Datasheet

LTC2495
27
2495fd
Figure 17. INL vs Differential Input
Voltage and Reference Source
Resistance for C
REF
> 1µF
Figure 19. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Rejection Mode
Figure 18. Input Normal Mode Rejection, Internal
Oscillator and 50Hz Rejection Mode
results (see Figure 17). 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 100W of reference resistance.
In addition to the reference sampling charge, the reference
ESD protection diodes have a temperature dependent leak-
age current. This leakage current, nominally 1nA (±10nA
max) results in a small gain error. A 100W 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 LTC2495 significantly
simplifies anti-aliasing filter requirements. Additionally,
the input current cancellation feature allows external
lowpass 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
) (see Figures
18 and 19). 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
.
applications inForMation
Figure 15. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 16. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2495 F15
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
2495 F16
–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
V
IN
/V
REF
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2495 F17
–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
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 f
S
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S
11f
S
12f
S
INPUT NORMAL MODE REJECTION (dB)
2495 F18
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 f
S
INPUT NORMAL MODE REJECTION (dB)
2495 F19
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S