ADA4530-1ARZ-R7 datasheet(44/52 Pages) AD

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Data Sheet

ADA4530-1

PHOTODIODE INTERFACE

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Rev. C | 44 of 52

The low input bias current and low input offset voltage makes

the ADA4530-1 an excellent choice for signal conditioning photodi-

odes at extremely low illumination levels. Figure 124 shows the

ADA4530-1 configured in a transimpedance amplifier interfacing

with a photodiode operating in photovoltaic mode (photodiode is

zero biased). A photodiode produces an output current proportional

to the illumination level. The amplifier converts the signal current,

IPD, into an output voltage with the following equation:

VOUT = IPD × RF

(16)

Figure 124. Transimpedance Amplifier with Photodiode

Figure 125 replaces the photodiode with an equivalent circuit

model. IPD is the photo current generated by incident light and

is proportional to the light level. The shunt capacitance (CSHUNT)

models the depletion capacitance of the diode. This capacitance

depends on the area of the photodiode and the voltage bias. The

shunt resistance (RSHUNT) represents the voltage vs. current slope

of the exponential diode curve near zero bias voltage.

Figure 125. Transimpedance Amplifier with Photodiode Model

DC ERROR ANALYSIS

All of the errors described in the High Impedance Measurements

section related to TIA circuits are applicable to photodiode interfa-

ces.

The inverting input bias current, IB−, sums directly with the photo-

diode current for a referred to input (RTI) error equal to IB−. This

current flows through the feedback resistor, creating a referred to

output (RTO) error, VIB_TRO, equal to

VIB_RTO = IB− × RF

(17)

The amplifier offset voltage, VOS, is a major error source in pho-

todiode interface circuits because of the relatively low shunt resist-

ance of large area photodiodes. Typical values are in the range of 1

GΩ to 100 GΩ at 25°C.

More importantly, the shunt resistance decreases by half for every

10°C increase in temperature. An error current is created because

the amplifier offset voltage is applied across this shunt resistance,

resulting in an RTI error (IVOS_RTI) equal to

IVOS_RTI = VOS/RSHUNT

(18)

It is equivalent to think that the shunt resistance increases the DC

noise gain (NG), which multiplies the offset voltage to the output.

The RTO error due to VOS is equal to

VOS_RTO = VOS × Noise Gain

(19)

VOS_RTO = VOS × (1 + RF/RSHUNT)

(20)

The amplifier input resistance and insulation resistance appear

in parallel with the photodiode shunt resistance. These additional

resistances reduce the effective shunt resistance, but they are

much larger than the photodiode shunt resistance and can usually

be ignored.

AC ERROR ANALYSIS

Photodiode TIA circuits typically require external compensation to

give satisfactory dynamic performance. The large feedback resistor

(RF) interacts with the large photodiode capacitance (CSHUNT) to

create a low frequency pole in the feedback network. Photodiode

shunt capacitance, amplifier input capacitance, and trace capaci-

tance are lumped into a single element, CSHUNT. The phase shift

due to this pole must be recovered prior to the crossover frequency

for the feedback loop to be stable. The usual method to recover

this phase shift is to create a zero in the feedback factor with the

addition of the feedback capacitor (CF).

The classic way of analyzing this circuit is by examining the noise

gain vs. frequency (see Figure 126). At low frequencies, the noise

gain is determined by the ratio of the feedback to the shunt resist-

ance.

NG1=1+ RFRSHUNT

(21)

The troublesome low frequency pole (which is a zero in the noise

gain) occurs at Frequency f1. From this frequency onward, the

noise gain increases. If there is no feedback capacitor in the circuit,

the noise gain follows the dotted line until it intersects with the

amplifier open-loop gain curve. If these curves intersect at the 20

dB/decade slopes shown in Figure 126, the circuit is unstable.

The addition of CF adds a zero to the feedback factor (which is a

pole in the noise gain) at Frequency f2. Beyond Frequency f2, the

noise gain is determined by the ratio of the shunt capacitance to the

feedback capacitance.

NG2=1+CSHUNTCF

(22)

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