ltc2493 Linear Technology Corporation, ltc2493 Datasheet - Page 26

• Manufacturer Part Number: ltc2493
• Description: 24-bit 2-/4-channel Delta Sigma Adc With Easy Drive Input Current Cancellation And I2c Interface
• Manufacturer: Linear Technology Corporation

LTC2493
applicaTions inForMaTion
When using the internal oscillator, the LTC2493 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
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
shown superimposed over the theoretical values in all
three rejection modes.

S
= f
N
• 256) only reject noise
Figure 20. Input Normal Mode Rejection at f
Figure 19. Input Normal Mode Rejection at DC
–100
–110
–120
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
–10
–20
–30
–40
–50
–60
–70
–80
–90
N
250f
0
0
, where f
0
N
N
f
252f
N
= 50Hz
INPUT SIGNAL FREQUENCY (Hz)
INPUT SIGNAL FREQUENCY (Hz)
N
2f
N
N
254f
is
3f
N
N
256f
4f
Traditional high order delta-sigma modulators suffer
from potential instabilities at large input signal levels.
The proprietary architecture used for the LTC2493 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 if peak-to-peak noise. Figures
24 and 25 show measurement results for the rejection
of a 7.5V peak-to-peak noise source (150% of full scale)
applied to the LTC2493. These curves show that the rejec-
tion performance is maintained even in extremely noisy
environments.
N
N
5f
258f
N
f
f
N
N
N
6f
= f
= f
N
EOSC/5120
EOSC/5120
260f
7f
N
N
2493 F19
2493 F20
S
262f
8f
= 256 • f
N
N
N
2493fa