LTC1605
9
1605fd
For more information www.linear.com/LTC1605
APPLICATIONS INFORMATION
Conversion Details
The LTC1605 uses a successive approximation algo-
rithm and an internal sample-and-hold circuit to convert
an analog signal to a 16-bit or two byte parallel output.
The ADC is complete with a precision reference and an
internal clock. The control logic provides easy interface
to microprocessors and DSPs. (Please refer to the Digital
Interface section for the data format.)
Conversion start is controlled by the CS and R/C inputs.
At the start of conversion the successive approximation
register (SAR) is reset. Once a conversion cycle has begun
it cannot be restarted.
During the conversion, the internal 16-bit capacitive DAC
output is sequenced by the SAR from the most significant
bit (MSB) to the least significant bit (LSB). Referring to
Figure 1, V
IN
is connected through the resistor divider to the
sample-and-hold capacitor during the acquire phase and
the comparator offset is nulled by the autozero switches.
In this acquire phase, a minimum delay of 2µs will provide
enough time for the sample-and-hold capacitor to acquire
the analog signal. During the convert phase, the autozero
switches open, putting the comparator into the compare
mode. The input switch switches C
SAMPLE
to ground,
injecting the analog input charge onto the summing junc-
tion. This input charge is successively compared with the
binary-weighted charges supplied by the capacitive DAC.
Bit decisions are made by the high speed comparator. At
Figure 1. LTC1605 Simplified Equivalent Circuit
V
DAC
+
–
C
DAC
DAC
SAMPLE
HOLD
C
SAMPLE
S
A
R
16-BIT
LATCH
COMPARATOR
SAMPLE
SI
R
IN2
R
IN1
V
IN
the end of a conversion, the DAC output balances the V
IN
input charge. The SAR contents (a 16-bit data word) that
represents the V
IN
are loaded into the 16-bit output latches.
Driving the Analog Inputs
The nominal input range for the LTC1605 is ±10V
or(±4•V
REF
) and the input is overvoltage protected to
±25V. The input impedance is typically 20kΩ, therefore, it
should be driven with a low impedance source. Wideband
noise coupling into the input can be minimized by placing
a 1000pF capacitor at the input as shown in Figure 2. An
NPO-type capacitor gives the lowest distortion. Place the
capacitor as close to the device input pin as possible. If
an amplifier is to be used to drive the input, care should
be taken to select an amplifier with adequate accuracy,
linearity and noise for the application. The following list
is a summary of the op amps that are suitable for driving
the LTC1605. More detailed information is available at
www.linear.com.
LT
®
1007: Low noise precision amplifier. 2.7mA supply
current ±5V to ±15V supplies. Gain bandwidth product
8MHz. DC applications.
LT1097: Low cost, low power precision amplifier. 300µA
supply current. ±5V to ±15V supplies. Gain bandwidth
product 0.7MHz. DC applications.
LT1227: 140MHz video current feedback amplifier. 10mA
supply current
. ±5V to ±15V supplies. Low noise and low
distortion.
LT1360: 37MHz voltage feedback amplifier. 3.8mA supply
current. ±5V to ±15V supplies. Good AC/DC specs.
LT1363: 50MHz voltage feedback amplifier. 6.3mA supply
current. Good AC/DC specs.
LT1364/LT1365: Dual and quad 50MHz voltage feedback
amplifiers. 6.3mA supply current per amplifier. Good AC/
DC specs.
1605 • F02
1000pF 33.2k
V
IN
CAP
A
IN
200Ω
Figure 2. Analog Input Filtering
