Application Hints–AD548
PHOTODIODE PREAMP
The performance of the photodiode preamp shown in Figure 27
is enhanced by the AD548’s low input current, input voltage
offset and offset voltage drift. The photodiode sources a current
proportional to the incident light power on its surface. R
F
converts
the photodiode current to an output voltage equal to R
F
×
I
S
.
Figure 27.
An error budget illustrating the importance of low amplifier
input current, voltage offset and offset voltage drift to minimize
output voltage errors can be developed by considering the equi-
valent circuit for the small (0.2 mm
2
area) photodiode shown in
Figure 27. The input current results in an error proportional to
the feedback resistance used. The amplifier’s offset will produce
an error proportional to the preamp’s noise gain (I + R
F
/R
SH
),
where R
SH
is the photodiode shunt resistance. The amplifier’s
input current will double with every 10°C rise in temperature,
and the photodiode’s shunt resistance halves with every 10°C
rise. The error budget in Figure 28 assumes a room temperature
photodiode R
SH
of 500 MΩ, and the maximum input current
and input offset voltage specs of an AD548C.
TEMP
C
R
SH
(M )
15,970
2,830
500
88.5
15.6
7.8
V
OS
( V) (1+ R
F
/R
SH
) V
OS
150
200
250
300
350
370
151
µV
207
µV
300
µV
640
µV
2.6 mV
5.1 mV
I
B
(pA)
0.30
2.26
10.00
56.6
320
640
I
B
R
F
30
µV
262
µV
1.0 mV
5.6 mV
32 mV
64 mV
TOTAL
181
µV
469
µV
1.30 mV
6.24 mV
34.6 mV
69.1 mV
Figure 29. Low Power Instrumentation Amplifier
Gains of 1 to 100 can be accommodated with gain nonlinearities
of less than 0.01%. Referred to input errors, which contribute
an output error proportional to in amp gain, include a maxi-
mum untrimmed input offset voltage of 0.5 mV and an input
offset voltage drift over temperature of 4
µV/°C.
Output errors,
which are independent of gain, will contribute an additional
0.5 mV offset and 4
µV/°C
drift. The maximum input current is
15 pA over the common-mode range, with a common-mode
impedance of over 1
×
10
12
Ω.
Resistor pairs R3/R5 and R4/R6
should be ratio matched to 0.01% to take full advantage of the
AD548’s high common-mode rejection. Capacitors C1 and C1′
compensate for peaking in the gain over frequency caused by
input capacitance when gains of 1 to 3 are used.
The –3 dB small signal bandwidth for this low power instru-
mentation amplifier is 700 kHz for a gain of 1 and 10 kHz for a
gain of 100. The typical output slew rate is 1.8 V/µs.
LOG RATIO AMPLIFIER
–
25
0
+25
+50
+75
+85
Figure 28. Photo Diode Pre-Amp Errors Over Temperature
The capacitance at the amplifier’s negative input (the sum of the
photodiode’s shunt capacitance, the op amp’s differential input
capacitance, stray capacitance due to wiring, etc.) will cause a
rise in the preamp’s noise gain over frequency. This can result in
excess noise over the bandwidth of interest. C
F
reduces the
noise gain “peaking” at the expense of bandwidth.
INSTRUMENTATION AMPLIFIER
Log ratio amplifiers are useful for a variety of signal condition-
ing applications, such as linearizing exponential transducer out-
puts and compressing analog signals having a wide dynamic
range. The AD548’s picoamp level input current and low input
offset voltage make it a good choice for the front-end amplifier
of the log ratio circuit shown in Figure 30. This circuit produces
an output voltage equal to the log base 10 of the ratio of the in-
put currents I
1
and I
2
. Resistive inputs R1 and R2 are provided
for voltage inputs.
Input currents I
1
and I
2
set the collector currents of Q1 and Q2,
a matched pair of logging transistors. Voltages at points A and
B are developed according to the following familiar diode
equation:
V
BE
=
(kT/q) ln (I
C
/I
ES
)
The AD548C’s maximum input current of 10 pA makes it an
excellent building block for the high input impedance instru-
mentation amplifier shown in Figure 29. Total current drain for
this circuit is under 600
µA.
This configuration is optimal for
conditioning differential voltages from high impedance sources.
The overall gain of the circuit is controlled by R
G
, resulting in
the following transfer function:
V
OUT
V
IN
REV. C
=
1
+
(R
1
+
R
2
)
R
G
In this equation, k is Boltzmann’s constant, T is absolute tem-
perature, q is an electron charge, and I
ES
is the reverse saturation
current of the logging transistors. The difference of these two
voltages is taken by the subtractor section and scaled by a factor
of approximately 16 by resistors R9, R10, and R8. Temperature
–7–
AD [ ANALOG DEVICES ]
