Datasheet

LTC2495
29
2495fd
Figure 26. Measure Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 150% (50Hz Notch)
Figure 25. Measure Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 150% (60Hz Notch)
applications inForMation
Figure 24. Input Normal Mode Rejection vs Input Frequency with
Input Perturbation of 100% (50Hz/60Hz Notch)
When using the internal oscillator, the LTC2495 is designed
to reject line frequencies. As shown in Figure 20, 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 21); if noise sources are present at
these frequencies anti-aliasing will reduce their effects.
The user can expect to achieve this level of performance
using the internal oscillator, as shown in Figures 22, 23,
and 24. Measured values of normal mode rejection are
shown superimposed over the theoretical values in all
three rejection modes.
Traditional high order delta-sigma modulators suffer from
potential instabilities at large input signal levels. The
proprietary architecture used for the LTC2495 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. Figures 25 and 26 show
measurement results for the rejection of a 7.5V peak-
to-peak noise source (150% of full scale) applied to the
LTC2495. These curves show that the rejection performance
is maintained even in extremely noisy environments.
INPUT FREQUENCY (Hz)
0 20 40 60 80 100 120 140 160 180 200 220
NORMAL MODE REJECTION (dB)
2495 F25
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)
2495 F26
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)
INPUT FREQUENCY (Hz)
0
NORMAL MODE REJECTION (dB)
2495 F27
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)
12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200