ADA4610-1ARJZ-R2 datasheet(21/27 Pages) AD

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Data Sheet

ADA4610-1/ADA4610-2/ADA4610-4

Rev. I | Page 21 of 27

APPLICATIONS INFORMATION

INPUT OVERVOLTAGE PROTECTION

The ADA4610-1/ADA4610-2/ADA4610-4 have internal protective

circuitry that allows voltages as high as 0.3 V beyond the supplies

to be applied at the input of either terminal without causing

damage. For higher input voltages, a series resistor is necessary

to limit the input current. Determine the resistor value by

≤

−

where:

VIN is the input voltage.

VS is the voltage of either V+ or V−.

RS is the series resistor.

With a very low bias current of <1.5 nA up to 125°C, higher

resistor values can be used in series with the inputs. A 5 kΩ

resistor protects the inputs from voltages as high as 25 V

beyond the supplies and adds less than 10 µV to the offset.

PEAK DETECTOR

The function of a peak detector is to capture the peak value of a

signal and produce an output equal to it. By taking advantage of

the dc precision and super low input bias current of the JFET input

amplifiers, such as the ADA4610-1/ADA4610-2/ADA4610-4, a

highly accurate peak detector can be built, as shown in Figure 58.

VCC

VIN

–

ADA4610-1/

ADA4610-2

ADA4610-4

ADA4610-1/

ADA4610-2

ADA4610-4

VEE

U2A

50pF

1µF

1kΩ

10kΩ

1N448

1N4148

+PEAK

1N4148

U2B

Figure 58. Positive Peak Detector

In this application, Diode D3 and Diode D4 act as unidirectional

current switches that open up when the output is kept constant (in

hold mode). To detect a positive peak, U2A drives C3 through D3

and D4 until C3 is charged to a voltage equal to the input peak

value. Feedback from the output of the U2B + peak through R6

limits the output voltage of U2A. After detecting the peak, the

output of U2A swings low but is clamped by D2. Diode D3

reverses bias and the common node of D3, D4, and R7 is held to a

voltage equal to + peak by R7. The voltage across D4 is 0 V;

therefore, its leakage is small. The bias current of U2B is also small.

With almost no leakage, C3 has a long hold time.

The ADA4610-1/ADA4610-2/ADA4610-4, shown in Figure 58,

are ideal for building a peak detector because U2A requires dc

precision and high output current during fast peaks, and U2B

requires low input bias current (IB) to minimize capacitance

discharge between peaks. A low leakage and low dielectric

absorption capacitor, such as polystyrene or polypropylene, is

required for C3. Reversing the diode directions causes the

circuit to detect negative peaks.

CURRENT TO VOLTAGE (I TO V) CONVERSION

APPLICATIONS

Photodiode Circuits

Common applications for I to V conversion include photodiode

circuits where the amplifier converts a current emitted by a diode

placed at the negative input terminal into an output voltage.

The low input bias current, wide bandwidth, and low noise of

the ADA4610-1/ADA4610-2/ADA4610-4 make them excellent

choices for various photodiode applications, including fax

machines, fiber optic controls, motion sensors, and barcode

readers.

The circuit shown in Figure 59 uses a silicon diode with zero

bias voltage. This setup is a photovoltaic mode, which uses

many large photodiodes. This configuration limits the overall

noise and is suitable for instrumentation applications.

1/2

VEE

VCC

ADA4610-1/

ADA4610-2

ADA4610-4

Figure 59. 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 30 pF to 40 pF) and

the amplifier input capacitance (<10 pF), which includes external

parasitic capacitance. CT creates a zero in the frequency response

that can lead to an unstable system. To ensure stability and

optimize the bandwidth of the signal, place a capacitor in the

feedback loop of the circuit shown in Figure 59. The capacitor

creates a pole and yields a bandwidth with a corner frequency of

1/(2π(RF

CF))

where:

RF is the feedback resistor.

CF is the feedback capacitor

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