OPA644U BURR-BROWN [Burr-Brown Corporation], OPA644U Datasheet - Page 8

• Manufacturer Part Number: OPA644U
• Description: Low Distortion Current Feedback OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

APPLICATIONS INFORMATION
THEORY OF OPERATION
This current feedback architecture offers the following im-
portant advantages over voltage feedback architectures: (1)
the high slew rate allows the large signal performance to
approach the small signal performance, and: (2) there is very
little bandwidth degradation at higher gain settings.
The current feedback architecture of the OPA644 provides
the traditional strength of excellent large signal response
with the unusual addition of very high open-loop
transimpedance. This high open-loop transimpedance al-
lows the OPA644 to be used in applications requiring
16 bits or more of accuracy and dynamic linearity.
DC GAIN TRANSFER CHARACTERISTICS
The circuit in Figure 1 shows the equivalent circuit for
calculating the DC gain. When operating the device in the
inverting mode, the input signal error current (I
fied by the open-loop transimpedance gain (T
signal generated is equal to T
applied through R
equal to –R
FIGURE 1. Equivalent Circuit.
For non-inverting operation, the input signal is applied to the
non-inverting (high impedance buffer) input. The output
(buffer) error current (I
inverting input. The signal generated at the output is fed
back to the inverting input such that the overall gain is (1 +
R
Where a voltage-feedback amplifier has two symmetrical
high impedance inputs, a current feedback amplifier has a
low inverting (buffer output) impedance and a high non-
inverting (buffer input) impedance.
The closed-loop gain for the OPA644 can be calculated
using the following equations:
FF
V
N
/R
I
V
).
I
R
FF
FB
®
/R
C
1
OPA644
FF
FB
.
+
I
–
E
such that the device operates at a gain
E
R
S
) is generated at the low impedance
L
S
O
x I
R
FB
E
. Negative feedback is
C
T
O
C
O
). The output
E
) is ampli-
V
O
8
Inverting Gain = (–R
Non-inverting Gain = (1 + R
At higher gains the small value inverting input impedance
(R
seen from the equation:
Factual = F
This loss in bandwidth at high gains can be corrected
without affecting stability by lowering the value of the
feedback resistor from the specified value of 402 .
OFFSET VOLTAGE AND NOISE
The output offset is the algebraic sum of the input voltage
and current sources that influence DC operation. The output
offset is calculated by the following equation:
Output Offset Voltage = Ib
If all terms are divided by the gain (1 + R
observed that input referred offsets improve as gain in-
creases.
The effective noise at the output of the amplifier can be
determined by taking the root sum of the squares of equation
4 and applying the spectral noise values found in the Typical
Performance Curve graph section. This applies to noise from
the op amp only. Note that both the noise figure and
equivalent input offset voltages improve as the closed-loop
gain increases (by keeping R
R
FIGURE 2. Output Offset Voltage Equivalent Circuit.
INCREASING BANDWIDTH AT HIGH GAINS
The closed-loop bandwidth can be extended at high gains by
reducing the value of the feedback resistor R
1). This bandwidth reduction is caused by the feedback
current being split between R
(for a fixed R
R
fied bandwidth, the following equations can be used to
approximate R
where: Loop Gain = T(o)/(R
N
FF
INV
, which reduces closed-loop bandwidth. To maintain speci-
= 0 ).
) causes an apparent loss in bandwidth. This can be
IDEAL
FB
F
), more feedback current is shunted through
and R
/(1 + (R
R
R
G
N
FB
I
for any gain from 1 to 15:
/R
INV
(1 + R
FF
)/(1+1/Loop Gain)
/R
Ib
S
N
Ib
FB
F
N
and R
I
FB
FB
x R
/R
fixed and reducing R
FB
) x (1/(1+T(o)/(R
) (1 + R
FF
R
N
/R
FB
FF
)/(1 + 1/Loop Gain) (2)
(1 + R
G
. As the gain increases
) Ib
FB
FB
/R
FB
I
F
x R
(refer to Figure
/R
/R
FF
))
G
G
FB
) it can be
) V
FB
/R
I
IO
FF
with
))
(1)
(3)
(4)