參數(shù)資料
型號(hào)： LTC2480
廠商： Linear Technology Corporation
英文描述： 16-Bit ADC with Easy Drive
中文描述： 16位ADC的易道
文件頁(yè)數(shù)： 26/40頁(yè)
文件大?。?/td> 535K
代理商： LTC2480
26
LTC2480
2480f
During the conversion period, the undershoot and/or
overshoot of a fast digital signal connected to the pins can
severely disturb the analog to digital conversion process.
Undershoot and overshoot occur because of the imped-
ance mismatch of the circuit board trace at the converter
pin when the transition time of an external control signal
is less than twice the propagation delay from the driver to
the LTC2480
. For reference, on a regular FR-4 board,
signal propagation velocity is approximately 183ps/inch
for internal traces and 170ps/inch for surface traces.
Thus, a driver generating a control signal with a minimum
transition time of 1ns must be connected to the converter
pin through a trace shorter than 2.5 inches. This problem
becomes particularly difficult when shared control lines
are used and multiple reflections may occur. The solution
is to carefully terminate all transmission lines close to
their characteristic impedance.
Parallel termination near the LTC2480 pin will eliminate this
problem but will increase the driver power dissipation. A
series resistor between 27
and 56
placed near the driver
output pin will also eliminate this problem without additional
power dissipation. The actual resistor value depends upon
the trace impedance and connection topology.
An alternate solution is to reduce the edge rate of the con-
trol signals. It should be noted that using very slow edges
will increase the converter power supply current during the
transition time. The differential input architecture reduces
the converter’s sensitivity to ground currents.
Particular attention must be given to the connection of the
F
O
signal when the LTC2480 is used with an external
conversion clock. This clock is active during the conver-
sion time and the normal mode rejection provided by the
internal digital filter is not very high at this frequency. A
normal mode signal of this frequency at the converter
reference terminals can result in DC gain and INL errors.
A normal mode signal of this frequency at the converter
input terminals can result in a DC offset error. Such
perturbations can occur due to asymmetric capacitive
coupling between the F
O
signal trace and the converter
input and/or reference connection traces. An immediate
solution is to maintain maximum possible separation
between the F
O
signal trace and the input/reference sig-
nals. When the F
O
signal is parallel terminated near the
converter, substantial AC current is flowing in the loop
formed by the F
O
connection trace, the termination and the
ground return path. Thus, perturbation signals may be
inductively coupled into the converter input and/or refer-
ence. In this situation, the user must reduce to a minimum
the loop area for the F
O
signal as well as the loop area for
the differential input and reference connections. Even
when F
0
is not driven, other nearby signals pose similar
EMI threats which will be minimized by following good
layout practices.
Driving the Input and Reference
The input and reference pins of the LTC2480 converter are
directly connected to a network of sampling capacitors.
Depending upon the relation between the differential input
voltage and the differential reference voltage, these ca-
pacitors are switching between these four pins transfer-
ring small amounts of charge in the process. A simplified
equivalent circuit is shown in Figure 11.
For a simple approximation, the source impedance R
S
driving an analog input pin (IN
+
, IN
–
, V
REF+
or GND) can be
considered to form, together with R
SW
and C
EQ
(see
Figure 11), a first order passive network with a time
constant
τ
= (R
S
+ R
SW
) C
EQ
. The converter is able to
sample the input signal with better than 1ppm accuracy if
the sampling period is at least 14 times greater than the
input circuit time constant
τ
. The sampling process on the
four input analog pins is quasi-independent so each time
constant should be considered by itself and, under worst-
case circumstances, the errors may add.
When using the internal oscillator, the LTC2480’s front-
end switched-capacitor network is clocked at 123kHz
corresponding to an 8.1
μ
s sampling period. Thus, for
settling errors of less than 1ppm, the driving source
impedance should be chosen such that
τ
≤
8.1
μ
s/14 =
580ns. When an external oscillator of frequency f
EOSC
is
used, the sampling period is 2.5/f
EOSC
and, for a settling
error of less than 1ppm,
τ
≤
0.178/f
EOSC
.
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is low
(up to 10k
with no external bypass capacitor or up to
500
with 0.001
μ
F bypass), complete settling of the input
APPLICATIU
W
U
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