Datasheet

LTC2494
32
2494fd
Figure 19. Input Normal Mode Rejection, Internal
Oscillator and 50Hz Rejection Mode
Figure 20. Input Normal Mode Rejection, Internal
Oscillator and 60Hz Rejection Mode
applications inForMation
In addition to the reference sampling charge, the reference
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 LTC2494 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
) (see Figures
19 and 20). 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 LTC2494 is designed
to reject line frequencies. As shown in Figure 21, 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
the modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 22), if noise sources are present at
these frequencies anti-aliasing will reduce their effects.
Figure 21. Input Normal Mode Rejection at DC
Figure 22. Input Normal Mode Rejection at f
S
= 256 • f
N
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)
2494 F19
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)
2494 F20
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
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2494 F21
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC/5120
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2494 F22
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120