LTC2487 Linear Technology, LTC2487 Datasheet - Page 25

• Manufacturer Part Number: LTC2487
• Description: 16-Bit 2-/4-Channel ADC
• Manufacturer: Linear Technology

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applicaTions inForMaTion
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversample ratio, the LTC2487 significantly
simplifies antialiasing filter requirements. Additionally,
the input current cancellation feature allows external low
pass filtering without degrading the DC performance of
the device.
The SINC
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
17 and 18). The modulator sampling frequency is f
15,360Hz while operating with its internal oscillator and
f
of frequency f
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
determined by the input control bits FA and FB (f
or 60Hz or 55Hz for simultaneous rejection). Multiples of
S
= f
EOSC
4
/20 when operating with an external oscillator
–10
10
–2
–4
–6
–8
digital filter provides excellent normal mode
8
6
4
2
0
Figure 16. INL vs Differential Input
Voltage and Reference Source
Resistance for C
– 0.5
EOSC
V
V
V
T
C
A
CC
REF
IN(CM)
REF
= 25°C
= 5V
= 5V
= 10µF
– 0.3
.
= 2.5V
– 0.1
V
REF
IN
/V
REF
> 1µF
0.1
R = 500Ω
R = 100Ω
R = 1k
0.3
2487 F16
S
) (see Figures
0.5
N
, where f
N
= 50Hz
N
S
is
=
the modulator sampling rate (f
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
Figure 17. Input Normal Mode Rejection, Internal
Oscillator and 50Hz Rejection Mode
Figure 18. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Rejection Mode
–100
–110
–120
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
0
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 f
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
f
S
S
2f
2f
S
S
3f
S
3f
4f
S
S
4f
5f
S
S
5f
6f
S
S
S
7f
6f
= f
S
S
8f
N
7f
S
S
• 256) only reject noise
9f
S
8f
10f
S
S
9f
11f
LTC2487
2487 F18
2487 F17
S
S
10f
12f
S
S

2487fa