LOG112 datasheet(14/18 Pages) BURR-BROWN

14 / 18 page

OPA380

OPA2380

SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004

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.

Guard ring

OUT

OPA380

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 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 10

Ω 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.

OPA380

Ω

6800pF

ADS8411

RC Values shown are optimized for the

ADS8411

 values may vary for other ADCs.

Figure 12. Driving 16-bit ADCs

OPA380

(Provides high−speed amplification

with very low offset and drift.)

OUT

Figure 13. OPA380 inverting gain configuration

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