25
LTC2424/LTC2428
APPLICATIONS INFORMATION
WUU
U
While a digital input signal is in the 0.5V to (V
CC
␣ –␣ 0.5V)
range, the CMOS input receiver draws additional current
from the power supply. It should be noted that, when any
one of the digital input signals (F
O
, CSADC, CSMUX, D
IN
,
CLK and SCK in External SCK mode of operation) is within
this range, the LTC2424/LTC2428 power supply current
may increase even if the signal in question is at a valid logic
level. For micropower operation and in order to minimize
the potential errors due to additional ground pin current,
it is recommended to drive all digital input signals to full
CMOS levels [V
IL
< 0.4V and V
OH
> (V
CC
– 0.4V)].
Severe ground pin current disturbances can also occur
due to the undershoot of fast digital input signals. Under-
shoot and overshoot can occur because of the imped-
ance mismatch at the converter pin when the transition
time of an external control signal is less than twice the
propagation delay from the driver to LTC2424/LTC2428.
For reference, on a regular FR-4 board, signal propaga-
tion 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 LTC2424/LTC2428 input
pins will eliminate this problem but will increase the driver
power dissipation. A series resistor between 27Ω and 56Ω
placed near the driver or near the LTC2424/LTC2428 pin
will also eliminate this problem without additional power
dissipation. The actual resistor value depends upon the
trace impedance and connection topology.
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched ca-
pacitor network. This network consists of capacitors switch-
ing between the analog input (ADCIN), ZS
SET
(Pin 5) and
the reference (FS
SET
). The result is small current spikes
seen at both ADCIN and V
REF
. A simplified input equivalent
circuit is shown in Figure 18.
The key to understanding the effects of this dynamic input
current is based on a simple first order RC time constant
model. Using the internal oscillator, the internal switched
capacitor network of the LTC2424/LTC2428 is clocked at
153,600Hz corresponding to a 6.5µs sampling period.
Fourteen time constants are required each time a capacitor
is switched in order to achieve 1ppm settling accuracy.
Therefore, the equivalent time constant at V
IN
and V
REF
should be less than 6.5µs/14 = 460ns in order to achieve
1ppm accuracy.
Figure 18. LTC2424/LTC2428 Equivalent Analog Input Circuit
FS
SET
CHX
ADCV
CC
(PIN 2)
R
SW
5k
AVERAGE INPUT CURRENT:
I
DC
= 0.25(V
IN
– 0.5 • V
REF
) • f • C
EQ
I
REF
I
REF
ADCV
CC
(PIN 2)
I
IN(LEAK)
I
IN(LEAK)
±I
DC
MUXV
CC
(PIN 8)
I
IN(MUX)
I
IN(MUX)
R
SW
5k
R
SW
75Ω
C
EQ
1pF (TYP)
R
SW
5k
SELECTED
CHANNEL
24248 F18
f
OUT
= 50Hz, INTERNAL OSCILLATOR: f = 128kHz
f
OUT
= 60Hz, INTERNAL OSCILLATOR: f = 153.6kHz
EXTERNAL OSCILLATOR: 2.56kHz ≤ f ≤ 307.2kHz
ZS
SET
ADCINMUXOUT
