AD8510ARM-R2 datasheet(17/20 Pages) AD

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AD8510/AD8512/AD8513

Rev. E | Page 17 of 20

I-V CONVERSION APPLICATIONS

Photodiode Circuits

Common applications for I-V conversion include photodiode

circuits, where the amplifier is used to convert a current emitted

by a diode placed at the positive input terminal into an output

voltage.

The AD8510/AD8512/AD8513’s low input bias current, wide

bandwidth, and low noise make them each an excellent choice

for various photodiode applications, including fax machines,

fiber optic controls, motion sensors, and bar code readers.

The circuit shown in Figure 53 uses a silicon diode with zero

bias voltage. This is known as a Photovoltaic Mode; this

configuration limits the overall noise and is suitable for

instrumentation applications.

AD8510

VEE

VCC

Figure 53. Equivalent Preamplifier Photodiode Circuit

A larger signal bandwidth can be attained at the expense of

additional output noise. The total input capacitance (Ct)

consists of the sum of the diode capacitance (typically 3 pF to

4 pF) and the amplifier’s input capacitance (12 pF), which

includes external parasitic capacitance. Ct creates a pole in the

frequency response, which may lead to an unstable system. To

ensure stability and optimize the bandwidth of the signal, a

capacitor is placed in the feedback loop of the circuit shown in

Figure 53. It creates a zero and yields a bandwidth whose corner

frequency is 1/(2π(R2Cf)).

The value of

R2 can be determined by the ratio V/ID, where V is

the desired output voltage of the op amp and ID is the diode

current. For example, if ID is 100 µA and a 10 V output voltage is

desired, R2 should be 100 kΩ. Rd is a junction resistance that

drops typically by a factor of 2 for every 10°C increase in

temperature. A typical value for Rd is 1000 MΩ. Since Rd >> R2,

the circuit behavior is not impacted by the effect of the junction

resistance. The maximum signal bandwidth is

MAX

2π

where ft is the unity gain frequency of the amplifier.

Using the parameters above,

Cf ≈ 1 pF, which yields a signal

bandwidth of about 2.6 MHz.

2π

where ft is the unity gain frequency of the op amp, achieves a

phase margin, Φm, of approximately 45°.

A higher phase margin can be obtained by increasing the value

of Cf. Setting Cf to twice the previous value yields approximately

Φm = 65° and a maximally flat frequency response, but reduces

the maximum signal bandwidth by 50%.

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