective RC time constants
are used, an external buffer amplifier may be required to
minimize the effects of input dynamic current.
S
), see Figure 27. The modulator sampling frequency is
O
, where F
–10
10
–2
–4
–6
–8
8
6
4
2
0
Figure 26. INL Error vs R
0
independent of capacitance at V
O
200
C
C
C
C
C
C
VREF
VREF
VREF
VREF
VREF
VREF
is the notch frequency (typically 50Hz
U
RESISTANCE AT V
= 22 F
= 10 F
= 1 F
= 0.1 F
= 0.01 F
= 0.001 F
400
U
600
REF
VREF
( )
W
V
V
T
800
A
CC
REF
(Large C)
= 25 C
= 5V
24212 F26
= 5V
1000
U
REF
REF
, see
ESD
The modulator contained within the LTC2421/LTC2422
can handle large-signal level perturbations without satu-
rating. Signal levels up to 40% of V
analog modulator. These signals are limited by the input
ESD protection to 300mV below ground and 300mV above
V
Simple Basic Program for Interfacing to the
LTC2421/LTC2422
”TINY.BAS V1.0 Copyright (C) 2000 by J. A. Dutra and LTC, All rights reseved'
NOTE this program generates 32 SCK’s for compatibility to 24-bit parts
'For use with most LTC24xy demo boards
designed for the PC Com Port, QBASIC
'Outputs are chan%,signneg%,d2400 (magnitude), PPM, and v (volts)
CLS : ON ERROR GOTO 4970
cport = 1: REM INPUT "com port number "; cport
GOSUB 1900: timestart$ = TIME$
mcr% = port + 4: msr% = port + 6
COLOR 15: LOCATE 3, 1: PRINT "Hit any key to stop…
FOR np = 1 TO 2000: OUT port, c0%: NEXT np: 'Power Via TxD
DO: '-------------------------START LOOP here--------
CC
.
–100
–140
–120
–20
–40
–60
–80
0
0
V
Figure 27. Sinc
IN
V
GND
REF
LTC2421
LTC2422
INPUT FREQUENCY
LTC2421/LTC2422
Figure 28
f
4
SDO
S
SCK
/2
CS
Filter Rejection
DTR
CTS
RTS
REF
do not saturate the
24212 F27
SERIAL
PORT
PC
24212 F28
f
S
";
25
24212f