LTC2431 Linear Technology, LTC2431 Datasheet - Page 27

• Manufacturer Part Number: LTC2431
• Description: (LTC2430 / LTC2431) 20-Bit No Latency Delta-Sigma ADCs
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Output Data Rate
When using the internal oscillator, the LTC2430/LTC2431
can produce up to 7.5 readings per second with a notch
frequency of 60Hz (F
second with a notch frequency of 50Hz (F
actual output data rate will depend upon the length of the
sleep and data output phases which are controlled by the
user and which can be made insignificantly short. When
operated with an external conversion clock (F
to an external oscillator), the LTC2430/LTC2431 output
data rate can be increased as desired. The duration of the
conversion phase is 20510/f
converter behaves as if the internal oscillator is used and
the notch is set at 60Hz. There is no significant difference
in the LTC2430/LTC2431 performance between these two
operation modes.
An increase in f
translate into a proportional increase in the maximum
output data rate. This substantial advantage is nevertheless
accompanied by three potential effects, which must be
carefully considered.
First, a change in f
in the internal notch position and in a reduction of the
converter differential mode rejection at the power line
frequency. In many applications, the subsequent perfor-
mance degradation can be substantially reduced by rely-
ing upon the LTC2430/LTC2431’s exceptional common
mode rejection and by carefully eliminating common
mode to differential mode conversion sources in the input
circuit. The user should avoid single-ended input filters
and should maintain a very high degree of matching and
symmetry in the circuits driving the IN
Second, the increase in clock frequency will increase
proportionally the amount of sampling charge transferred
through the input and the reference pins. If large external
input and/or reference capacitors (C
previous section provides formulae for evaluating the
effect of the source resistance upon the converter perfor-
mance for any value of f
or reference capacitors (C
the external source resistance upon the LTC2430/LTC2431
typical performance can be inferred from Figures 13, 14
EOSC
EOSC
U
O
over the nominal 153600Hz will
will result in a proportional change
EOSC
= LOW) and 6.25 readings per
U
IN
EOSC
, C
. If small external input and/
REF
. If f
) are used, the effect of
W
IN
EOSC
, C
+
REF
and IN
= 153600Hz, the
O
) are used, the
= HIGH). The
O
U
connected
–
pins.
and 17 in which the horizontal axis is scaled by
153600/f
Third, an increase in the frequency of the external oscilla-
tor above 1.6MHz (a more than 10 increase in the output
data rate) will start to decrease the effectiveness of the
internal autocalibration circuits. This will result in a progres-
sive degradation in the converter accuracy and linearity.
Typical measured performance curves for output data rates
up to 100 readings per second are shown in Figures 20 to
27. In order to obtain the highest possible level of accuracy
from this converter at output data rates above 50 readings
per second, the user is advised to maximize the power
supply voltage used and to limit the maximum ambient
operating temperature. The accuracy is also sensitive to the
clock signal levels and edge rate as discussed in the sec-
tion Digital Signal Levels. In certain circumstances, a re-
duction of the differential reference voltage may be
beneficial.
Input Bandwidth
The combined effect of the internal sinc
of the analog and digital autocalibration circuits deter-
mines the LTC2430/LTC2431 input bandwidth. When the
internal oscillator is used, the 3dB input bandwidth of the
LTC2430/LTC2431 is 3.63Hz for 60Hz notch frequency
(F
(F
frequency f
bandwidth is 2.36 • 10
Due to the complex filtering and calibration algorithms
utilized, the converter input bandwidth is not modeled very
accurately by a first order filter with the pole located at the
3dB frequency. When the internal oscillator is used, the
shape of the LTC2430/LTC2431 input bandwidth is shown
in Figure 28. When an external oscillator of frequency
f
bandwidth can be derived from Figure 28, F
of the LTC2411 in which the horizontal axis is scaled by
f
The conversion noise (2.8 V
be modeled as a white noise source connected to a noise
free converter. The noise spectral density is 67nV/ Hz for
EOSC
EOSC
O
O
= HIGH). If an external conversion clock generator of
= LOW) and 3.02Hz for 50Hz notch frequency
/153600.
is used, the shape of the LTC2430/LTC2431 input
EOSC
EOSC
.
is connected to the F
LTC2430/LTC2431
–5
• f
EOSC
RMS
typical for V
.
O
pin, the 3dB input
4
digital filter and
O
REF
= LOW curve
= 5V) can
27
24301f