1

eOUT = √4k TBRk: Boltzman’s constant = 1.38 x 10–23 J/KT: temperature (°K)B: noise bandwidth (Hz)R: feedback resistor (Ω)

eOUT: noise voltage (Vrms)

while transimpedance gain (signal) increases as:

eOUT = i (signal) R

Signal-to-noise improves by √R.

• A low bias current op amp is needed to achieve highestsensitivity. Bias current causes voltage offset errors withlarge-feedback resistors. Wide bandwidth circuits withsmaller feedback resistors are less subject to bias currenterrors, but even in these circuits, bias current must be

The OPA128 ultra-low bias current operational amplifierachieves its 75fA maximum bias current without compro-mise. Using standard design techniques, serious perfor-mance trade-offs were required which sacrificed overallamplifier performance in order to reach femtoamp (fA = 10–15

A) bias currents.

UNIQUE DESIGN MINIMIZESPERFORMANCE TRADE-OFFS

Small-geometry FETs have low bias current, of course, butFET size reduction reduces transconductance and increasesnoise dramatically, placing a serious restriction on perfor-mance when low bias current is achieved simply by makinginput FETs extremely small. Unfortunately, larger geom-etries suffer from high gate-to-substrate isolation diode leak-age (which is the major contribution to BIFET® amplifierinput bias current).

Replacing the reverse-biased gate-to-substrate isolation di-ode structure of BlFETs with dielectric isolation removesthis large leakage current component which, together with anoise-free cascode circuit, special FET geometry, and ad-vanced wafer processing, allows far higher Difet ® perfor-mance compared to BIFETs.

HOW TO IMPROVE PHOTODIODEAMPLIFIER PERFORMANCE

An important electro-optical application of FET op amps isfor photodiode amplifiers. The unequaled performance ofthe OPA128 is well-suited for very high sensitivity detectordesigns. A few design tips for photodiode amplifiers may behelpful:

• Photodiode capacitance should be as low as possible. SeeFigure 1: CJ affects not only bandwidth but noise as well.This is because CJ and the op amp’s feedback resistor forma noise-gain zero (feedback pole).

• Photodiode active area should be as small as possible sothat CJ is small and RJ is high. This will allow a highersignal-to-noise ratio. If a large area is needed, considerusing optical “gain” (lens, mirror, etc.) rather than a largearea diode. Optical “gain” is essentially noise-free.

• Use as large a feedback resistor as possible (consistentwith bandwldth requirements) to minimize noise. Thisseems paradoxical, but remember, resistor thermal noiseincreases as:

FIGURE 1. Photodiode Equivalent Circuit.

RS

IP = photocurrentRJ = shunt resistance of diode junctionCJ = junction capacitanceRS = series resistance

IP CJRJ

OPA128LM

2

3

6

HP5082-4204

8

109Ω

5pF Responsivity ≈ 109V/WBandwidth: DC to ≈ 30HzOffset Voltage ≈ ±485µV

109Ω

FIGURE 2. High-Sensitivity Photodiode Amplifier.

DESIGNING PHOTODIODE AMPLIFIER CIRCUITS WITH OPA128

©1994 Burr-Brown Corporation AB-077 Printed in U.S.A. January, 1994

®

SBOA061

2

ISIGNAL

RF

If: RJ = ∞ and IS = 0 EOUT = –ISIGNAL RF + VOS

EOUTVOS

ISIGNAL

RF

If: RJ = ∞, IS = 0 and R'F >> (R1/R2) EOUT = –ISIGNAL R'F(1 + R1/R2) + VOS(1 + R1/R2)

EOUTVOS

R2

R1

FIGURE 3. Wide-Temperature Range Photodiode Amplifier.

• For highest sensitivity use the photodiode in a “photovol-taic mode”. With zero-bias operation, dark current offseterrors are not generated by this (photodiode leakage)current. Zero bias is a slower but higher sensitivity modeof operation. Most photodiodes work quite effectively withzero bias, even those originally designed for reverse-biasedoperation.

