5962-8964401PA(2001) Просмотр технического описания (PDF) - Analog Devices
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5962-8964401PA Datasheet PDF : 16 Pages
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UNDERSTANDING THE AD844
The AD844 can be used in ways similar to a conventional op
amp while providing performance advantages in wideband
applications. However, there are important differences in the
internal structure which need to be understood in order to
optimize the performance of the AD844 op amp.
Open Loop Behavior
Figure 1 shows a current feedback amplifier reduced to essen-
tials. Sources of fixed dc errors such as the inverting node bias
current and the offset voltage are excluded from this model and
are discussed later. The most important parameter limiting the
dc gain is the transresistance, Rt, which is ideally infinite. A finite
value of Rt is analogous to the finite open loop voltage gain in a
conventional op amp.
The current applied to the inverting input node is replicated by
the current conveyor so as to flow in resistor Rt. The voltage
developed across Rt is buffered by the unity gain voltage follower.
Voltage gain is the ratio Rt/ RIN. With typical values of Rt = 3 MΩ
and RIN = 50 Ω, the voltage gain is about 60,000. The open
loop current gain is another measure of gain and is determined
by the beta product of the transistors in the voltage follower
stage (see Figure 4); it is typically 40,000.
+1
IIN
Rt
Ct
+1
RIN
IIN
Figure 1. Equivalent Schematic
The important parameters defining ac behavior are the trans-
capacitance, Ct, and the external feedback resistor (not shown).
The time constant formed by these components is analogous to
the dominant pole of a conventional op amp, and thus cannot
be reduced below a critical value if the closed loop system is to
be stable. In practice, Ct is held to as low a value as possible
(typically 4.5 pF) so that the feedback resistor can be maximized
while maintaining a fast response. The finite RIN also affects the
closed loop response in some applications as will be shown.
The open loop ac gain is also best understood in terms of the
transimpedance rather than as an open loop voltage gain. The
open loop pole is formed by Rt in parallel with Ct. Since Ct is
typically 4.5 pF, the open loop corner frequency occurs at
about 12 kHz. However, this parameter is of little value in
determining the closed loop response.
AD844
Response as an Inverting Amplifier
Figure 2 shows the connections for an inverting amplifier.
Unlike a conventional amplifier the transient response and the
small signal bandwidth are determined primarily by the value of
the external feedback resistor, R1, rather than by the ratio of
R1/R2 as is customarily the case in an op amp application. This
is a direct result of the low impedance at the inverting input. As
with conventional op amps, the closed loop gain is –R1/R2.
The closed loop transresistance is simply the parallel sum of R1
and Rt. Since R1 will generally be in the range 500 Ω to 2 kΩ
and Rt is about 3 MΩ the closed loop transresistance will be
only 0.02% to 0.07% lower than R1. This small error will often
be less than the resistor tolerance.
When R1 is fairly large (above 5 kΩ) but still much less than Rt,
the closed loop HF response is dominated by the time constant
R1Ct. Under such conditions the AD844 is over-damped and
will provide only a fraction of its bandwidth potential. Because
of the absence of slew rate limitations under these conditions,
the circuit will exhibit a simple single pole response even under
large signal conditions.
In Figure 2, R3 is used to properly terminate the input if desired.
R3 in parallel with R2 gives the terminated resistance. As R1 is
lowered, the signal bandwidth increases, but the time constant
R1Ct becomes comparable to higher order poles in the closed
loop response. Therefore, the closed loop response becomes
complex, and the pulse response shows overshoot. When R2 is
much larger than the input resistance, RIN, at Pin 2, most of the
feedback current in R1 is delivered to this input; but as R2
becomes comparable to RIN, less of the feedback is absorbed at
Pin 2, resulting in a more heavily damped response. Conse-
quently, for low values of R2 it is possible to lower R1 without
causing instability in the closed loop response. Table I lists
combinations of R1 and R2 and the resulting frequency response
for the circuit of Figure 2. TPC 13 shows the very clean and fast
± 10 V pulse response of the AD844.
VIN
R2
R3
OPTIONAL
R1
AD844
RL
VOUT
CL
Figure 2. Inverting Amplifier
REV. D
–7–
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