Datasheet

Wideband, Inverting Gain Operation
Wideband, High-Sensitivity, Transimpedance
OPA659
R
OUT
0.1 Fm 10 Fm
0.1 Fm 10 Fm
50 LoadW
50 SourceW
R
T
R
G
V
IN
V
OUT
R
F
+6V
-6V
OPA659
R
OUT
0.1 Fm 10 Fm
0.1 Fm 10 Fm
50 LoadW
I
D
C
D
C
F
V
OUT
R
F
Photo
Diode
-V
B
l
+6V
-6V
OPA659
www.ti.com ............................................................................................................................................ SBOS342B – DECEMBER 2008 – REVISED AUGUST 2009
Figure 36 shows the noninverting input tied directly to
ground. Often, a bias current-cancelling resistor to
The circuit of Figure 36 shows the inverting gain test
ground is included here to nullify the dc errors caused
circuit used for most of the inverting Typical
by input bias current effects. For a JFET input op
Characteristics graphs. As with the noninverting
amp such as the OPA659, the input bias currents are
applications, most of the curves were characterized
so low that dc errors caused by input bias currents
using signal sources with 50 Ω driving impedance,
are negligible. Thus, no bias current-cancelling
and with measurement equipment that presents a
resistor is recommended at the noninverting input.
50 Ω load impedance. In Figure 36 , the shunt resistor
R
T
at V
IN
should be set so the parallel combination of
the shunt resistor and R
G
equals 50 Ω to match the
Design
source impedance of the test generator and cable,
while the series output resistor R
OUT
at V
OUT
should
The high GBP and low input voltage and current
also be set to 50 Ω to provide matching impedance for
noise for the OPA659 make it an ideal wideband,
the measurement equipment load and cable.
transimpedance amplifier for low to moderate
Generally, data sheet voltage swing specifications are
transimpedance gains. Higher transimpedance gains
measured at the output pin, V
OUT
, in Figure 36 .
(above 100k Ω ) can benefit from the low input noise
current of a JFET input op amp such as the OPA659.
Designs that require high bandwidth from a large
area detector can benefit from the low input voltage
noise for the OPA659. This input voltage noise is
peaked up over frequency by the diode source
capacitance, and in many cases, may become the
limiting factor to input sensitivity. The key elements to
the design are the expected diode capacitance (C
D
)
with the reverse bias voltage ( – V
B
) applied, the
desired transimpedance gain, R
F
, and the GBP for
the OPA659 (350MHz). Figure 37 shows a general
transimpedance amplifier circuit, or TIA, using the
OPA659. Given the source diode capacitance plus
parasitic input capacitance for the OPA659, the
transimpedance gain, and known GBP, the feedback
Figure 36. General Inverting Test Circuit
capacitor value, C
F
, may be calculated to avoid
excessive peaking in the frequency response.
The inverting circuit can also use a wide range of
resistor values to set the gain; Table 2 lists several
recommended resistor values for inverting gains with
a 50 Ω input/output match.
Table 2. Resistor Values for Inverting Gains with
50 Ω Input/Output Match
INVERTING
GAIN R
F
R
G
R
T
R
OUT
– 1 249 249 61.9 49.9
– 2 249 124 84.5 49.9
– 5 249 49.9 Open 49.9
– 10 499 49.9 Open 49.9
Figure 37. Wideband, Low-Noise, Transimpedance
Amplifier (TIA)
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