Application Hints–AD548
P H O TO D IO D E P REAMP
T he 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. T he photodiode sources a current
proportional to the incident light power on its surface. RF converts
the photodiode current to an output voltage equal to RF × IS.
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 mm2 area) photodiode shown in
Figure 27. T he input current results in an error proportional to
the feedback resistance used. T he amplifier’s offset will produce
an error proportional to the preamp’s noise gain (I + RF/RSH),
where RSH is the photodiode shunt resistance. T he 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. T he error budget in Figure 28 assumes a room temperature
photodiode RSH of 500 MΩ, and the maximum input current
and input offset voltage specs of an AD548C.
Figure 29. Low Power Instrum entation Am plifier
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. T he maximum input current is
15 pA over the common-mode range, with a common-mode
impedance of over 1 × 1012 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.
TEMP
؇C
RSH (M⍀)
VOS (V) (1+ RF/ RSH) VOS IB (pA) IBRF
TOTAL
T he –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. T he typical output slew rate is 1.8 V/µs.
–
0
25
15,970
2,830
500
88.5
15.6
150
200
250
300
350
151 µV
207 µV
300 µV
640 µV
2.6 m V
0.30
2.26
10.00
56.6
320
30 µV 181 µV
262 µV 469 µV
1.0 m V 1.30 m V
5.6 m V 6.24 m V
32 m V 34.6 m V
+25
+50
+75
LO G RATIO AMP LIFIER
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. T he 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. T his circuit produces
an output voltage equal to the log base 10 of the ratio of the in-
put currents I1 and I2. Resistive inputs R1 and R2 are provided
for voltage inputs.
+85
7.8
370
5.1 m V
640
64 m V 69.1 m V
Figure 28. Photo Diode Pre-Am p Errors Over Tem perature
T he 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. T his can result in
excess noise over the bandwidth of interest. CF reduces the
noise gain “peaking” at the expense of bandwidth.
Input currents I1 and I2 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:
INSTRUMENTATIO N AMP LIFIER
T he 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. T otal current drain for
this circuit is under 600 µA. T his configuration is optimal for
conditioning differential voltages from high impedance sources.
VBE = (kT/q) ln (IC /IES
)
In this equation, k is Boltzmann’s constant, T is absolute tem-
perature, q is an electron charge, and IES is the reverse saturation
current of the logging transistors. T he 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. T emperature
T he overall gain of the circuit is controlled by RG, resulting in
the following transfer function:
VOUT
VIN
(R1 + R2 )
= 1 +
RG
REV. C
–7–
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