Datasheet

LTC2493
28
2493fd
Using the 2× speed mode of the LTC2493 alters the rejec-
tion characteristics around DC and multiples of f
S
. The
device bypasses the offset calibration in order to increase
the output rate. The resulting rejection plots are shown
in Figures 26 and 27. 1× type frequency rejection can be
achieved using the 2× mode by performing a running aver-
age of the previous two conversion results (see Figure 28).
Output Data Rate
When using its internal oscillator, the LTC2493 produces
up to 15 samples per second (sps) with a notch frequency of
60Hz. 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 LTC2493 output data rate
can be increased. The duration of the conversion cycle is
41036/f
EOSC
. If f
EOSC
= 307.2kHz, the converter behaves
as if the internal oscillator is used.
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 rejec-
tion. When using the integrated temperature sensor, the
internal oscillator should be used or an external oscillator
f
EOSC
= 307.2kHz maximum.
A change in f
EOSC
results in a proportional change in the
internal notch position. This leads to reduced differential
mode rejection of line frequencies. The common mode
rejection of line frequencies remains unchanged, thus fully
differential input signals with a high degree of symmetry
on both the IN
+
and IN
–
pins will continue to reject line
frequency noise.
An increase in f
EOSC
also increases the effective dynamic
input and reference current. External RC networks will
continue to have zero differential input current, but the
time required for complete settling (580ns for f
EOSC
=
307.2kHz) is reduced, proportionally.
Once the external oscillator frequency is increased
above 1MHz (a more than 3× increase in output rate)
the effectiveness of internal auto-calibration circuits
begins to degrade. This results in larger offset errors,
full-scale errors, and decreased resolution, as seen in
Figures 29-36.
Figure 26. Input Normal Mode Rejection 2× Speed Mode Figure 27. Input Normal Mode Rejection 2× Speed Mode
applicaTions inForMaTion
INPUT SIGNAL FREQUENCY (f
N
)
INPUT NORMAL REJECTION (dB)
2493 F26
0
–20
–40
–60
–80
–100
–120
0
f
N
2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
INPUT SIGNAL FREQUENCY (f
N
)
INPUT NORMAL REJECTION (dB)
2493 F27
0
–20
–40
–60
–80
–100
–120
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