ADA4001-2

Data Sheet

Rev. C | Page 10 of 12

APPLICATIONS INFORMATION

TOTAL NOISE INCLUDING SOURCE RESISTORS

The low input current noise and input bias current of the

ADA4001-2 makes it the ideal amplifier for circuits with

substantial input source resistance. Input offset voltage

increases by less than 15 nV per 500 Ω of source resistance

at room temperature. The total noise density of the circuit is



S

S

n

n

nTOTAL

kTR

R

i

e

e

4

2

2







where:

en

is the input voltage noise density of the part.

in

is the input current noise density of the part.

RS

is the source resistance at the noninverting terminal.

k

is Boltzmann’s constant (1.38 × 10–23 J/K).

T

is the ambient temperature in Kelvin (T = 273 + °C).

For RS < 4 kΩ, en dominates and enTOTAL ≈ en. The current

noise of the ADA4001-2 is so low that its total density does

not become a significant term unless RS is greater than

100 MΩ, an impractical value for most applications.

The total equivalent rms noise over a specific bandwidth is

expressed as

BW

e

e

nTOTAL

nTOTAL 

where BW is the bandwidth in hertz.

Note that the previous analysis is valid for frequencies larger

than 150 Hz and assumes flat noise above 10 kHz. For lower

frequencies, flicker noise (1/f) must be considered.

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 negative input terminal into an output

voltage.

The ADA4001-2 low input bias current, wide bandwidth, and

low noise makes it an excellent choice for various photodiode

applications, including fax machines, fiber optic controls,

motion sensors, and bar code readers.

The circuit shown in Figure 28 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

8

3

1

2

ADA4001-2

Cf

R2

Rd

Ct

VEE

VCC

Figure 28. 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 and the amplifier’s

input capacitance (8 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 28. 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.

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 (see Figure 28) 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

2



where ft is the unity gain frequency of the amplifier.

Cf can be calculated by

ft

R

Ct

Cf

2

2



where ft is the unity gain frequency of the op amp, and it 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 maximal flat frequency response, but it reduces the

maximum signal bandwidth by 50%.