ltc2435-1 Linear Technology Corporation, ltc2435-1 Datasheet - Page 25

• Manufacturer Part Number: ltc2435-1
• Description: 20-bit No Latency Delta Sigma Adcs With Differential Input And Differential Reference
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Driving the Input and Reference
The input and reference pins of the LTC2435/LTC2435-1
converters are directly connected to a network of sampling
capacitors. Depending upon the relation between the
differential input voltage and the differential reference
voltage, these capacitors are switching between these
four pins transferring small amounts of charge in the
process. A simplified equivalent circuit is shown in
Figure 14.
For a simple approximation, the source impedance R
driving an analog input pin (IN
considered to form, together with R
Figure 14), a first order passive network with a time
constant τ = (R
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 (F
LTC2435’s front-end switched-capacitor network is clocked
at 76800Hz corresponding to a 13μs sampling period and
the LTC2435-1’s front end is clocked at 69900Hz corre-
sponding to 14.2μs. Thus, for settling errors of less than
1ppm, the driving source impedance should be chosen
such that τ ≤ 13μs/14 = 920ns (LTC2435) and τ <14.2μs/
14 = 1.01μs (LTC2435-1). When an external oscillator of
frequency f
and, for a settling error of less than 1ppm, τ ≤ 0.14/f
Input Current
If complete settling occurs on the input, conversion re-
sults will be unaffected by the dynamic input current. An
incomplete settling of the input signal sampling process
may result in gain and offset errors, but it will not degrade
the INL performance of the converter. Figure 14 shows the
mathematical expressions for the average bias currents
flowing through the IN
sampling charge transfers when integrated over a sub-
stantial time period (longer than 64 internal clock cycles).
EOSC
S
is used, the sampling period is 2/f
+ R
U
SW
+
) • C
and IN
U
EQ
+
, IN
. The converter is able to
–
–
pins as a result of the
, REF
O
W
= LOW or HIGH), the
SW
+
or REF
and C
U
–
EQ
) can be
EOSC
EOSC
(see
S
.
The effect of this input dynamic current can be analyzed
using the test circuit of Figure 15. The C
includes the LTC2435/LTC2435-1 pin capacitance (5pF
typical) plus the capacitance of the test fixture used to
obtain the results shown in Figures 16 and 17. A careful
implementation can bring the total input capacitance (C
+ C
than the one predicted by Figures 16 and 17. For simplic-
ity, two distinct situations can be considered.
For relatively small values of input capacitance (C
0.01μF), the voltage on the sampling capacitor settles
almost completely and relatively large values for the
source impedance result in only small errors. Such values
for C
performance without significant benefits of signal filtering
and the user is advised to avoid them. Nevertheless, when
small values of C
of input multiplexers, wires, connectors or sensors, the
LTC2435/LTC2435-1 can maintain their exceptional accu-
racy while operating with relative large values of source
resistance as shown in Figures 16 and 17. These mea-
sured results may be slightly different from the first order
approximation suggested earlier because they include the
effect of the actual second order input network together
with the nonlinear settling process of the input amplifiers.
For small C
almost independently and there is little benefit in trying to
match the source impedance for the two pins.
Larger values of input capacitors (C
required in certain configurations for antialiasing or gen-
eral input signal filtering. Such capacitors will average the
input sampling charge and the external source resistance
will see a quasi constant input differential impedance.
When F
typical differential input resistance is 22MΩ (LTC2435) or
24MΩ (LTC2435-1) which will generate a +FS gain error
of approximately 0.023ppm (LTC2435) or 0.021ppm
(LTC2435-1) for each ohm of source resistance driving
IN
oscillator and 50Hz notch), the typical differential input
resistance is 26MΩ which will generate a +FS gain error of
approximately 0.019ppm for each ohm of source resis-
+
PAR
or IN
IN
) closer to 5pF thus achieving better performance
O
will deteriorate the converter offset and gain
= LOW (internal oscillator and 60Hz notch), the
–
. For the LTC2435, when F
IN
values, the settling on IN
LTC2435/LTC2435-1
IN
are unavoidably present as parasitics
IN
O
> 0.01μF) may be
+
= HIGH (internal
and IN
PAR
capacitor
–
25
occurs
24351fb
IN
IN
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