THS4631_14 Datasheet, PDF (9/35 Pages) Texas Instruments – HIGH-VOLTAGE, HIGH SLEW RATE, WIDEBAND FET-INPUT OPERATIONAL AMPLIFIER

THS4631
www.ti.com
SLOS451B – DECEMBER 2004 – REVISED AUGUST 2011
APPLICATION INFORMATION
INTRODUCTION
The THS4631 is a high-speed, FET-input operational
amplifier. The combination of: high gain bandwidth
product of 210 MHz, high slew rate of 1000 V/µs, and
trimmed dc precision makes the device an excellent
design option for a wide variety of applications,
including test and measurement, optical monitoring,
transimpedance gain circuits, and high-impedance
buffers. The applications section of the data sheet
discusses these particular applications in addition to
general information about the device and its features
TRANSIMPEDANCE FUNDAMENTALS
FET-input amplifiers are often used in
transimpedance applications because of their
extremely high input impedance. A transimpedance
block accepts a current as an input and converts this
current to a voltage at the output. The high-input
impedance associated with FET-input amplifiers
minimizes errors in this process caused by the input
bias currents, IIB, of the amplifier.
DESIGNING THE TRANSIMPEDANCE
CIRCUIT
Typically, design of a transimpedance circuit is driven
by the characteristics of the current source that
provides the input to the gain block. A photodiode is
the most common example of a capacitive current
source that interfaces with a transimpedance gain
block. Continuing with the photodiode example, the
system designer traditionally chooses a photodiode
based on two opposing criteria: speed and sensitivity.
Faster photodiodes cause a need for faster gain
stages, and more sensitive photodiodes require
higher gains in order to develop appreciable signal
levels at the output of the gain stage.
These parameters affect the design of the
transimpedance circuit in a few ways. First, the speed
of the photodiode signal determines the required
bandwidth of the gain circuit. Second, the required
gain, based on the sensitivity of the photodiode, limits
the bandwidth of the circuit. Third, the larger
capacitance associated with a more sensitive signal
source also detracts from the achievable speed of the
gain block. The dynamic range of the input signal
also places requirements on the amplifier dynamic
range. Knowledge of the source output current levels,
coupled with a desired voltage swing on the output,
dictates the value of the feedback resistor, RF. The
transfer function from input to output is VOUT = I_INR_F.
The large gain-bandwidth product of the THS4631
provides the capability for simultaneously achieving
both high-transimpedance gain, wide bandwidth, high
slw rate, and low noise. In addition, the high-power
supply rails provide the potential for a very wide
dynamic range at the output, allowing for the use of
input sources which possess wide dynamic range.
The combination of these characteristics makes the
THS4631 a design option for systems that require
transimpedance amplification of wideband, low-level
input signals. A standard transimpedance circuit is
shown in Figure 32.
Photodiode Circuit
C_F
R_F
λ
_
+
R_L
−V_(Bias)
Figure 32. Wideband Photodiode
Transimpedance Amplifier
As indicated, the current source typically sets the
requirements for gain, speed, and dynamic range of
the amplifier. For a given amplifier and source
combination, achievable performance is dictated by
the following parameters: the amplifier
gain-bandwidth product, the amplifier input
capacitance, the source capacitance, the
transimpedance gain, the amplifier slew rate, and the
amplifier output swing. From this information, the
optimal performance of a transimpedance circuit
using a given amplifier is determined. Optimal is
defined here as providing the required
transimpedance gain with a maximized flat frequency
response.
For the circuit shown in Figure 32, all but one of the
design parameters is known; the feedback capacitor
(C_F) must be determined. Proper selection of the
feedback capacitor prevents an unstable design,
controls pulse response characteristics, provides
maximized flat transimpedance bandwidth, and limits
broadband integrated noise. The maximized flat
frequency response results with CF calculated as
shown in Equation 1, where CF is the feedback
capacitor, R_F is the feedback resistor, CS is the total
source capacitance (including amplifier input
capacitance and parasitic capacitance at the inverting
node), and GBP is the gain-bandwidth product of the
amplifier in hertz.
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