LTC1400
U U
W U
APPLICATIO S I FOR ATIO
Input signal leads to A and signal return leads from GND
theLTC1400GNDpin.ThegroundreturnfromtheLTC1400
Pin 4 to the power supply should be low impedance for
noise free operation. Digital circuitry grounds must be
connected to the digital supply common.
IN
(Pin 4) should be kept as short as possible to minimize
noisecoupling.Inapplicationswherethisisnotpossible,a
shielded cable between source and ADC is recommended.
Also,sinceanypotentialdifferenceingroundsbetweenthe
signalsourceandADCappearsasanerrorvoltageinseries
with the input signal, attention should be paid to reducing
the ground circuit impedance as much as possible.
InapplicationswheretheADCdataoutputsandcontrolsig-
nalsareconnectedtoacontinuouslyactivemicroprocessor
bus, it is possible to get errors in the conversion results.
These errors are due to feedthrough from the micropro-
cessor to the successive approximation comparator. 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.
ANALOG SUPPLY
GND
DIGITAL SUPPLY
–5V
5V
GND
5V
Power-Down Mode
+
+
+
Upon power-up, the LTC1400 is initialized to the active
state and is ready for conversion. However, the chip can
be easily placed into the Nap or Sleep mode by exercising
the right combination of CLK and CONV signal. In the Nap
modeallpowerisoffexcepttheinternalreference,whichis
still active and provides 2.42V output voltage to the other
circuitry. In this mode, the ADC draws only 6mW of power
instead of 75mW (for minimum power, the logic inputs
must be within 500mV of the supply rails). The wake-up
time from the Nap mode to the active mode is 350ns.
V
GND
V
GND
V
CC
SS
CC
LTC1400
DIGITAL CIRCUITRY
1400 F11
Figure 11. Power Supply Connection
Figure11showstherecommendedsystemgroundconnec-
tions. All analog circuitry grounds should be terminated at
t
t
11
11
CLK
t
t
1
1
CONV
NAP
SLEEP
V
REF
REFRDY
1400 F12
NOTE: NAP AND SLEEP ARE INTERNAL SIGNALS. REFRDY APPEARS AS A BIT IN THE D
WORD.
OUT
Figure 12. Nap Mode and Sleep Mode Waveforms
1400fa
12
Linear [ Linear ]
