Datasheet

OPA656
10
SBOS196G
www.ti.com
APPLICATIONS INFORMATION
WIDEBAND, NONINVERTING OPERATION
The OPA656 provides a unique combination of a broadband,
unity gain stable, voltage-feedback amplifier with the DC
precision of a trimmed JFET-input stage. Its very high Gain
Bandwidth Product (GBP) of 230MHz can be used to either
deliver high signal bandwidths for low-gain buffers, or to
deliver broadband, low-noise transimpedance bandwidth to
photodiode-detector applications. To achieve the full perfor-
mance of the OPA656, careful attention to printed circuit
board (PCB) layout and component selection is required as
discussed in the remaining sections of this data sheet.
Figure 1 shows the noninverting gain of +2 circuit used as the
basis for most of the Typical Characteristics. Most of the curves
were characterized using signal sources with 50Ω driving im-
pedance, and with measurement equipment presenting a 50Ω
load impedance. In Figure 1, the 50Ω shunt resistor at the V
I
terminal matches the source impedance of the test generator,
while the 50Ω series resistor at the V
O
terminal provides a
matching resistor for the measurement equipment load. Gener-
ally, data sheet voltage swing specifications are at the output pin
(V
O
in Figure 1) while output power specifications are at the
matched 50Ω load. The total 100Ω load at the output combined
with the 500Ω total feedback network load, presents the OPA656
with an effective output load of 83Ω for the circuit of Figure 1.
WIDEBAND, INVERTING GAIN OPERATION
The circuit of Figure 2 shows the inverting gain of –1 test
circuit used for most of the inverting Typical Characteristics.
In this case, an additional resistor R
M
is used to achieve the
50Ω input impedance required by the test equipment using in
characterization. This input impedance matching is optional
in a circuit board environment where the OPA656 is used as
an inverting amplifier at the output of a prior stage.
FIGURE 1. Noninverting G = +2 Specifications and Test
Circuit.
Voltage-feedback op amps, unlike current feedback prod-
ucts, can use a wide range of resistor values to set their gain.
To retain a controlled frequency response for the noninverting
voltage amplifier of Figure 1, the parallel combination of
R
F
|| R
G
should always < 200Ω. In the noninverting configu-
ration, the parallel combination of R
F
|| R
G
will form a pole
with the parasitic input capacitance at the inverting node of
the OPA656 (including layout parasitics). For best perfor-
mance, this pole should be at a frequency greater than the
closed loop bandwidth for the OPA656. For this reason, a
direct short from output to inverting input is recommended for
the unity gain follower application.
In this configuration, the output sees the feedback resistor as
an additional load in parallel with the 100Ω load used for test.
It is often useful to increase the R
F
value to decrease the
loading on the output (improving harmonic distortion) with the
constraint that the parallel combination of R
F
|| R
G
< 200Ω.
For higher inverting gains with the DC precision provided by
the FET input OPA656, consider the higher gain bandwidth
product OPA657.
Figure 2 also shows the noninverting input tied directly to
ground. Often, a bias current canceling resistor to ground is
included here to null out the DC errors caused by input bias
current effects. This is only useful when the input bias
currents are matched. For a JFET part like the OPA656, the
input bias currents do not match but are so low to begin with
(< 5pA) that DC errors due to input bias currents are
negligible. Hence, no resistor is recommended at the
noninverting inputs for the inverting signal path condition.
WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE
DESIGN
The high GBP and low input voltage and current noise for the
OPA656 make it an ideal wideband transimpedance ampli-
fier for low to moderate transimpedance gains. Higher
transimpedance gains (> 100kΩ) will benefit from the low
input noise current of a FET input op amp such as the
OPA656. One transimpedance design example is shown on
the front page of the data sheet. Designs that require high
bandwidth from a large area detector will benefit from the low
input voltage noise for the OPA656. This input voltage noise
FIGURE 2. Inverting G = –1 Specifications and Test Circuit.
OPA656
+5V
–5V
–V
S
+V
S
50Ω
V
O
V
I
50Ω
+
0.1µF
+
6.8µF
6.8µF
R
G
250Ω
R
F
250Ω
50Ω Source
50Ω Load
0.1µF
OPA656
+5V
–5V
+V
S
–V
S
R
M
57.6Ω
50ΩV
O
V
I
+
6.8µF0.1µF
+
6.8µF0.1µF
R
F
402Ω
R
G
402Ω
50Ω Source
50Ω Load