ltc2428 Linear Technology Corporation, ltc2428 Datasheet - Page 28

• Manufacturer Part Number: ltc2428
• Description: 4-/8-channel 20-bit ?power No Latency Delta-sigma Adc
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC2424/LTC2428
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 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
28
S
), see Figure 29. The modulator sampling frequency is
–10
–20
–10
10
–2
–4
–6
–8
50
40
30
20
10
Figure 27. INL Error vs R
Figure 28. INL Error vs R
8
6
4
2
0
0
1
0
V
V
T
A
CC
REF
= 25 C
= 5V
= 5V
200
10
C
C
C
C
C
C
VREF
VREF
VREF
VREF
VREF
VREF
U
C
RESISTANCE AT V
RESISTANCE AT V
VREF
C
C
VREF
VREF
= 22 F
= 10 F
= 1 F
= 0.1 F
= 0.01 F
= 0.001 F
400
100
= 0.01 F
INFORMATION
= 1000pF
= 100pF
U
600
1k
REF
REF
VREF
VREF
( )
( )
C
W
VREF
V
V
T
800
10k
A
CC
REF
(Small C)
(Large C)
= 25 C
= 0pF
24248 F28
24248 F27
= 5V
= 5V
1000
100k
U
REF
ESD
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.
As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2424/LTC2428. If passive RC components are
placed in front of the LTC2424/LTC2428, the input dy-
namic current should be considered. 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.
The modulator contained within the LTC2424/LTC2428
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
Using a Low Power Precision Reference
The circuit in Figure 30 shows the connections and by-
passing for an LT1461-2.5 as a 2.5V reference. The
LT1461 is a bandgap reference capable of 3ppm/ C tem-
perature stability yet consumes only 45 A of current. The
1k resistor between the reference and the ADC reduces the
transient load changes associated with sampling and
produces optimal results. This reference will not impact
the noise level of the LTC2424/LTC2428 if signals are less
CC
.
O
, where F
–100
–140
–120
–20
–60
–40
–80
0
0
Figure 29. Sync
f
S
= 15,360Hz
O
is the notch frequency (typically 50Hz
INPUT FREQUENCY
f
S
4
/2
Filter Rejection
REF
do not saturate the
24248 F29
f
S