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
n
nTOTAL
kTR
R
i
e
4
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
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.
low noise makes it an excellent choice for various photodiode
applications, including fax machines, fiber optic controls,
motion sensors, and bar code readers.
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
103
75
-034
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
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%.