F
i GBW
F
F GBW
1
8 R C f
C
4 R f
+
p
=
p
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Hardware Description
3.5.3
Feedback Capacitor
Using an external resistor may result in a noisier output signal because there is a noise reducing capacitor
calibrated for the onboard feedback resistor’s resistance. Selecting a feedback capacitor value is
determined by
Equation 2
(see the MSP430F2274 datasheet for details).
where
•
R
F
is the feedback resistance
•
C
i
is 10 pF
•
f
GBW
is the gain-bandwidth product of the op-amp (determined by its slew rate mode)
(2)
In general, a larger feedback capacitance will result in a slightly smaller bandwidth.
For more information on selecting a feedback capacitor, see the
Transimpedance Considerations for High-
Speed Amplifiers
and
Compensate Transimpedance Amplifiers Intuitively
resources in
Section 6
.
3.6
Operational Amplifier
The TIDM-TIA uses the MSP430F2274’s integrated op-amp with a feedback resistor to provide the
current-to-voltage conversion. Using the op-amp is preferred to simply feeding the photodiode’s current
across a resistor. A lone feedback resistor has a tradeoff between a large gain and a small response time.
Using the onboard op-amp with a feedback resistor allows for a faster response time, large gain, and
better signal-to-noise ratio of output signals. There is also a feedback capacitor added across the
feedback resistor. Its job is to act as a low-pass filter, reducing noise and oscillation created by the op-
amp (see
Section 3.5.3
for details on changing the capacitor). The complete circuit involving the op-amp
can be seen in
Figure 4
where IS1 simulates the photodiode as a current source.
Figure 4. Operational Amplifier Circuit
5
TIDU443 – August 2014
MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide
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