Datasheet

Offset (V) +
Charge (pC)
C
H
Total (pF)
OPA615
www.ti.com ...................................................................................................................................... SBOS299E –FEBRUARY 2004–REVISED SEPTEMBER 2009
SAMPLING COMPARATOR The additional offset voltage or switching transient
induced on a capacitor at the current source output
The OPA615 sampling comparator features a very
by the switching charge can be determined by the
short switching (2.5ns) propagation delay and utilizes
following formula:
a new switching circuit architecture to achieve
excellent speed and precision.
(2)
It provides high impedance inverting and noninverting
analog inputs, a high-impedance current source The switching stage input is insensitive to the low
output and a TTL-CMOS-compatible Hold Control slew rate performance of the hold control command
Input. and compatible with TTL/CMOS logic levels. With
TTL logic high, the comparator is active, comparing
The sampling comparator consists of an operational
the two input voltages and varying the output current
transconductance amplifier (OTA), a buffer amplifier,
accordingly. With TTL logic low, the comparator
and a subsequent switching circuit. This combination
output is switched off, showing a very high
is subsequently referred to as the Sampling
impedance to the hold capacitor.
Operational Transconductance Amplifier (SOTA). The
OTA and buffer amplifier are directly tied together at
DESIGN-IN TOOLS
the buffer outputs to provide the two identical
high-impedance inputs and high open-loop
Demonstration Fixture
transconductance. Even a small differential input
voltage multiplied with the high transconductance
Two printed circuit boards (PCBs) are available to
results in an output current—positive or
assist in the initial evaluation of circuit performance
negative—depending upon the input polarity. This
using the OPA615. The demonstration fixture is
characteristic is similar to the low or high status of a
offered free of charge as an unpopulated PCB,
conventional comparator. The current source output
delivered with a user's guide. The summary
features high output impedance, output bias current
information for this fixture is shown in Table 2.
compensation, and is optimized for charging a
capacitor in DC restoration, nanosecond integrators,
Table 2. OPA615 Demonstration Fixtures
peak detectors and S/H circuits. The typical
ORDERING LITERATURE
comparator output current is ±5mA and the output
PRODUCT PACKAGE NUMBER NUMBER
bias current is minimized to typically ±10μA in the
OPA615ID SO-14 DEM-OPA-SO-1C SBOU039
sampling mode.
OPA615IDGS MSOP-10 DEM-OPA-MSOP-1A SBOU042
This innovative circuit achieves the high slew rate
representative of an open-loop design. In addition,
The demonstration fixture can be requested at the
the acquisition slew current for a hold or storage
Texas Instruments web site (www.ti.com) through the
capacitor is higher than standard diode bridge and
OPA615 product folder.
switch configurations, removing a main contributor to
the limits of maximum sampling rate and input
Macromodel and Applications Support
frequency.
Computer simulation of circuit performance using
The switching circuits in the OPA615 use current
SPICE is often a quick way to analyze the
steering (versus voltage switching) to provide
performance of analog circuits and systems. This is
improved isolation between the switch and analog
particularly true for video and RF amplifier circuits
sections. This design results in low aperture time
where parasitic capacitance and inductance can have
sensitivity to the analog input signal, reduced power
a major effect on circuit performance. A SPICE model
supply and analog switching noise. Sample-to-hold
for the OPA615 is available through the TI web page
peak switching charge injection is 40fC.
(www.ti.com). This model predicts typical small-signal
AC, transient steps, DC performance, and noise
under a wide variety of operating conditions. The
model includes the noise terms found in the electrical
specifications of the data sheet. However, the model
does not attempt to distinguish between package
types in their small-signal AC performance. The
applications department is also available for design
assistance.
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Product Folder Link(s): OPA615