Datasheet

LTC2496
26
2496fb
applicaTions inForMaTion
In cases where large bypass capacitors are required on
the reference inputs (C
REF
> 0.01µF), full-scale and linear-
ity errors are proportional to the value of the reference
resistance. Every ohm of reference resistance produces
a full-scale error of approximately 0.5ppm, see Figures
14 and 15. If the input common mode voltage is equal to
the reference common mode voltage, a linearity error of
approximately 0.67ppm per 100Ω of reference resistance
results, see Figure 16. In applications where the input and
reference common mode voltages are different, the errors
increase. A 1V difference in between common mode input
and common mode reference results in a 6.7ppm INL error
for every 100Ω of reference resistance.
In addition to the reference sampling charge, the reference
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2496 F14
200
100
0
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
REF
= 1µF, 10µF
C
REF
= 0.1µF
C
REF
= 0.01µF
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–200
–100
0
800
2496 F15
–300
–400
–500
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
REF
= 1µF, 10µF
C
REF
= 0.1µF
C
REF
= 0.01µF
Figure 14. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 15. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
V
IN
/V
REF
(V)
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2496 F16
–2
–6
0
4
8
–4
–8
–10
–0.3
–0.1
0.1
0.5
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
T
A
= 25°C
C
REF
= 10µF
R = 1k
R = 100Ω
R = 500Ω
Figure 16. INL vs Differential Input Voltage and Reference
Source Resistance for C
REF
> 1µF
ESD projection diodes have a temperature dependent leak-
age current. This leakage current, nominally 1nA (±10nA
max) results in a small gain error. A 100Ω reference
resistance will create a 0.5µV full scale error.
Normal Mode Rejection and Anti-aliasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversample ratio, the LTC2496 significantly
simplifies anti-aliasing filter requirements. Additionally,
the input current cancellation feature allows external low
pass filtering without degrading the DC performance of
the device.
The SINC
4
digital filter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
). The modulator
sampling frequency is f
S
= 15,360Hz while operating with
its internal oscillator and f
S
= f
EOSC
/20 when operating
with an external oscillator of frequency f
EOSC
.
When using the internal oscillator, the LTC2496 is designed
to reject line frequencies. As shown in Figure 17, rejection
nulls occur at multiples of frequency f
N
, where f
N
is 55Hz
for simultaneous 50Hz/60Hz rejection). Multiples of the
modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 18), if noise sources are present at
these frequencies anti-aliasing will reduce their effects.