• Fastest response and greatest bandwidth are obtained inthe “photoconductive mode”. Reverse bias reduces CJ

substantially and also reduces or eliminates the slow risetime diffusion “tail” which is troublesome at longer wave-lengths. Disadvantages of biased operation are: dark cur-rent, 1/F noise component is introduced, and the occasionalneed for an extra bias supply.

• A very high resistance feedback resistor is MUCH betterthan a low resistance in a T network. See Figure 5. Althoughtransimpedance gain (eOUT/iSIGNAL) is equivalent, the T net-work will sacrifice performance. The low feedback resis-tance will generate higher current noise (iN) and the voltagedivider formed by R1/R2 multiply input offset voltage, drift,and amplifier voltage noise by the ratio of 1+R1/R2. In most electrometer amplifiers, these input specifica-tions are not very good to start with. Multiplying an alreadyhigh offset and drift (sometimes as high as 3mV and 50µV/°C) by use of a T network becomes impractical. By using afar better amplifier, such as the OPA128, moderate T net-work ratios can be accommodated and the resulting multi-plied errors will be far smaller. Although a single very-highresistance will give better performance, the T network canovercome such problems as gain adjustment and difficulty infinding a large value resistor.

OPA128LM

2

3

6

HamamatsuG1735

8

400MΩ

≈ 0.4pF

400MΩ1000pF

Responsivity ≈ –1.6 X 108V/WSpectral Response ≈ 400 – 760nmBandwidth ≈ 1kHzOffset Voltage ≈ ±150µV at 25°C ≈ ±325µV at 60°C

OPA128

2

3

6

UDT Pin-040A orSDC SD-041-11-21-011

8

1MΩ

≈ 0.5pF Responsivity ≈ –5 X 105V/WBandwidth ≈ 100kHzOffset Voltage ≈ ±1mV

0.1µF

Bias Voltage+10V to +50V

FIGURE 4. Wider-Bandwidth Photodiode Amplifier.

FIGURE 5. Feedback Resistors for Transimpedance Ampli-fiers.

considered if wide temperature range operation is ex-pected. The OPA128LM specs only ±2pA max at +70°C.Bias current also causes shot noise.

iS = √2qi

q: 1.602 x 10–19 coulombsi: bias (or signal) current (A)iS: noise current (A rms)

In most circuits, the dominant noise source will be thethermal (Johnson) noise of the feedback resistor.

• Diode shunt resistance (RJ) should be as high as possible.If RJ » RF, then the circuit DC gain (noise gain) is 1V/V.Low resistance diodes will cause noise, voltage offset, anddrift to be amplified by 1+ RF/RJ.

Since diode shunt resistance decreases at a higher tempera-ture, it can cause unexpected errors. In Figure 3 a diffused-junction GaAsP photodiode is used to maintain RJ =3000MΩ at +60°C. Due to its higher bandgap, GaAsP hasa flatter RJ versus temperature slope than silicon.

3

• Shield the photodetector circuit in a metal housing. It is avery high impedance, high sensitivity circuit and it requiresgood shielding and effective power supply bypassing. Thisis not optional.

• A small capacitor across RF is frequently required tosuppress oscillation or gain peaking. Although it can affectbandwidth, a small amount of capacitance will usually berequired to ensure loop stability. This capacitor can bemade larger for bandwidth limitation if desired.

The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumesno responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to changewithout notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrantany BURR-BROWN product for use in life support devices and/or systems.

KEY OPA128 SPECIFICATIONS

Bias current .......................................................... 75fA maxOffset voltage .................................................... 500µV maxDrift ................................................................. 5µV/°C maxNoise .................................................... 15nV/√Hz at 10kHz

BIFFET® National Semiconductor Corp.; Difet ® Burr-Brown Corp.

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