28
LTC2424/LTC2428
APPLICATIONS INFORMATION
WUU
U
Figure 29. Sync
4
Filter Rejection
INPUT FREQUENCY
0
–60
–40
0
f
S
= 15,360Hz
24248 F29
–80
–100
f
S
/2 f
S
–120
–140
–20
REJECTION (dB)
In addition to the dynamic reference current, the V
REF
ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA max),
results in a fixed full-scale shift of 10µV 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
S
), see Figure 29. The modulator sampling frequency is
256 • F
O
, where F
O
is the notch frequency (typically 50Hz
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
REF
do not saturate the
analog modulator. These signals are limited by the input
ESD protection to 300mV below ground and 300mV above
V
CC
.
Figure 27. INL Error vs R
VREF
(Small C)
Figure 28. INL Error vs R
VREF
(Large C)
RESISTANCE AT V
REF
(Ω)
1
30
40
50
1k 10k
24248 F27
20
10
10 100 100k
0
–10
–20
INL ERROR (ppm)
V
CC
= 5V
V
REF
= 5V
T
A
= 25°C
C
VREF
= 0.01µF
C
VREF
= 1000pF
C
VREF
= 0pF
C
VREF
= 100pF
RESISTANCE AT V
REF
(Ω)
0
6
8
10
600 800
24248 F28
4
2
–2
–4
–6
–8
–10
200 400 1000
0
INL ERROR (ppm)
C
VREF
= 22µF
C
VREF
= 10µF
C
VREF
= 1µF
C
VREF
= 0.1µF
C
VREF
= 0.01µF
C
VREF
= 0.001µF
V
CC
= 5V
V
REF
= 5V
T
A
= 25°C
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 45µA 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
