AD8510/AD8512/AD8513

Rev. H | 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.

4

7

3

6

2

AD8510

Cf

R2

Rd

Ct

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 that can 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

where:

V

is the desired output voltage of the op amp.

is the diode current.

R2 should be 100 kΩ. Rd (see Figure 53) 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. Because Rd >> R2, the

circuit behavior is not impacted by the effect of the junction

resistance. The maximum signal bandwidth is

Ct

R

ft

f

MAX

2

2π

=

where ft is the unity gain frequency of the amplifier.

Cf can be calculated by

ft

R

Ct

Cf

2

2π

=

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

A higher phase margin can be obtained by increasing the value

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

maximum signal bandwidth by 50%.

Using the previous parameters with a Cf ≈ 1 pF, the signal

bandwidth is approximately 2.6 MHz.

Signal Transmission Applications

One popular signal transmission method uses pulse-width

modulation. High data rates may require a fast comparator

rather than an op amp. However, the need for sharp, undistorted

signals may favor using a linear amplifier.

The AD8510/AD8512/AD8513 make excellent voltage

comparators. In addition to a high slew rate, the AD8510/

AD8512/AD8513 have a very fast saturation recovery time. In

the absence of feedback, the amplifiers are in open-loop mode

(very high gain). In this mode of operation, they spend much of

their time in saturation.

The circuit shown in Figure 54 was used to compare two signals

of different frequencies, namely a 100 Hz sine wave and a 1 kHz

triangular wave. Figure 55 shows a scope plot of the resulting

output waveforms. A pull-up resistor (typically 5 kΩ) can be

reach the positive rail. The trade-off is that power consumption

is higher.

V1

V2

4

2

6

7

3

–15V

+15V

Figure 54. Pulse-Width Modulator