ADA4530-1ARZ-R7 datasheet(45/51 Pages) AD

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ADA4530-1

Data Sheet

Rev. A | Page 44 of 50

PHOTODIODE INTERFACE

The low input bias current and low input offset voltage makes

the ADA4530-1 an excellent choice for signal conditioning

photodiodes at extremely low illumination levels. Figure 124

shows the ADA4530-1 configured in a transimpedance ampli-

fier 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

VOUT

PHOTODIODE

IPD

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.

VOUT

PHOTODIODE

RSHUNT

CSHUNT

IPD

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

interfaces.

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

photodiode 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 equal to

VIB_RTO = IB− × RF

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

photodiode interface circuits because of the relatively low shunt

resistance of large area photodiodes. Typical values are in the

range of 1 GΩ to100 GΩ at 25°C. More importantly, the shunt

resistance decreases by half for every 10°C increase in tempera-

ture. An error current is created because the amplifier offset

voltage is applied across this shunt resistance, resulting in an

RTI error equal to

IVOS_RTI = VOS/RSHUNT

It is equivalent to think that the shunt resistance increases the

DC noise gain which multiplies the offset voltage to the output.

The RTO error due to VOS is equal to:

VOS_RTO = VOS × Noise Gain

VOS_RTO = VOS × (1 + RF/RSHUNT)

The amplifier input resistance and insulation resistance appear

in parallel with the photodiode shunt resistance. These addi-

tional 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 capacitance 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 resistance.

SHUNT



 1

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.

SHUNT



 1

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