ad549sh-883b Analog Devices, Inc., ad549sh-883b Datasheet - Page 15

• Manufacturer Part Number: ad549sh-883b
• Description: Ultralow Input Bias Current Operational Amplifier
• Manufacturer: Analog Devices, Inc.

Input current, I
proportional to the feedback resistance
The op amp’s input voltage offset causes an error current
through the photodiode’s shunt resistance, R
The error current results in an error voltage (V
amplifier’s output equal to
Given typical values of photodiode shunt resistance (on the
order of 10
if a large feedback resistance is used. Also, R
temperature because photodiode shunt resistance typically
drops by a factor of 2 for every 10°C rise in temperature. An op
amp with low offset voltage and low drift must be used in order
to maintain accuracy. The AD549K offers a guaranteed maxi-
mum 0.25 mV offset voltage and 5 mV/°C drift for very
sensitive applications.
Photodiode Preamp Noise
Noise limits the signal resolution obtainable with the preamp.
The output voltage noise divided by the feedback resistance is
the minimum current signal that can be detected. This mini-
mum detectable current divided by the responsivity of the
photodiode represents the lowest light power that can be
detected by the preamp.
Noise sources associated with the photodiode, amplifier, and
feedback resistance are shown in Figure 43; Figure 44 is the
spectral density vs. frequency plot of the contribution of each of
the noise sources to the output voltage noise (circuit parameters
in Figure 42 are assumed). Each noise source’s rms contribution
to the total output voltage noise is obtained by integrating the
square of its spectral density function over frequency. The rms
value of the output voltage noise is the square root of the sum of
all contributions. Minimizing the total area under these curves
optimizes the preamplifier’s resolution for a given bandwidth.
The photodiode preamp in Figure 41 can detect a signal current
of 26 fA rms at a bandwidth of 16 Hz, which, assuming a
photodiode responsivity of 0.5 A/W, translates to a 52 fW rms
minimum detectable power. The photodiode used has a high
source resistance and low junction capacitance. C
signal bandwidth with R
V
I = V
V
I
S
E1
E2
= I
= (1 + R
OS
9
B
/R
B
Ω), R
R
× R
Figure 43. Photodiode Preamp Noise Sources
S
S
B
, contributes an output voltage error, V
B
F
F
/R
F
/R
C
S
)V
S
S
can easily be greater than 1, especially
OS
F
and also limits the peak in the noise
IN
R
IF
EN
F
A
F
S
/R
E2
S
) at the
increases with
C
F
F
sets the
E1
,
Rev. G | Page 15 of 20
gain that multiplies the op amp input voltage noise contribu-
tion. A single-pole filter at the output of the amplifier limits the
op amp output voltage noise bandwidth to 26 Hz, comparable
to the signal bandwidth. This greatly improves the signal-to-
noise ratio of the preamplifier (in this case, by a factor of 3).
LOG RATIO AMPLIFIER
Logarithmic ratio circuits are useful for processing signals with
wide dynamic range. The AD549L’s 60 fA maximum input
current makes it possible to build a log ratio amplifier with
1% log conformance for input currents ranging from 10 pA
to 1 mA, a dynamic range of 160 dB.
The log ratio amplifier in Figure 45 provides an output voltage
proportional to the log base 10 of the ratio of Input Current I
and Input Current I
for voltage inputs. Because NPN devices are used in the feed-
back loop of the front-end amplifiers that provide the log
transfer function, the output is valid only for positive input
voltages and input currents. The input currents set the collector
currents IC1 and IC2 of a matched pair of log transistors, Q1
and Q2, to develop voltages VA and VB
where IES is the transistors’ saturation current.
The difference of VA and VB is taken by the subtractor section
to obtain
VC is scaled up by the ratio of (R9 + R10)/R8, which is equal to
approximately 16 at room temperature, resulting in the output
voltage
R8 is a resistor with a positive 3500 ppm/°C temperature
coefficient to provide the necessary temperature compensation.
The parallel combination of R15 and R7 is provided to keep the
subtractor section’s gain for positive and negative inputs
matched over temperature.
Figure 44. Photodiode Preamp Noise Sources' Spectral Density vs. Frequency
VA, VB = –(kT/q)ln IC/IES
VC = (kT/q)ln(IC2/IC1)
V
OUT
100n
10n
10µ
1µ
= 1 V × log(IC2/IC1)
1
EN
CONTRIBUTION,
WITH FILTER
10
2
. Resistor R1 and Resistor R2 are provided
IF AND CS, NO FILTERS
100
IF AND CS, WITH FILTERS
FREQUENCY (Hz)
OPEN-LOOP GAIN
1k
AD549
EN CONTRIBUTION,
NO FILTER
10k
100k
AD549
1M
1