1With their low-input currents, FET input op amps are uni-versally used in monitoring photodetectors, the most com-mon of which are photodiodes. There are a variety ofamplifier connections for this purpose and the choice isbased on linearity, offset, noise and bandwidth consider-ations. These same factors influence the selection of theamplifier with newer devices offering very low-input cur-rents, low noise and high speed.Photodetectors are the bridge between a basic physicalindicator and electronics resulting in the largest single usageof FET op amps. As a measure of physical conditions, lightis secondary to temperature and pressure until the measure-ment is made remotely with no direct contact to the moni-tored object. Then, the signals of a CAT scanner, star-tracking instrument or electron microscope depend on lightfor the final link to signal processing. Photodiodes havemade that link economical and expanded usage to detectorarrays that employ more than 1000 light sensors. Focus thenturns to accurate conversion of the photodiode output to alinearly related electrical signal. As always, this is a contestbetween speed and resolution with noise as a basic limitingelement. Central to the contest is the seemingly simplecurrent-to-voltage converter which displays surprising mul-tidimensional constraints and suggests alternative configura-tions for many optimizations.CURRENT-TO-VOLTAGEThe energy transmitted by light to a photodiode can bemeasured as either a voltage or current output. For a voltageresponse, the diode must be monitored from a high imped-ance that does not draw significant signal current. Thatcondition is provided by Figure 1a. Here, the photodiode isin series with the input of an op amp where ideally zerocurrent flows. That op amp has feedback set by R1 and R2 toestablish amplification of the voltage diode just as if it wasan offset voltage of the amplifier. While appealing to morecommon op amp thinking, this voltage mode is nonlinear.The response has a logarithmic relationship to the lightenergy received since the sensitivity of the diode varies withits voltage.Constant voltage for a fixed sensitivity suggests currentoutput instead and that response is linearly related to theincident light energy. A monitor of that current must havezero input impedance to respond with no voltage across thediode. Zero impedance is the role of an op amp virtualground as high-amplifier loop gain removes voltage swingfrom the input. That is the key to the basic current-to-voltageconverter connection of Figure 1b. It provides an inputresistance of R1/A where A is the open-loop gain of the opamp. Even though R1 is generally very large, the resultinginput resistance remains negligible in comparison to theoutput resistance of photodiodes.FIGURE 1a. Photodiode Output Can be Monitored as aVoltage; or, 1b, as a Current.Diode current is not accepted by the input of the op amp asits presence stimulates the high amplifier gain to receive thatcurrent through the feedback resistor, R1. To do so, theamplifier develops an output voltage equal to the diodecurrent times the feedback resistance, R1. For that current-to-voltage gain to be high, R1 is made as large as otherconstraints will permit. At higher resistance levels, thatresistor begins to develop significant thermal DC voltagePHOTODIODE MONITORING WITH OP AMPSR2100kΩA1D1R1100kΩeOR1100MΩA1eOeO = (1 + R2/R1) (KT/q) In (1 + IP/IS)A1: OPA128D1: HP5082-4204eO = IPR1A1: OPA111D1: HP5082-4204D1IPR1100MΩ0.1µF(a)(b)©1994 Burr-Brown Corporation AB-075 Printed in U.S.A. January, 1995APPLICATION BULLETIN®Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132SBOA0352a response zero due to CD and begins a rise that is terminatedonly because of a second parasitic capacitance. Stray capaci-tance, CS, shunts the feedback resistor resulting in a responsepole leveling the gain at 1+ CD/CS. For large area diodes CDcan be hundreds of picofarads causing the noise gain to peakin the hundreds as well. That gain continues to higherfrequencies until rolled off by the op amp bandwidth limit.As feedback resistance increases, the pole and zero of thisgain peaking move together to lower frequencies encom-passing a greater spectrum with high gain.First signs of this gain peaking phenomena are familiar toanyone who has used high resistance op amp feedback inmore general circuits. High output to input resistance withan op amp results in overshoot, response peaking, poorsettling or even oscillation all due to the resistance interac-tion with amplifier input capacitance. Together the resis-tance and capacitance form another pole in the feedbackloop resulting in the classic differentiator feedback response.Shown by the dashed line for more general op amp cases, theassociated feedback factor reciprocal intercepts the ampli-fier open loop magnitude response with a 12dB/octave rateFIGURE 2a. Due to Diode Capacitance in the Feedback ofthe Basic Current-to-Voltage Converter, 2b,Op Amp Noise Receives Gain and BandwidthNot Available to the Signal.R1100MΩA1eOR2100MΩ0.01µFCDRDenInIPCSeO = IPR1 + InR1 + enA1: OPA1111 + JωR1CD1 + JωR1CSfCLog f(CS/CD) fC1/(2πR1 CD)1/(2πR1 CS)1 + CD/CSNoise GainI to V GainOpen-Loop Gain|A| (dB)(a)(b)drift due to the temperature coefficient of the amplifier inputcurrent. To compensate this error, an equal resistance R2 iscommonly connected in series with the op amp noninvertinginput, as shown, and capacitively bypassed to remove mostof its noise. The remaining DC error is determined by themismatches between the amplifier input currents and be-tween the two resistors. A drawback of this error correctionis the voltage drop it creates across the diode and theresulting diode leakage current. That leakage can overridethe correction achieved with R2, as photodiodes typicallyhave large junction areas for high sensitivity. Leakage cur-rent is proportional to that area which can become muchlarger than the op amp input currents.Only zero diode voltage can eliminate this new error sourcebut that is in conflict with control of a second attribute oflarge diode area. Large parasitic capacitance is also presentcreating often severe amplification of noise as will bedescribed. To reduce that capacitance, a large