OPA380AID Datasheet, PDF (14/22 Pages) Texas Instruments – Precision, High-Speed Transimpedance Amplifier

OPA380
OPA2380
SBOS291G − NOVEMBER 2003 − REVISED SEPTEMBER 2007
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, as shown in
Figure 11.
R_F
λ
OPA380
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, and 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.
www.ti.com
CAPACITIVE LOAD AND STABILITY
The OPA380 series op amps can drive up to 500pF pure
capacitive load. Increasing the gain enhances the
amplifier’s ability to drive greater capacitive loads (see
the Typical Characteristic curve, Small-Signal
Overshoot vs Capacitive Load).
One method of improving capacitive load drive in the
unity-gain configuration is to insert a 10Ω to 20Ω
resistor in series with the load. This reduces ringing with
large capacitive loads while maintaining DC accuracy.
DRIVING FAST 16-BIT ANALOG-TO-DIGITAL
CONVERTERS (ADC)
The OPA380 series is optimized for driving a fast 16-bit
ADC such as the ADS8411. The OPA380 op amp
buffers the converter’s input capacitance and resulting
charge injection while providing signal gain. Figure 12
shows the OPA380 in a single-ended method of
interfacing the ADS8411 16-bit, 2MSPS ADC. For
additional information, refer to the ADS8411 data sheet.
C_F
R_F
OPA 38 0
15Ω
ADS8411
6800pF
RC Values shown are optimized for the
ADS8411 values may vary for other ADCs.
Figure 12. Driving 16-Bit ADCs
R₁
V_IN
C_F
R_F
OPA 38 0
V_OUT
(Provides high−speed amplification
with very low offset and drift.)
Figure 13. OPA380 Inverting Gain Configuration
14