OPA380 Datasheet, PDF (11/22 Pages) Burr-Brown (TI) – Precision, High-Speed Transimpedance Amplifier

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BIASING PHOTODIODES IN SINGLE-SUPPLY
CIRCUITS
The +IN input can be biased with a positive DC voltage
to offset the output voltage and allow the amplifier
output to indicate a true zero photodiode measurement
when the photodiode is not exposed to any light. It will
also prevent the added delay that results from coming
out of the negative rail. This bias voltage appears
across the photodiode, providing a reverse bias for
faster operation. An RC filter placed at this bias point will
reduce noise, as shown in Figure 4. This bias voltage
can also serve as an offset bias point for an ADC with
range that does not include ground.
C_F(1)
< 1pF
R_F
10MΩ
V+
λ
OPA380
V_OUT
0.1µF 100kΩ
+V_Bias
NOTE: (1) C_F is optional to prevent gain peaking.
It includes the stray capacitance of R_F.
Figure 4. Filtered Reverse Bias Voltage
TRANSIMPEDANCE AMPLIFIER
Wide bandwidth, low input bias current, and low input
voltage and current noise make the OPA380 an ideal
wideband photodiode transimpedance amplifier.
Low-voltage noise is important because photodiode
capacitance causes the effective noise gain of the
circuit to increase at high frequency.
The key elements to a transimpedance design are
shown in Figure 5:
the total input capacitance (C_TOT), consisting of the
photodiode capacitance (C_DIODE) plus the parasitic
common-mode and differential-mode input
capacitance (3pF + 1.1pF for the OPA380);
OPA380
OPA2380
SBOS291G − NOVEMBER 2003 − REVISED SEPTEMBER 2007
the desired transimpedance gain (R_F);
the Gain Bandwidth Product (GBW) for the
OPA380 (90MHz).
With these three variables set, the feedback capacitor
value (C_F) can be set to control the frequency response.
C_STRAY is the stray capacitance of R_F, which is 0.2pF for
a typical surface-mount resistor.
To achieve a maximally flat, 2nd-order, Butterworth
frequency response, the feedback pole should be set
to:
Ǹ ¹
2pR_F^ǒC_F ) C_STRAY^Ǔ
+
GBW
4pR_FC_TOT
(1)
Bandwidth is calculated by:
Ǹ f_*3dB +
GBW
2pR_FC_TOT
Hz
(2)
These equations will result in maximum
transimpedance bandwidth. For even higher
transimpedance bandwidth, the high-speed CMOS
OPA300 (SBOS271 (180MHz GBW)), or the OPA656
(SBOS196 (230MHz GBW)) may be used.
For additional information, refer to Application Bulletin
AB−050 (SBOA055), Compensate Transimpedance
Amplifiers Intuitively, available for download at
www.ti.com.
C_F(1)
R_F
10MΩ
C_STRAY(2)
+5V
λ
C_TOT(3) OPA380
V_OUT
R_P (optional
pulldown resistor)
−5V
NOTE: (1) C_F is optional to prevent gain peaking.
(2) C_STRAY is the stray capacitance of R_F
(typically, 0.2pF for a surface−mount resistor).
(3) C_TOT is the photodiode capacitance plus OPA380
input capacitance.
Figure 5. Transimpedance Amplifier
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