Datasheet
f
-3dB
=
UGBW
2 R (C )
F
TOT
p
Hz
R
F
1MW
C
F
(1)
<1pF
+V
S
-V
S
V =I R
OUT D F
C
TOT
I
D
(1)C isoptionaltopreventgainpeaking.
F
(2)C isthestraycapacitanceofR
STRAY F
(typically,2pFforasurface-mountresistor).
C
STRAY
(2)
NOTES:
OPA827
C
F
1
4 R UGBW
F
p
( )
(8 C
TOT
R UGBW
F
p
( )
1+ 1+
IN-
IN+
OUT
V+
V-
OPA827
SBOS376H –NOVEMBER 2006–REVISED MAY 2012
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TRANSIMPEDANCE AMPLIFIER Bandwidth (f
–3dB
) calculated by Equation 2:
The gain bandwidth, low voltage noise, and current
noise of the OPA827 series make them ideal wide
(2)
bandwidth transimpedance amplifiers in a photo-
These equations result in maximum transimpedance
conductive application. High transimpedance gains
bandwidth. For additional information, refer to
with feedback resistors greater than 100kΩ benefit
Application Bulletin SBOA055, Compensate
from the low input current noise (2.2fA/Hz) of the
Transimpedance Amplifiers Intuitively, available for
JFET input. Low voltage noise is important because
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photodiode capacitance causes the effective noise
gain in the circuit to increase at high frequencies.
Total input capacitance of the circuit limits the overall
gain bandwidth of the amplifier and is addressed
below. Figure 46 shows a photodiode
transimpedance application.
Key Transimpedance Points
• The total input capacitance (C
TOT
) consists of the
photodiode junction capacitance, and both the
common-mode and differential input capacitance
of the operational amplifier.
• The desired transimpedance gain, V
OUT
= I
D
R
F
.
• The Unity Gain Bandwidth Product (UGBW)
(22MHz for the OPA827).
With these three variables set, the feedback capacitor
value (C
F
) can be calculated to ensure stability.
C
STRAY
is the parasitic capacitance of the PCB and
passive components, which is approximately 0.5pF.
To ensure 45° phase margin, the minimal amount of
feedback capacitance can be calculated using
Figure 46. Transimpedance Amplifier
Equation 1:
(1)
Figure 47. Equivalent Schematic (Single Channel)
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