Datasheet

LTC2487
25
2487fd
Figure 16. INL vs Differential Input
Voltage and Reference Source
Resistance for C
REF
> 1μF
V
IN
/V
REF
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2487 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 18. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Rejection Mode
Figure 17. Input Normal Mode Rejection, Internal
Oscillator and 50Hz Rejection Mode
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital fi ltering. Combined
with a large oversample ratio, the LTC2487 signifi cantly
simplifi es antialiasing fi lter requirements. Additionally,
the input current cancellation feature allows external low
pass fi ltering without degrading the DC performance of
the device.
The SINC
4
digital fi lter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
) (see Figures
17 and 18). 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 LTC2487 is designed
to reject line frequencies. As shown in Figure 19, rejec-
tion nulls occur at multiples of frequency f
N
, where f
N
is
determined by the input control bits FA and FB (f
N
= 50Hz
or 60Hz or 55Hz for simultaneous rejection). Multiples of
APPLICATIONS INFORMATION
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
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)
2487 F17
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0f
S
INPUT NORMAL MODE REJECTION (dB)
2487 F18
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
the modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 20); if noise sources are present at
these frequencies antialiasing will reduce their effects.
The user can expect to achieve this level of performance
using the internal oscillator, as shown in Figures 21, 22,
and 23. Measured values of normal mode rejection are