OPA2381 Datasheet, PDF (13/19 Pages) Burr-Brown (TI) – Precision, Low Power, 18MHz Transimpedance Amplifier

www.ti.com
BOARD LAYOUT
Minimize photodiode capacitance and stray
capacitance at the summing junction (inverting input).
This capacitance causes the voltage noise of the op
amp to be amplified (increasing amplification at high
frequency). Using a low-noise voltage source to
reverse-bias a photodiode can significantly reduce its
capacitance. Smaller photodiodes have lower
capacitance. Use optics to concentrate light on a small
photodiode.
Circuit board leakage can degrade the performance of
an otherwise well-designed amplifier. Clean the circuit
board carefully. A circuit board guard trace that
encircles the summing junction and is driven at the
same voltage can help control leakage. See Figure 11.
R_F
λ
OPA381
V_OUT
Guard ring
Figure 11. Connection of Input Guard
OTHER WAYS TO MEASURE SMALL
CURRENTS
Logarithmic amplifiers are used to compress extremely
wide dynamic range input currents to a much narrower
range. Wide input dynamic ranges of 8 decades, or
100pA to 10mA, can be accommodated for input to a
12-bit ADC. (Suggested products: LOG101, LOG102,
LOG104, LOG112.)
Extremely small currents can be accurately measured
by integrating currents on a capacitor. (Suggested
product: IVC102.)
Low-level currents can be converted to high-resolution
data words. (Suggested product: DDC112.)
For further information on the range of products
available, search www.ti.com using the above specific
model names or by using keywords transimpedance
and logarithmic.
OPA381
OPA2381
SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004
CAPACITIVE LOAD AND STABILITY
The OPA381 series op amps can drive greater than
100pF pure capacitive load. Increasing the gain
enhances the amplifier’s ability to drive greater
capacitive loads. See the Phase Margin vs Load
Capacitance typical characteristic curve.
One method of improving capacitive load drive in the
unity-gain configuration is to insert a 10Ω to 20Ω
resistor inside the feedback loop, as shown in
Figure 12. This reduces ringing with large capacitive
loads while maintaining DC accuracy.
R_F
C_F(3)
V+
R_S
λ
20Ω
OPA381
V_OUT
V_B(1)
V−
C_L
R_L
V_PD(2)
NOTES: (1) V_B = GND or pedestal voltage to reverse bias the photodiode.
(2) V_PD = GND or −5V.
(3) C_F x R_F ≥ 2C_L x R_S.
Figure 12. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
DRIVING 16-BIT ANALOG-TO-DIGITAL
CONVERTERS (ADC)
The OPA381 series is optimized for driving a 16-bit ADC
such as the ADS8325. The OPA381 op amp buffers the
converter input capacitance and resulting charge
injection while providing signal gain. Figure 13 shows
the OPA381 in a single-ended method of interfacing the
ADS8325 16-bit, 100kSPS ADC. For additional
information, refer to the ADS8325 data sheet.
13