OPA381 datasheet(13/24 Pages) TI1

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OPA381

OPA2381

SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004

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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.

Guard ring

OUT

OPA381

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.

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 10

Ω to 20Ω

resistor inside the feedback loop, as shown in

Figure 12. This reduces ringing with large capacitive

loads while maintaining DC accuracy.

OPA381

OUT

(1)

V+

Ω

(3)

NOTES: (1) V

B = GND or pedestal voltage to reverse bias the photodiode.

(2) V

PD =GND or −5V.

(3) C

F xRF ≥ 2CL xRS.

(2)

−

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.

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