22
LTC2400
connection resistance. The LTC2400’s power supply cur-
rent flowing through the 0.01Ω resistance of the common
ground pin will develop a 2.5µV offset signal. For a
reference voltage V
REF
= 2.5V, this represents a 1ppm
offset error.
In an alternative configuration, the GND pin of the converter
can be the single-point-ground in a single point grounding
system. The input signal ground, the reference signal
ground, the digital drivers ground (usually the digital
ground) and the power supply ground (the analog ground)
should be connected in a star configuration with the com-
mon point located as close to the GND pin as possible.
The power supply current during the conversion state
should be kept to a minimum. This is achieved by restrict-
ing the number of digital signal transitions occurring
during this period.
While a digital input signal is in the range 0.5V to
(V
CC
␣ –␣ 0.5V), 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
, CS and SCK
in External SCK mode of operation) is within this range, the
LTC2400 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 impedance
mismatch at the converter pin when the transition time of
an external control signal is less than twice the propaga-
tion delay from the driver to LTC2400. For reference, on
a regular FR-4 board, signal propagation velocity is ap-
proximately 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 diffi-
cult when shared control lines are used and multiple
reflections may occur. The solution is to carefully termi-
nate all transmission lines close to their characteristic
impedance.
APPLICATIONS INFORMATION
WUU
U
V
REF
V
IN
V
CC
R
SW
5k
AVERAGE INPUT CURRENT:
I
IN
= 0.25(V
IN
– 0.5 • V
REF
)fC
EQ
I
REF(LEAK)
I
REF(LEAK)
V
CC
R
SW
5k
C
EQ
10pF (TYP)
R
SW
5k
I
IN(LEAK)
I
IN
2400 F15
I
IN(LEAK)
SWITCHING FREQUENCY
f = 153.6kHz FOR INTERNAL OSCILLATOR (f
O
= LOGIC LOW OR HIGH)
f = f
EOSC
FOR EXTERNAL OSCILLATORS
GND
Figure 15. LTC2400 Equivalent Analog Input Circuit
Parallel termination near the LTC2400 pin 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 LTC2400 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
switching between the analog input (V
IN
), ground (Pin 4)
and the reference (V
REF
). The result is small current spikes
seen at both V
IN
and V
REF
. A simplified input equivalent
circuit is shown in Figure 15.
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 LTC2400’s inter-
nal switched capacitor network 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.
