Datasheet

LTC2489
19
2489fa
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 LTC2489 is designed
to reject line frequencies. As shown in Figure 15, rejection
nulls occur at multiples of frequency f
N
, where f
N
= 55Hz
for simultaneous 50Hz/60Hz rejection. Multiples of the
modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 16); if noise sources are present at
these frequencies antialiasing will reduce their effects.
The user can expect to achieve this level of performance us-
ing the internal oscillator, as shown in Figure 17. Measured
values of normal mode rejection are shown superimposed
over the theoretical values.
Traditional high order delta-sigma modulators suffer from
potential instabilities at large input signal levels. The
proprietary architecture used for the LTC2489 third order
modulator resolves this problem and guarantees stability
with input signals 150% of full scale. In many industrial
applications, it is not uncommon to have microvolt level
signals superimposed over unwanted error sources with
several volts if peak-to-peak noise. Figure 18 shows mea-
surement results for the rejection of a 7.5V peak-to-peak
noise source (150% of full scale) applied to the LTC2489.
This curve shows that the rejection performance is main-
tained even in extremely noisy environments.
APPLICATIONS INFORMATION
Figure 16. Input Normal Mode Rejection at f
S
= 256 • f
N
Figure 15. Input Normal Mode Rejection at DC
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2489 F15
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)
2489 F16
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
= f
EOSC/5120
INPUT FREQUENCY (Hz)
0
12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
NORMAL MODE REJECTION (dB)
2489 F17
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
V
IN(P-P)
= 5V
T
A
= 25°C
MEASURED DATA
CALCULATED DATA
INPUT FREQUENCY (Hz)
0
15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240
NORMAL MODE REJECTION (dB)
2489 F18
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
T
A
= 25°C
V
IN(P-P)
= 5V
V
IN(P-P)
= 7.5V
(150% OF FULL SCALE)
Figure 17. Input Normal Mode Rejection vs Input Frequency with
Input Perturbation of 100% (50Hz/60Hz Notch)
Figure 18. Measure Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 150% (60Hz Notch)