Datasheet

LTC2496
27
2496fb
applicaTions inForMaTion
The user can expect to achieve this level of performance us-
ing the internal oscillator, as shown in Figure 19. Measured
values of normal mode rejection are shown superimposed
over the theoretical rejection.
Traditional high order delta-sigma modulators suffer
from potential instabilities at large input signal levels.
The proprietary architecture used for the LTC2496 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 mi-
crovolt level signals superimposed over unwanted error
sources with several volts of peak-to-peak noise. Figure
20 shows measurement results for the rejection of a 7.5V
peak-to-peak noise source (150% of full scale) applied
to the LTC2496. From these curves, it is shown 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)
2496 F19
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 SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2496 F17
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
Figure 17. Input Normal Mode Rejection at DC
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2496 F18
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
Figure 18. Input Normal Mode Rejection at f
S
= 256 • f
N
Figure 19. Input Normal Mode Rejection vs Input Frequency with
Input Perturbation of 100% (50Hz/60Hz Notch)
Output Data Rate
When using its internal oscillator, the LTC2496 produces up
to 6.9 samples per second (sps) with a notch frequency of
55Hz. The actual output data rate depends upon the length
of the sleep and data output cycles which are controlled
by the user and can be made insignificantly short. When
operating with an external conversion clock (f
O
connected
to an external oscillator), the LTC2496 output data rate
can be increased. The duration of the conversion cycle
is 41036/f
EOSC
. If f
EOSC
= 307.2kHz, the converter notch
frequency is 60Hz.
An increase in f
EOSC
over the nominal 307.2kHz will trans-
late into a proportional increase in the maximum output
data rate (up to a maximum of 100sps). The increase in
output rate leads to degradation in offset, full-scale error,
and effective resolution as well as a shift in frequency
rejection.
INPUT FREQUENCY (Hz)
0
15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240
NORMAL MODE REJECTION (dB)
2496 F20
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 20. Input Normal Mode Rejection vs Input Frequency with
Input Perturbation of 150% (60Hz Notch)