12
LTC1603
1603f
APPLICATIONS INFORMATION
WUU
U
starts (settling time must be 200ns for full throughput
rate).
Choosing an Input Amplifier
Choosing an input amplifier is easy if a few requirements
are taken into consideration. First, to limit the magnitude
of the voltage spike seen by the amplifier from charging
the sampling capacitor, choose an amplifier that has a
low output impedance (<100Ω) at the closed-loop band-
width frequency. For example, if an amplifier is used in a
gain of +1 and has a unity-gain bandwidth of 50MHz, then
the output impedance at 50MHz should be less than
100Ω. The second requirement is that the closed-loop
bandwidth must be greater than 15MHz to ensure
adequate small-signal settling for full throughput rate. If
slower op amps are used, more settling time can be
provided by increasing the time between conversions.
The best choice for an op amp to drive the LTC1603 will
depend on the application. Generally applications fall into
two categories: AC applications where dynamic specifi-
cations are most critical and time domain applications
where DC accuracy and settling time are most critical.
The following
list is a summary of the op amps that are
suitable for driving the LTC1603. More detailed informa-
tion is available in the Linear Technology databooks, the
LinearView
TM
CD-ROM and on our web site at:
www.linear-tech. com.
Figure 10. t
ACQ
vs Source Resistance
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 sup-
ply current, ±5V to ±15V supplies, good AC/DC specs.
LT1363: 50MHz Voltage Feedback Amplifier. 6.3mA sup-
ply current, good AC/DC specs.
LT1364/LT1365: Dual and Quad 50MHz Voltage Feedback
Amplifiers. 6.3mA supply current per amplifier, good AC/
DC specs.
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1603 noise and distortion. The small-signal band-
width of the sample-and-hold circuit is 15MHz. Any noise
or distortion products that are present at the analog inputs
will be summed over this entire bandwidth. Noisy input
circuitry should be filtered prior to the analog inputs to
minimize noise. A simple 1-pole RC filter is sufficient for
many applications. For example, Figure 11 shows a 3000pF
capacitor from A
IN
+
to ground and a 100Ω source resistor
to limit the input bandwidth to 530kHz. The 3000pF
capacitor also acts as a charge reservoir for the input
sample-and-hold and isolates the ADC input from sam-
pling glitch sensitive circuitry. High quality capacitors and
resistors should be used since these components can add
distortion. NPO and silver mica type dielectric capacitors
have excellent linearity. Carbon surface mount resistors can
also generate distortion from self heating and from damage
that may occur during soldering. Metal film surface mount
resistors are much less susceptible to both problems.
LinearView is a trademark of Linear Technology Corporation.
SOURCE RESISTANCE (Ω)
1 10 100 1k 10k
ACQUISITION TIME (µs)
10
1
0.1
0.01
1603 F10
