15
LTC1404
1404fa
APPLICATIONS INFORMATION
WUU
U
BOARD LAYOUT AND BYPASSING
To obtain the best performance from the LTC1404, a
printed circuit board is required. Layout for the printed
circuit board should ensure that digital and analog signal
lines are separated as much as possible. In particular, care
should be taken not to run any digital traces alongside an
analog signal trace or underneath the ADC. The analog
input should be screened by GND.
High quality 10µF surface mount AVX capacitor with a
0.1µF ceramic should be used at the V
CC
, V
SS
and V
REF
pins. For better results, another 10µF AVX capacitor can be
added to the V
CC
pin. At 600ksps, the CLK frequency can
be as high as 9.6MHz. A poor quality capacitor can lose
more than 80% of its capacitance at this frequency range.
Therefore, it is important to consult the manufacturer’s
data sheet before the capacitor is used. For the LTC1404,
at 600ksps, every bit decision must be determined within
104ns (9.6MHz). During this short time interval, the
supply disturbance due to a CLK transition needs to settle.
The ADC must update its DAC, make a comparator deci-
sion based on sub-mV overdrive, latch the new DAC
information and output the serial data. This ADC provides
one power supply, V
CC
, which is connected to both the
internal analog and digital circuitry. Any ringing due to
poor supply or reference bypassing, inductive trace runs,
CLK and CONV over- or undershoot, or unnecessary D
OUT
loading can cause ADC errors. Therefore, the bypass
capacitors must be located as close to the pins as possible.
The traces connecting the pins and the bypass capacitors
must be kept short and should be made as wide as
possible. In unipolar mode operation, V
SS
must be con-
nected to the GND pin directly.
Input signal leads to A
IN
and signal return leads from GND
(Pin 4) should be kept as short as possible to minimize
noise coupling. In applications where this is not possible,
a shielded cable between the analog input signal and the
ADC is recommended. Also, any potential difference in
grounds between the analog signal and the ADC appears
as an error voltage in series with the analog input signal.
Attention should be paid to reducing the ground circuit
impedance as much as possible.
Figure 11 shows the recommended system ground con-
nections. All analog circuitry grounds should be termi-
nated at the LTC1404 GND pin. The ground return from the
LTC1404 Pin 4 to the power supply should be low imped-
ance for noise free operation. Digital circuitry grounds
must be connected to the digital supply common. As an
alternative, instead of a direct short between the digital and
analog circuitry, a 10Ω or a ferrite bead jumper helps
reduce the digital noise.
ANALOG SUPPLY
–5V GND 5V
+ +
LTC1404
V
SS
V
CC
GND
DIGITAL SUPPLY
GND 5V
+
DIGITAL CIRCUITRY
V
CC
GND
1404 F11
Figure 11. Power Supply Connection
In applications where the ADC data outputs and control
signals are connected to a continuously active micropro-
cessor bus, it is possible to get errors in the conversion
results. These errors are due to feedthrough from the
microprocessor to the successive approximation com-
parator. The problem can be eliminated by forcing the
microprocessor into a Wait state during conversion or by
using three-state buffers to isolate the ADC data bus.
Power-Down Mode
Upon power-up, the LTC1404 is initialized to the active
state and is ready for conversion. However, the chip can be
easily placed into Nap or Sleep mode by exercising the
right combination of CLK and CONV signals. In Nap mode,
all power is off except for the internal reference, which is
still active and provides 2.43V output voltage to the other
circuitry. In this mode, the ADC draws only 7.5mW of
power instead of 75mW (for minimum power, the logic
